1377 mod questions??

Mig Rod:
I will direct these to you. I have two of the older 1377 and need a summer project. I would like all the advice you can offer up to me on how to modify these little buddies.

The older one would not pump until I cleaned and lubed it but the other was fine. ( hell Ive had the old one for 15+ years) Any way I put them on the chrony to see the numbers. Old topped out at 467 with a spread of about 10-12 fps @ ten pumps. The newer one / less fired, topped out at 478 with a spread of about 6-8 fps @ ten pumps. All were with RWS Superdomes 8.3g.
I did not group any of them on paper though.

I would like to get a longer barrel and some other mods to speed it up along with improving how it loads. All the new ones have a bolt instead of the trap door thing.

One other thing I do not have a machine shop at my disposal. So turning parts will be very hard to impossible. I do have a full wood shop on the other hand and am thinking of redoing the forearm so it fits my hands.

The two (old @ bottom)

Trap door

Other Guns

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Viewing 12 replies - 1 through 12 (of 12 total)

Looks great….

a little trick to getting the piston in without nicking it… when deburring the tube, take a small rod and wrap some 600grit sandpaper on it to make a loop… you’ll then insert the rod into the tube where you can then polish down the edged slot where the linkage rides. This is a spot that regularly nicks orings on the valve and piston when assembling/disassembling. Dont know if I’ve explained it well, but hopefully you get the idea.

After deburring everything thoroughly, take a 12 gauge shotgun cleaning swab and put some light lapping compound on it and work it through the tube to polish it up nice. Feel as far inside as you can with your finger…use your fingernail, if a burr catches on your fingernail, it will catch on the O-ring.

Hope this helps. The 13xx platforms are great for modding and tinkering…lots of fun!

Ok I got the parts in and have assembled them. Here is a few pic of the almost complete 1377. I got the crossman stock till I build one out of wood. I have ordered a sweet piece of Cocobolo. It will be a time consuming project but the final out come will be worth it.

I did throw it over the chrony and it is right at 597 to 604 with 15 pumps and RWS superdomes.

With the stock attached.

The only shit thing about it was that I nicked one of the O-rings when I installed it. So I might be getting a little blow by. Need to change it out and will chrony it again.
Know its time to do the shroud. And pointers will be greatly appreciated.

Well Pinguino, I am more knowledgeable with the 22XX C02 models than the pump models when it comes to modifying the valve.
But for what I see, you got all the right stuff like flat top piston. That will give you plenty of power right there.
Now comes the spring part, Make sure you get a stronger Hammer Spring, and get a lighter or weaker spring for the valve. Get the right combo, the best is to get a bunch of different sizes and test out the combo that gives out the higher fps.( important, make sure you don’t get a spring too weak and a hammer spring too strong, it could damage the valve stem.

I don’t know if the valve you got from Roy already has this, but drill out the port hole to 110 degrees for better air flow. (I’ve done this with 22XX valve, not 13XX, but I think it is the same. But still check)

Now, work on the Stem( again , don’t know if Roy already sold it to you with this done) Grind out the Valve Stem with a grinder, be careful to not melt the seal, as you grind dip the whole stem in cold water as you go. Only grind where the air flows when the Valve is open. Dont grind too much either, it could weaken the Stem. Just grind out like about 1/32 inch the size of the transfer port of the valve. Make sure when you put the valve together the section of the Stem you grinded is facing the Transfer Port when valve is open or struck by the hammer.

This is what I did to my 22XX and got Amazing Power. I’m sure it can work on the 13XX,( but check anyways to be sure.) The C02 models are much easier to mod than the 13XX, but looks like you are headed in the right direction.

Hope you build a Monster!!

OK Mig: I got my parts from Roy at Mountain air and I’m going to do the install soon. So my question is, is there any thing ells that you recommend to do to the stock parts for that extra little something? Here is the parts list that I got from Roy. The 2240 breach I got from Crossman.

2240 Breach Kit
Extended bolt w/ handle
Barrel band
Barrel cut to 16″
flat top valve
Flat top piston
Pump pin w/ clips
And the transfer port seal

Walkno: I still would like a few pointers on the barrel shroud that you have on yours. I’m going to do some thing like that but want to know how you dealt with the barrel band??


I just traded a Bushnell 3-9 AO airgun scope for one of these guns, I am waiting anxiously. That is a source I found on how do to mods at home and seemed like a decent write-up. I have a free to me Crosman 760 that used to shoot