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Mrodair Airmax Compressor Review: Prep, Poor Build Quality, Fixes, Ultimately Unsafe for Use

Home Forums General Compressors, tanks and pumps Mrodair Airmax Compressor Review: Prep, Poor Build Quality, Fixes, Ultimately Unsafe for Use

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    For those working on their MrodAir Airmax Extreme compressor, this thread is both a resource and report of my own experience. Work on my compressor evolved over time. Initially, the focus was preparing the compressor for a longer life, easier maintenance scheduling, and safer operation. As issues were discovered, this devolved into troubleshooting, fixes, and currently finding my compressor unsafe to use until an oil-in-air problem fix is found at Mrodair.

    You can follow my journey beginning in October last year
    This thread is that Guinea Pig thread’s more useful successor.

    The first few posts in this thread cover major topics. The major topic posts continually undergo editing to keep information up to date. They do not attempt to preserve chronology. Posts after the major topics are chronologic, just as in any normal thread.

    NB. Changes in the major topic posts are not flagged as new by the BBS system.

    Disclaimer: Material presented here may be incomplete or inaccurate.
    Work you undertake on a compressor is expressly at your own risk.


    I pre-ordered my Airmax compressor from MrodAir after watching their product introduction video and reading the website. My impression was that the compressor would be a mid-priced, mid-performance machine suitable for my limited, single shooter needs. The promise that a US outfit would be going over the units and making sure they worked was reassuring. With its advertised “silicon bronze rings”, “3 cylinders”, and real pistons, this seemed a happy medium between an o-ring based compressor and the “overkill” of a dive compressor.

    We were reassured by MrodAir that each unit would be tested and that they had arranged many upgrades. These compressors would be long lasting, “real deal” compressors. As an airgun newbie, I was unfamiliar with MrodAir. There were some negative online reviews, but I also know that online reviews often trend towards the negative – even unfairly. I took a chance, ordered a compressor, and promised to report my experience on this forum.

    My desire was a compressor that would let me fill or top up a tank reliably. I was only a single shooter, but the convenience of ready air was enticing. Also, the ability to use high air consuming items, like regulator testers, made a compressor/tank combo a nice step up from hand pumping.

    My experience has not been consistent with a ready to go, fully developed product. Others have fared both better and worse than I. There are issues potential buyers should know before purchase.

    False Advertising
    There were factual inaccuracies in the product advertising at MrodAir. These incorrect details affected my decision to place an order. Had they been accurate, I might not have placed an order.

    As described on the product page when I ordered the machine…

    quote :

    Finally a real 110 compressor…….True 3 cylinder, with real pistons and rings made from silicon bronze for long service life…

    No, this is a TWO stage compressor. Only two cylinders do actual compression. Although the 2nd stage piston rides atop a carrier piston, that carrier piston does zero compression work. It is drilled through and lacks air inlet or outlet. It is simply not a compression cylinder. The compressor can still achieve 4500 psi by making the two stages work harder, but the 3 cylinder count is a factual error. I am only counting cylinders that perform compression. Otherwise, one could lash a six-pack of beer to the compressor and dub it a 9 cylinder compressor. Two cylinders means each must do more work than in a three stage machine. There is also less chance for interstage cooling of the compressed air.

    Silicon bronze piston rings were prominently mentioned as a feature of the compressor. This gave the impression that rebuild intervals would be consistent with that of metal piston rings. I was surprised to find my 2nd stage piston rings were not metallic. The rings resisting the greatest heat and pressure, are polymer
    Several other owners confirmed that the high pressure cylinder rings. I informed MrodAir and the initial reaction was that I was wrong. Their website continued to advertise these compressors as having silicon bronze rings for several days more.

    quote :

    ….the new Airmax Extreme and it IS a true 3 cylinder HPA compressor, with real pistons and high pressure rings made from silicon bronze, suspended in phenolic resin for long service life…..The low pressure cylinder, has traditional cast iron rings.

    Piston ring description was adjusted as of 4/22/2016. The high pressure rings are now described as silicon bronze suspended in phenolic resin and the low pressure rings as traditional cast iron. The number of cylinders advertised remains three.

    Because the 2nd stage rings are now known to be a less durable material than silicon bronze, having replacements is even more important. The high pressure rings look like angle cut wear rings. They are soft and easily indent with a fingernail. Heat, flame and smell testing of one from my compressor makes me think these are actually silicon bronze filled PTFE wear rings. I would like to find a second source. I have yet to find a source for bronze filled phenolic resin rings, but silicon bronze filled PTFE wear rings do exist.

    Durability and duty cycle should be scaled back in your mind. Filling a 88 cf tank in one session stresses the compressor to its limit. A reasonable expectation would be to top off a tank once in a while or fill a gun directly. Long term torture testing suggests 20 minute max run times.

    Technically, the compressors are 120 volt units, but they need a 30 amp service outlet to run properly. A 20 amp circuit is insufficient. My own unit draws about 24-27 amps during operation. It is simply too large a current draw to safely run on a 20 amp circuit. Perhaps one could get away with it for filling a gun directly, but topping or filling a tank is asking a lot of a 20 amp circuit.

    Electrical Hazard
    My unit arrived with two major electrical components (relay and contactor) hanging loose. Each was held in place only by the wires connected to them. A consumer who receives the machine reasonably expects safe and secure wiring. If your unit arrives in the state mine did, disaster will happen in short order.

    The compressor is not grounded. Grounding pin of its AC power socket is connected to ….. nothing. This machine has a semi-open, metal frame, uses water for cooling, and is not grounded.

    An under capacity 15 amp IEC receptacle connects the AC power cord to the compressor. The IEC connector is only 15 amp rated, well below the actual current draw of the compressor. The IEC power connector can overheat or starve the machine of adequate power.

    Electrical wring skills can correct these shortcomings. It is doable, but for a vetted design and build, the average buyer should not need to do electrical rework.

    Air Quality with Heavy Oil Contamination, (Critical Go/No Go Safety Item)
    My unit, and that of some other owners, continually passes oil from the crankcase into the low pressure cylinder. Oil that gets into the air path oxidizes (maybe even diesels) at the high pressure cylinder, fouling that cylinder’s rings and valves. The remaining oil goes on to the water separator and MUST be filtered out before it reaches a gun or tank. A little oil is not uncommon for a compressor, but it must be removed with a filter before it gets into your tank or gun. My unit splatters oil all about in the low pressure cylinder. Bleeding the separator filled my garage with suspended oil vapor. Not all units have this problem. Mine does and so do some other owner’s. This is distinct from being shipped with oil in the crankcase. It’s actively putting more oil into the cylinder with each stroke.

    This oiling issue should be tested before one puts the compressor into use. An affected compressor will still fill a tank. Merely testing whether the compressor will fill a tank is insufficient. You should disconnect the air output of the 1st stage and specifically check for continued oil output before you try a pressurized run. Without my large, Alpha filter, this would have been an oil in the tank disaster. With my Alpha, it is still a problem. A filter can only handle do so much. Filters are meant to deal with the the last traces of oil, not heavy contamination. Oil in your high pressure air system is an explosion hazard and may negatively affect seals in your guns.

    There are indeed MrodAir upgrades like the automatic cutoff gauge and radiator. Those are useful, but one needs to go over these units carefully before use. Expect to do electrical and mechanical work to keep them running. Mine, as delivered, was neither turnkey nor ready to use. Ultimately, mine had such severe oil contamination issues that it was unusable and no fix was forthcoming from Mrodair. On the up side, the compressor is easy to tear down and work upon. If good parts were available, and fixes developed, one could conceivably keep a unit running until one could afford a more robust solution. Just anticipate needing your mechanical and electrical skills.

    The compressor does fill fast – perhaps too fast for its own good, Takes about 80-90 minutes to fill a Great White from empty to 4500 that’s with frequent bleeding. It’s just a tad over 1 CFM. You can actually see the pressure gauge of a Great White move as the compressor works. However, there are reliability, longevity, duty cycle, and oil contamination issues that need working through. I never got mine sorted after months of patient work.

    My story follows. You will learn about initial inspection, electrical, air leaks, oil in air contamination, and blowouts. There is even a down trodden me “throwing in the towel” and ready to scrap the machine after first discovering heavy oil contamination and suffering another o-ring blowout while filling a tank.

    I think one might get this compressor to marginally work directly filling a gun or a small cylinder, but expect very slow to no customer support if you encounter real issues. This is a machine built to the barest margins possible to still run. It has no pressure safety releases and should not be run more than 15 – 20 minutes at a time. You need electrical and mechanical skills to keep things in good order. This is of course, if you get a good unit in the first place. Test thoroughly when it arrives. You are the factory’s last quality control step.


    Main Topic Posts Index

    30 amp 120 volt RV service outlet Installation.

    Delivered Components

    Initial Inspection

    inspection of cylinders for oil seepage during shipment

    Opening Crank Case for Complete Oil Change (No, not for routine changes!)

    Electrical Fixes and Upgrades

    High Pressure Valve and Water Separator

    Automatic Pressure Switch Gauge Glycerin

    Water Cooling System

    Piston and Piston Ring Measurements

    O-ring Sizes

    Latest changes
    No Mrodair fix for the oil in air contamination issues. Lower pressure cylinder consumes oil severely. Small engine shop evaluation suggested piston and cylinder tolerance are too large to ever effect a seal. Without a way to fix that problem, the compressor is unsafe to use. My unit is now going to another victim/owner for use as a parts donor.

    If you are contemplating this compressor… my painfully earned advice is to buy from a different dealer and get a Shoebox or save up for a full scale dive compressor. The Mrodair Airmax Extreme compressor is a poor quality product you will most likely regret.


    Issues with Solutions
    Most of these have resolutions described in more detail in this thread.

    Found and Corrected on My Compressor

    Relay & Contactor not on rail. Arrived hanging just by wires.
    Remount on DIN rail. Check wiring is well attached

    Underrated AC IEC connector.
    Replace supplied AC cord & IEC connector with strain relief plate
    and directly wired power cord & RV service plug.

    No electrical grounding
    Attach ground of new power cord to compressor frame

    Exposed 125 electrical wiring
    Shock hazard if anyone reaches under top panel. Needs enclosure for wiring.

    High current demand
    Install 30 amp 115 volt RV service circuit to power compressor
    Install power meter to monitor current draw

    Shipped with oil in crankcase & seeped into cylinders
    Blow out oil from system by letting compressor run
    with air pipes disconnected. Check and clean poppet valves and water separator.

    Oil contamination through entire air system. Apparently during test fills at MrodAir.
    Disassemble and clean cylinders, valves, water separator

    Fouling of high temperature cylinder piston
    Tear down high pressure cylinder & clean. Re-clock gaps

    Fouling of high pressure poppet spring
    Remove & clean poppet from top of high pressure cylinder.

    Compressor has 2 compression cylinders, not three as originally advertised
    Informed dealer of inaccuracy.

    Corrosion in water separator’s media chamber
    Disassemble and clean with nylon bristle brush
    Avoid use of silica media in media chamber

    Corrosion in water separator’s main chamber
    Corrosion level inside water separator, when new, already matches appearance of 5-10 year
    old separator per SCUBA compressor expert. No safe cycle life info provided for unit.

    Deformed, incorrect size o-ring at separator inlet fitting
    Replace with N1.50X011 or N1.80X011.2 buna D70 rings

    Female QD on fill whip does not fit most male QD’s.
    Replace fill whip with one with connection in spec.
    No glycerin in pressure gauge —> needle bounce
    Remove pressure gauge and fill with glycerin

    Water pump leak at impeller housing
    Replace supplied water pump with submersible unit

    Air Leak at water separator bleed valve body (straight taper, no seal)
    Remove valve body and use teflon tape to create air seal

    High pressure rings discovered not to be silicon bronze as originally advertised
    Informed dealer
    Decrease expected time before ring replacement

    No Air Intake Filter
    New low pressure cylinder cap, filter, and bolts (McKeown)

    Durability and Duty Cycle – Failure of o-rings and gaskets with prolonged runs despite adequate water cooling
    Limit on time to 15-20 minutes max. Let cool completely before restart
    Periodically inspect o-rings at high pressure head, water separator, filter cap

    Correctable Issues on Other Owner Compressors
    Blowout of high pressure cylinder head o-ring at 4100 psi
    Replace with N3.00X016 buna D70
    Silicone 3.00X016 D70 probable better replacement due to heat.
    Lap cylinder body top flat & thermal compound
    Torque adequately to reinforce seal

    Air Filter o-ring failure / too large to completely fit inside channel size
    Replace with N3.00X17 o-ring

    Low Pressure Head Gasket Failure
    Replace with new gasket
    Ensure adequate cylinder head bolt torque
    Larger valve plate & gasket assembly (McKeown)

    Very little pressurization
    Check low pressure stage reed valve alignment
    Check low pressure poppet valves
    Verify o-rings are in correct order and clocked correctly

    Pressure only 350 psi and oil vapor coming out of crankcase vent
    Probably high pressure piston ring failure
    Replace high pressure rings
    Check high outlet and water separator inlet poppets for fouling
    Is low pressure cylinder leaking oil and causing premature
    failure of high pressure rings?

    Ongoing Concerns
    Items not yet resolved or still planned for investigation

    Some low pressure cylinders do not control oil during runtime
    BEFORE first pressurized run, test for continual oil entering cylinder by letting compressor run
    with air pipes disconnected. Check oil level is at or just below center of observation port.
    If oil output persists > 10 min of test contact dealer & do NOT pressurize.

    Re-clocking piston rings and reducing oil level in crankcase did NOT help.
    Ring gaps measured and identified as possible root cause of oil leaking into low pressure side.
    Reed valve thickness or transition hole size also implicated

    Examination of low pressure cylinder by small engine shop reports piston too small for cylinder
    and is likely breaking ring seal by canting piston.

    (Currently awaiting results of MrodAir testing alternative piston rings. Compressor unusable until resolved)

    If only a little bit of oil issue, clean little, Mrodair supplied filter every 20 minutes
    Use larger filter (Alpha) with activated carbon to remove oil contamination.

    High pressure ring durability – polymer, not the expected metallic rings.
    Informed dealer. Corrected on website.
    2nd source wear rings possible? Are these actually PTFE rather than phenolic resin?

    Piston rods have no bearings.
    The wear surfaces of the piston rods are the bare aluminum of the rods themselves.

    Water Separation is AFTER media unit.
    Mechanical water separation should happen before compressed air reaches media.
    Airmax separator has its inner media cartridge before mechanical water separation.
    As a result, any absorbent media is immediately overwhelmed.

    Manual Switch Prone to Contact Welding or Burnout.
    The 15 amp manual switch is forced to handle switching off the entire 25 amp load when compressor is switched off manually. Rather than cutting of the motor via the contactor, the manual switch gets the brunt of breaking contact. Over time, the heavy arcing welds or burns out the switch. The manual cut off should have been implemented with a separate, normally open, push button breaking current to the contactor coil.




    The Airmax Extreme compressor requires a high amperage 120 VAC supply. Initial current draw was reported by MrodAir as 18 amps and building to 20-22 amps as pressure builds. My unit pulls nearly 25 – 27 amps, nearly all the time.

    This is too high a load for most home 120 volt outlets. A brief overload of a 20 amp circuit is said by the seller to be OK. I would not recommend overloading a 20 amp circuit intentionally. One should install a 30 amp 120 volt service for this compressor.

    Some owners of the compressor have taken advantage of their existing 30 AMP 240 V clothes dryer outlet. They build themselves an adapter to tap a one leg of the 220 volt, 30 amp dryer circuit. This avoids wiring in a new service, but my dryer outlet is difficult to reach and I preferred to add a new dedicated, “RV Service” 30 amp 120 v circuit in my garage

    WARNING. Never do your own wiring if you don’t have the experience to do it safely. WARNING.

    Lowe’s had the required 120 volt, 30 amp “RV service” receptacle and plug (TT-30R & TT-30P)


    Here are the goodies before and after installation.

    I intend to use the compressor for filling tanks. My wiring must support prolonged, full current load. I used 10 gauge Romex for a three foot run. That is more than sufficient ampacity for the distance.

    The MrodAir supplied AC power cord and IEC connector are underrated for the load. The connector is IEC 60320 C13 / C14, hence only 15 amp rated. My original plan was to use the existing cord and connector and monitor for heat, but eventually replaced the entire power cord and eliminated the under capacity IEC connector. That is detailed in another portion of this thread.

    For more info about installing 30 amp 120 volt service, see


    There are reports of compressors arrived in good working order, but some are arriving with significant issues. Inspecting the package for obvious damage is not enough. One should “look under the hood”

    1. Check whether oil is already in the crankcase. If so, you need to clean out oil that has seeped into the cylinders.
    2. Electrical components and wiring may be loose.
    3. Check low pressure cylinder for oil seal.
    4. Check high pressure outlet valve for fouling and water separator for oil contamination

    Multiple owners have reported their compressors arriving with electrical components loose or wiring disconnected. My own unit had both the relay and contactor hanging only by their wiring. Another owner had a loose relay. Yet another not only had loose relay, contractor and disconnected wiring, but also damage to some of those components.

    Here is mine in the condition it arrived. Both relay and contactor are loose

    If an owner didn’t notice this problem and fired up the compressor, there could be a short circuit, shock hazard, or compressor failure.

    Relay and contactor should be mounted like this

    Look under the top panel and verify that both the relay and contactor are well secured on the DIN rail. They should be snapped firmly into place and not pop off with a light push.
    Check that each wire is well secured. None should be completely free. All should be snug in their terminals. If any are loose or disconnected, seek advice from the seller for proper reconnection directions.

    Even if your compressor did not ship with oil in its crankcase, I would check it for low pressure cylinder oil leakage BEFORE your first pressurized run. You can test without tearing down the low pressure cylinder. Simply disconnect the air pipe from from the low pressure cylinder output. Start up your cooling pump and then the compressor. Let the outgoing air from the low pressure cylinder go into a paper towel. Careful, the air is hot. If you see oil staining and this persistently happens after five or ten minutes of run time, you likely have an oil seal issue in the lower pressure assembly. You can confirm this by looking into the low pressure cylinder. If copious oil coats the interior of the cylinder and the piston, you have a definite problem. Contact the dealer before attempting further use of the compressor.

    A good low pressure cylinder assembly limits how much oil gets from crankcase into cylinder. If you run the compressor with an oil in air contamination issue, the high pressure cylinder with foul quickly and you will have oil contaminating your air. Oil in your tank and equipment not only is an explosion hazard but that oil may attack the seals and materials or your guns.

    This is what happens with my compressor. It clearly has an oil contamination issue. You need to check for this before using the compressor.

    As of 4/30/2016, I am awaiting word from Mrodair whether some alternative piston rings will help. If this can’t be solved, one can’t safely use the compressor.


    My Mrodair Airmax Extreme was ordered with the full upgrade package. It arrived in two boxes. The compressor itself was double boxed. A plastic bag enclosed the compressor and a small box of accessories was taped inside the compressor’s frame. The 2nd box contained the larger accessories like the water jug and radiator.

    Pieces of three different diameter water tubing (sizes OD x ID: 8 x 5 mm, 10 x 6.5 mm, & 1/2″ x 3/8″), high pressure cylinder rebuild rings, fill whip and dowty washer, 90 degree brass water fittings for the cooling system.

    [NOTE: The high pressure rings that were prominently featured as silicone bronze are not actually metallic rings. They are polymer rings currently described as “silicon bronze suspended in phenolic resin.” Based on my examining a ring from my compressor, I think these are actually PTFE rings with suspended silicon bronze

    There are only two cooling fans included. This is intentional despite the radiator itself having space for three fans. If one mounts the fans interior to the radiator, the motor fan house would interfere with the 3rd position. However, radiator heat transfer is largely dominated by air flow, not water flow.I have ordered a 3rd fan from Amazon and plan to mount my fans on the outside of the radiator.

    Included fill whip’s female QD connector is out of spec. It won’t connect to any of my stainless steel Brancato male foster fittings. It barely connects onto my Vulcan fill probe. I thought it was an o-ring thickness issue, but no. Even at full metal to metal compression, the fitting just cannot get far enough on to lock into place. The face of the inner portion is too forward from the bearings. The inside face meets the male foster fitting when the bearings are still too far back to engage.

    I borrowed the fill whip from my Hill pump so It’s fitting could be used instead of the bad one shipped with the compressor.

    The air filter is tiny. Although captured in separate photo, I have kept the relative scaling constant in the below image.

    The included water jug is 5 gallons. If you thought your compressor will be a nice compact, single integrated unit, throw out that idea. This size jug is going to be tough to integrate. Mine is going onto a hand cart so I can keep my compressor, water jug, and J Brancato air dryer all together.


    You normally do not need to tear down and inspect the compressor, but if it was one shipped with oil in its crankcase like mine was, it is relatively easy to do so on this compressor. Inspection may also be needed if your compressor has a low pressure cylinder with an excess oiling issue.

    Airmax compressors should not be shipped with oil in the crankcase. If the compressor is tipped on it side, oil will seep into the low pressure cylinder. Once oil gets into the cylinder, it will contaminate the entire air system. The oil also undergoes high temperature breakdown and fouls your high pressure piston rings and water separator. In severe cases, it can get into your tanks and guns.

    MrodAir had problems with this in at least two ways. Some units arrived from China with oil in their crank case. Some units were not drained of oil after their test run at MrodAir. In either case, there was risk of oil seepage into the cylinder(s). Reportedly, the compressors are now shipping WITHOUT oil in the crankcase, but early owners suffered this issue (lucky me).

    A separate issue of poor piston oil seal was discovered with my unit’s low pressure cylinder. If your compressor has this issue, it will continually suck oil from the crank case into the low pressure cylinder. I strongly recommend checking for this fault by disconnecting the low pressure output line and doing some extended tests for continuing oil output from the low pressure cylinder. Initial inspection isn’t enough. Do at least a 5 minute run and verify that oil isn’t continuing to be output before allowing the compressor to perform a pressurized run. Otherwise excess oil will foul the output stage and contaminate output air. When this post was originally written, the only oiling issue know was due to shipping with oil in the crank case.

    For those affected, one method to check for and clear out oil is to disconnect the air line between the low and high pressure cylinders, run the compressor and let the air movement hopefully clear out the low pressure cylinder. Then, do further run time with the tubing connected, but no outlet load would clear out the high pressure end. MrodAir suggests checking the water separator filter elements during this process.

    I wasn’t convinced that was adequate an clean out. Armed with the promise that this compressor is easy to tear down, I opted to open the cylinder heads, inspect and clean my cylinders after I saw this upon my compressor’s arrival.

    Mine was one of those that WAS shipped with oil.

    The shipping bolt on the crankcase is merely a plug to prevent leakage during shipment. It does not engage or lock anything inside the crankcase in place.

    The crankcase breather is a simple filter element that lets the crankcase equalize pressure and yet not spew out oil.

    Removing the two cover panels is simple

    First, detach the plastic tubing then the metal air pipe.

    Four bolts are easily removed to free the cooling block, valve plate/cylinder head.

    Caution to anyone else doing this. The valve plate is very easy to accidentally drop once the bolts are out. It barely sticks to the cooling block and unless you support BOTH parts, the valve plate can suddenly drop off. Also, note the gasket between the cooling block and valve plate has narrow regions that must seal against the full pressure of the cylinder. If this gasket is damaged or the head inadequately torqued down, the gasket can blow out in those thin regions.

    The reed valve is held in position by two small metal pins. These pins merely sit in their mounting holes and readily fall out if one removes and inverts the cylinder. Since, we are only doing an inspection and clean out, there is no need to remove the actual cylinder. I did not for my inspection. Just the cylinder head needs to be opened. Leave the cylinder bolted to the crank case unless you have a unit that has an oiling issue and are instructed to inspect the rings.

    Here you see oil has seeped into my compressor’s low pressure side. It is pooled at the piston’s lower portion.

    Oil has also stained the inside of the cylinder head and the air intake reed valve (long metallic flap).

    Some alcohol soaked towels easily wiped away all the oil inside the cylinder and off the valves & valve plate.

    When reassembling the low pressure cylinder make sure the metallic reed valve, valve plate/cylinder head, cooling block detent together. If you align them correctly, they detent together.

    The low pressure outlet poppet valve is inside the air outlet fitting. Mine had a touch of oil on it, of course

    Onward to the high pressure cylinder….

    After removing the single red panel, Remove the high pressure air line that connects the high pressure cylinder to the water separator. Use two wrenches so you can apple counter torque to the avoid twisting the fittings.

    Detach air line and remove four bolts to free the cylinder head. Don’t break the thermal probe or its wire.

    This reveals three o-rings that seal cooling water. Two small ones are at the water inlet and outlet. A large one seals the top of the water jacket. A medium size o-ring in the center is the high pressure seal for the 2nd stage. That o-ring is at considerable risk of blowout with prolonged runs.

    The small, central, metallic disc with a hole is the high pressure cylinder’s air inlet reed valve wafer. It sits loose in its recess. Don’t lose that inlet reed valve! It is very thin and easily missed. I didn’t even realize what it was during my first tear down. Luckily, I never inverted the high pressure cylinder so it never fell out. Another owner of this compressor did lose this piece and had to machine a new reed valve wafer.

    The tubular cooling jacket can be slid off the cylinder. Take care to preserve its large o-rings. When you pull the cooling jacket off, take it gently and straight off the cylinder.

    Four bolts hold the cylinder in place.

    The small diameter, high pressure cylinder piston rides atop a carrier piston. The carrier piston performs no compression. It is only a mount that moves the actual high pressure piston up and down. The carrier piston is also drilled through and communicates with the crankcase. There are only TWO compressor stages. I originally mistook the large diameter, carrier piston as a coaxial, medium pressure stage piston. There is a similar compressor that adds an intervening head, some valves and uses that carrier piston as a compression stage. This compressor does not. There are only two compression stages.

    The high pressure cylinder is inside the long, thin portion of this piece. The cylinder has a steel liner held in place by a retaining ring.

    The reed valve wafer is attracted to magnets and very flat. Probably spring steel of some sort.
    Per my trusty old analog Mitutoyo, the reed valve wafer measures….

    outer diameter 0.5076 inches
    inner hole 0.1120 inches
    thickness 0.0208 inches

    Oil got onto my high pressure carrier piston as well. You can see a little pooled atop the carrier piston.

    Cleaned up the carrier piston and high pressure piston. My high pressure rings were already fouled with black carbon during the short test run at MrodAir.

    Cleaned and ready to put back together.

    Detail of high pressure piston and its rings after cleaning. These are not rotated into proper clocking yet.

    The high pressure rings are soft polymer (probably PTFE with suspended bronze particles) with a steel tensioning spring inside each ring. Align the gap of each tension spring with the gap of its corresponding PTFE wear ring. The gaps of the rings should not align with each other. Compress each ring with your fingers as they enter the cylinder. Take care to keep each ring well within their groove until they are fully inside the cylinder. Despite the entry taper of the cylinder liner, the wear rings are easily damaged unless you compress and support each as they enter the cylinder.

    Tips: it is easier to recompress the high pressure wear rings with your fingers if you EXTEND the piston out. If you don’t extend the piston, you won’t have enough finger clearance to compress each ring as it enters the high pressure cylinder. You can move the piston by pulling the piston out. With the piston in its extended position, the carrier piston nearly comes up to top of the finned cylinder.

    It is easier to achieve good air seal by attaching the 2nd stage cylinder end of the air pipe BEFORE seating the water separator end of the air pipe.

    Some warnings here….
    Don’t try to remove the high pressure cylinder air inlet fitting. That fitting is glued in place and does not house a poppet valve. Only the LOW pressure air outlet fitting houses a poppet valve.

    When tightening the cylinder heads, pay particular attention to having adequate torque on the heads, especially on the high pressure head. While you don’t want to strip the threads, you must torque sufficiently to seal against high air pressure and support the gaskets. Failure to tighten adequately could lead to leaks, or gasket failure.


    An air intake filter is not included in the MrodAir upgrade package. It was originally part of the spec, but was deleted some time after I ordered my compressor. I purchased a kit of materials for adding an air filter, replacing the oil drain plug, and uprating the power cord available from a fellow Airmax owner.

    The new, low pressure cylinder cover is threaded for the filter. Longer cap screws, gasket and anti-seize help everything fit together.
    The strain relief is used for bypassing the 15 amp (under-capacity) IEC connector that comes on the compressor.

    Here is how the filter screws onto the new cylinder cover. Threads go about 4/5 of the way through the cylinder cover. The last 1/5 of the hole is smaller diameter so the filter cannot screw in too far.

    Separating the original low pressure cylinder cover from the water cooling block took a chisel and light whack.

    Be mindful of the black gasket at the bottom of the cooling block! Don’t damage its tabs by improperly positioning the assembly before your “diamond cleaving” operation.

    Put a chisel right at the seam and lightly tap with a mallet to part the red RTV

    I temporarily removed the o-ring to make cleaning off all the red RTV from the cooling block easier. Once all the red RTV was gone, the o-ring went back into its groove.

    Prepped the new, longer cap screws by transferring the washers from the original bolts. A bit of anti-seize on the threads and it is ready to go.

    Verify the alignment of metal valve flap, intermediate air guide, and water cooling block. When properly positioned, each detents into places. Once all are aligned, add the new gasket atop the cooling block.

    Four bolts and the new cylinder cap are in place. Time to celebrate!
    Oops, the air filter won’t fit because of mechanical interference.

    One metal tab needed to be trimmed. I protected the air inlet with tape during the trimming.

    Now the air filter threads right in. Perfect. This is how the “upgraded” compressor should have been.


    You normally won’t need to inspect inside crankcase, but since I wanted to document as much as possible and completely switch over to Chemlube 501 I elected to follow Michael McKeown and open the crankcase and clean it out thoroughly. This is not a routine maneuver.

    HPA Compressors expose their lubricant to both high temperature and pressure. The seller supplied oil definitely undergoes breakdown into black residue in the high pressure cylinder. This fouls the rings. The situation is worse if you had oil seepage that wasn’t prevented or cleaned out.

    There are specialized compressor lubes like Royal Purple or the one suggested by Michael McKeown, Chemlube 501. These have a higher temperature before break down and are purported to extend intervals between compressor rebuilds. Mind you. You still need to regularly change the oil. We haven’t been quoted any oil change intervals or oil type for that matter. I am reading the diving forums and oil change interval seems to be somewhere around 50 to 100 hours (or one year if that comes first) for conservative owners.

    Chemlube 501 is a high temperature , synthetic, compressor lubricant. It should suffer less carbonization than the original oil. The original oil was already breaking down in the high pressure cylinder. Black carbon deposit on the high pressure piston had developed with just the short to check out the compressor in Arkansas. This was BEFORE I have even powered up the compressor.

    Lucky for me, the original oil drain plug was easy to remove with a light touch of a wrench. It did not disintegrate on me as it did for Michael McKeown.

    The original oil was surprisingly dark. This compressor is new and has ONLY been run at MrodAir. I have never run it. I’ll let the oil settle for a while and see if the dark settles to the bottom. [Edit: It does not]

    Draining the oil took about five minutes. Had to tilt the compressor to get more out.

    Five bolts hold the crankcase cover in place.

    Gently remove the crankcase cover to avoid damaging the gasket/membrane that forms the seal between the cover and the crankcase.

    This is the gasket/membrane partially removed. It peels off easily. The membrane partially isolates the cover from the main crankcase volume. The main crankcase undergoes very heavy oil splashing. The membrane helps limit oil reaching the crankcase breather. The small hole at the top of the membrane allows air exchange.

    Inside the crankcase, you can see the connecting rods, splash dipper, and counterweight. The splash dipper is actually part of the low pressure piston rod.

    Cleaned out 99% of the original oil by wiping down the interior with mild solvent. I continued to be shocked by how dark the oil was. Look at this paper towel that was merely wiped through a little oil. This isn’t soaked through, just a wipe along the bottom after most of the oil as out. [Even Chemlube 501 gets this dark after 90 minutes run time in this compressor]

    Washed the gasket/membrane using Dawn and some cotton balls to get it really clean. Because I plan to change oil fairly frequently, I also enlarged the hole at the bottom of the membrane to permit more complete drainage of the crankcase. The membrane makes it difficult to completely empty the compressor of oil. For this reason, I would be suspicious for oil seepage of every unit even it was drained before shipment.

    Putting it back together. The gasket bead fits into the crankcase cover’s groove. Be sure the grooves are clean!

    WARNING FOR LATER: BE VERY CAREFUL WHEN TIGHTENING THEM! It is easy to strip the holes into which they go. In fact, I had one that came already stripped at the factory. The original threading on that hole was too shallow. End of the bolt would bottom out in the threads before it tensioned its head. Hence, it had stripped when assembled at the factory. I was dismayed when the bolt simply spun at the bottom of its travel. Had to fix this problem by cutting new threads and using a larger diameter bolt.

    AGAIN, be super careful and don’t over-torque the bolts. The threads of the crankcase are thin. It’s aluminum. The wall of the crankcase is only 0.1 inches thick. So, be gentle. You only need to compress the gasket enough to seal against oil leakage. Doesn’t torque too much. If you use the HANDLE of your wrench, hold it with just two fingers and your thumb right at the socket. Don’t go cranking via the end of the wrench. I wish we knew the proper torque spec.

    Once the cover was back in place, the oil drain was reinstalled with just barely more than finger tightness. I used teflon tape on its threads to create a seal with minimal torque.

    Fill only to level of inspection port/oil drain center at most. It is a very efficient splash lubrication system. Under-filling causes insufficient lubrication, but overfilling causes over lubrication. Because my unit has a problem with over oiling when filled to center dot, I am keeping mine at a lower level in an attempt to reduce oil in the compressed air. The splash system is very effective at splattering oil all about.


    I traced out the wiring on my unit and Michael Esch confirmed my understanding of how it works.
    When power is applied to the compressor, the contactor coil is energized via a normally closed contact of the relay.
    The contactor then engages and supplies power to the motor.

    When the pressure gauge hits target pressure, its contacts close and energize the relay’s coil.
    This OPEN’s the NC contact that is powering the contactor coil. The contactor releases and cuts off the motor.
    Energizing the relay coil also latches the RELAY on via a N.O. contact in the relay. This is in parallel with the pressure gauge.

    The compressor will NOT power the motor again until
    1. The power is completely turned off
    2. The pressure gauge drops below its cutoff setting.

    Here are the changes I am planning…

    Another power switch upstream from the original one will help ensure my cooing pump and fans are on before the main motor can power up.

    Adding an hour meter to track how long the compressor has been running.


    The IEC connector supplied with the compressor is only rated for 15 amps. Michael McKeown’s kit includes a strain relief so one can completely eliminate that problem and connect a power cord directly to the compressor’s switch. You also have to do this because the red panel that original holds the IEC connector physically interferes with the air filter.

    A new 10 foot length of 12 gauge power cord replaces the original power cord.
    Here I have terminated it to fit my 30 amp 120 volt RV outlet. The cord then runs through McKeown’s strain relief. Beyond that, the outer jacket needed to be stripped back about a foot to access its wires.

    Grounding the compressor is a must in my book. Removed one of the feet and scraped the frame to expose bare metal. The grounding wire was then wrapped around the foot’s bolt and secured with the foot’s washers and nut. To ensure contact, none of the grounding wire’s insulation was allowed under the washer.


    Earlier in this adventure, I didn’t know about the relay and contactor mounting rail. I fashioned my own bracket. Here is how the relay and contactor should be mounted on the rail. They each snap into position. If properly snapped on, they won’t fall off even if you tug on them. It is possible to partially snap them into position. If you don’t get them all the way snapped in, they CAN and will fall off the rail.

    The finding that the units do indeed stay in place despite firm tugs is pretty good evidence that if these were loose at time of delivery, they simply weren’t properly snapped into position or were simply not even snapped into position. They didn’t fall off during shipping.

    The normal, central mounting position for the relay and contractor leaves no space for the extra switch and hour meter I want on my compressor.
    So, I moved the mounting rail about 2 inches rearward. This leaves plenty of space for my upgrades.

    I’m replacing the original, single switch with a pair of switches. Also, my hour meter needs a mounting position. That means cutting new panel holes and enlarging the existing one. I followed McKeown’s advice for cutting the holes.

    First, drill holes tangential to the corners. Then grind with a dremel tool with a cut-off wheel.

    The finished panel holes.

    Of course, one must have nice labels and hide the old rail mounting holes. Color laser printers are pretty handy.
    Clear plastic tape secures the label.

    Now, the switches and hour meter are mounted and ready for wiring.

    Once wired and rechecked several times, I made a successful test run of a few seconds. My wiring is correct!


    The high pressure outlet valve and water separator are inspected and cleaned in this post.
    Bob La Londe posts about his bad o-ring at the water separator further encouraged me to tear down, inspect, clean and rebuild these parts of my compressor.

    Beginning at the high pressure cylinder outlet, we examine the high pressure outlet poppet valve. This is accessed by first removing the high pressure air pipe from the top of the cylinder. The threaded piece at the top of the cylinder is the poppet valve housing. The housing unscrews to reveal the poppet valve.

    My poppet valve and spring were definitely fouled. In this montage of images you can see the gunk on the spring and flaking off onto the paper towel.
    All this needs to be cleaned. Of interest is the dowty washer that forms the air seal for the housing. A dowty washer is needed here because an unsupported o-ring would rapidly fail.

    Disassembled the components. Cleaned off most of the build-up. During reassembly, I coated the o-ring and spring with silicone grease. Anti-seize was applied to the threads. Here it is ready for re-installation.

    NB: The poppet housings at the high pressure cylinder and the water separator can both be inadvertently spun when you attach/detach the high pressure pipe. I recommend using a wrench to stabilize the poppet housings so they don’t rotate when you manipulate the pipe securing nuts. Otherwise, you can easily alter the torque on the poppet housings.

    TIP: It is easier to get an air tight seal on the high pressure tubing if you insert the cylinder end and finger tighten its B-nut first before engaging the water separator end. Doing it in reverse order makes it harder to properly seat the conical tube ends.

    Onward to the water separator….

    Atop the water separator are a poppet valve and the top of the air dryer’s media cylinder. Here, the poppet housing is partially unscrewed.
    The large hex at the top of the separator is the media cylinder’s top.

    The bleed valve’s brass body is prone to leak air where it threads into the water separator. This can occur even with the brass valve body fully tightened into the water separator. Mine had a big air leak here despite my never taking the brass body out of the water separator. Funny how that got past the tank fill testing in AR. Teflon tape on the brass valve body threads solves this leak. Just take out the entire valve body. Add 3-4 wraps of teflon and screw back into the aluminum water separator body.

    My poppet valve’s o-ring was mangled. This will require replacement.

    The original o-ring is actually too large for this application. One of the o-rings I have for my Vulcan fits and seals well here. It goes completely in the groove, seals well, and doesn’t get mangled.

    O-ring Store model: N1.50X011 (1.5 mm x 11 mm metric Buna N70)
    Per Michael McKeown, a N1.80X011.2 might be an even better fit.

    The N1.50X011 ring is small enough in cross section to fit entirely within the metal channel at the top of the water separator body. Place the o-ring all the way up past the poppet housing threads. When you screw in the poppet valve body, its metal flange should meet the metal of the water separator WITHOUT squeezing out the smaller o-ring. The o-ring expands under pressure to form the seal and doesn’t blow out like a larger o-ring.

    Removal of the media cylinder was challenging. Mine was nearly seized in position. Examination of its threads showed why.

    What about unscrewing the sintered bronze cap and looking inside the media cylinder?
    Yuck! Definitely got oil contaminated. The darkest beads were at the physical bottom.

    Inside the media cylinder I could definitely see aluminum corrosion. Joe Brancatos’s warnings about media being in direct contact should be heeded. Here it is in contact with just the media cylinder. The outer, main body is NOT in contact. Does that make a difference?

    Yes, the main body of the separator’s interior is pretty clean.

    Cleaned the components and reassembled. Again, silicone grease for the o-rings and anti-seize for the threads.

    Poppet ready to go back into the media/dryer cylinder.

    Whole assembly is now re-filled and ready to go back into the water separator’s main body. I tried to limit torque and let the o-rings do the work.

    Tearing down and cleaning the high pressure valves and media/dryer cylinder were essential on my compressor. It was also good to replace the mangled o-ring at the water separator’s poppet valve with a N1.50X011 (1.5 mm x 11 mm metric Buna N70)

    For now, I have refilled with a filter element in the media cylinder instead of silica beads. The volume is pitifully small there AND in front of the water separation. At any rate, I have a MUCH larger J Brancato Alpha filter that will do a better job. No reason to encourage further corrosion in the compressor’s water separator with silica beads.


    The upgraded Airmax Extreme comes with an automatic shutoff switch gauge. The gauge actually is capable of both low and high limit sensing, but only the high limit contact is used in this compressor. When pressure reaches the high limit, the electrical contact closes. This shuts off the motor by energizing the small relay. The relay cuts off the motor contactor’s coil and that is what removes power from the motor.

    The pressure switch gauge is more stable if filled with glycerin. Some units arrive with the gauge pre-filled. Others, like mine, arrive without glycerin.
    It is desirable to add glycerin to stabilize the sensed pressure to produce a more consistent cut-off pressure.

    Filling the gauge is much easier if one removes the gauge.

    Detach the two gauge wires (brown and black) from the relay block. Note where the wires go and that these wire each share their terminals with another wire. Here is a picture in case you forget where on the relay block the wires go.

    Detach the pressure sense tubing from the gauge. Use two wrenches, one to hold the fitting of the gauge to avoid stressing the gauge. I recommend detaching the GAUGE end of the tubing rather than the water separator end. The end at my separator does not swivel, whereas the end at my gauge does swivel. It is much easier to work on the swiveling end of the tubing.

    Remove three bolt and nuts to complete removal of the gauge.

    Filling the gauge is simple. It took about 100 ml of glycerin to fill my gauge. I left about 2 ml empty so there would be space for expansing and pressure changes. The rubber stopper comes out with just a trim flick of my fingernails.

    Reverse the process to reattach your gauge. it was was easier to reattach the wires with both the relay and contactor dismounted from the DIN rail. Take extra care to ensure both gauge wires AND the wire that accompanies each in their respective terminals are firmly in place.


    My compressor included the upgraded cooling kit with radiator. If one follows the Mrodair mounting video instructions, only the two included fans will fit. The third space is left without a fan because the fan would impinge against the motor fan fairing. Because radiator performance is known from the CPU overclocking world, to be mostly air flow dependent, I opted to use three fans instead of just the included two.

    NB: My mounting configuration does NOT match that in the Mrodair video. If you are only going to use two fans, you can follow the video’s instructions. What is described here is a modification needed to accommodate three fans.

    I have ordered a “AC Infinity AXIAL 1238, Muffin Cooling Fan, 115V AC 120mm by 120mm by 38mm High Speed” from Amazon to be the third fan. That
    That should include the power cord and I definitely wanted a dual ball bearing unit.

    Using three fans means they need to be installed on the outside of the radiator. I also wanted the electrical connectors up on top were I could easily access them. My install, therefore uses the mounting straps and hardware a bit differently from the Mrodair video. I only use three strap. Two are at the top of the radiator. One at the bottom secures the bottom of the radiator to the compressor base plate. It’s nicely secure and leaves room for all three fans. Another bonus is that all the strap and fan bolts are accessible in this orientation.

    Only two fans are currently installed, but you can see where the 3rd will go once it arrives.

    The straps are attached between the fans and the radiator. The bottom one needs to be bent so it projects straight rearward from radiator. Once all three strap were on the radiator, I test fitted using the upper two straps and marked where the lower strap will attach to the base plate. Then, I drilled a hole in the base plate to accept a self tapping machine screw. The screw was pre-fitted and the removed.

    NOTE: It is easier to attach tubing to the radiator barbs BEFORE you do final mounting of the radiator.

    In this picture, you can see how the lower strap has been secured to the compressor base plate. Up close detail of how the straps go between my fans and the radiator is shown.

    [Edited to indicate that one CAN screw in the adapters, albeit very cautiously because they are so tight]
    Two 90 degree, brass elbow connectors are provided in the kit. One is used to redirect the tubing of the 1st stage cooling jacket outlet towards the radiator. Unfortunately, they would easily screw into my unit. The cooling jacket is threaded to accept the tapered thread of the quick disconnect fitting, but the elbows are straight thread — really tight to thread into the cooling jacket. If it doesn’t thread in easily, I’m not going to use more force and possibly strip out the aluminum threads. Instead I used an alternative tube routing. The tube sticks out a little, but nothing is strained.

    A later experiment showed that it is possible to thread in the 90 degree connections, but the fitment is exceedingly tight. Requires a wrench to gradually thread in. Takes a gradual in/out motion like using a tap. Align perfectly before applying wrench. It would be very easy to cross thread.

    After it was all hooked up, I tested the water system.
    Unfortunately, not all was well….

    Pump is not self priming. You must mount your reservoir above the pump so it can prime. You cannot run hoses down to a reservoir. The empty pump cannot “suck” water up a tubing. After manually priming the pump, it started pumping water through the system. No leaks at the cylinder or radiator connections. Oh, wait what? No……

    My pump dribbles water from its impeller housing. Some others have reported even worse leaks. Although, Mrodair will send a replacement pump, I didn’t see any advantage to repeating with the same pump. Instead, I am ordering a higher output, submersible pump like some other compressor owners have used. The supplied pump might be submersible. It says “amphibious” on its casing, but I’m moving on to a unit others have had success with.

    Uniclife Submersible Pump Indoor Outdoor Water Fountain Pool Pump Aquarium Quiet US Plug

    The above, fully submersible pump solves the self-priming and leak issues. I placed the entire pump into my reservoir and used longer tubing for both my new pump outflow and radiator outflow.

    The pump’s outflow barb unscrews and accepts a 1/2 inch male NPT thread. Home depot has a 1/2 MTP to 1/4 ID brass barb that fits.
    Also, 3/8″ OD (1/4″ ID) PV tubing fits that brass barb and the high pressure cylinder water inlet quick connect.


    Third cooling fan and power meter arrived today.

    Third fan fits nicely. Had to loosen the other two fan’s bolts to slip new one into place. Then bolted up nicely.

    Here is the power draw with three fans (and nothing else) running…

    As measured on my newly installed volts, amp, and power meter.

    NB: NEVER power up a load if your amp meter’s current transformer is not hooked up. An unloaded CT can generate high voltages – enough to fry its own insulation. Always connect the CT to the meter before power is applied.

    Parts used were…
    bayite AC 80-260V 100A BYT-VAEM-034 Digital Current Voltage Power Energy Meter Ammeter Voltmeter with Open-close Current Transformer

    AC Infinity AXIAL 1238, Muffin Cooling Fan, 115V AC 120mm by 120mm by 38mm High Speed

    New submersible water pump arrived and achieved first pressure runs to 250 bar.

    No wonder people are popping their 20 amp circuits. Here is my current draw with the compressor running unloaded. The air outlet didn’t even have a fill whip screwed into it. Granted, this is real amp draw, not apparent amps.


    Dear, industrious Guykuo, I applaud your efforts and engineering skills. When you finally produce some high pressure air, you will look back on all your efforts with a sense of accomplishment and pride! You have taken a marginal machine and made it a useful tool. I wish all airgunners had your skill set and tools!

    LarryW 🙄

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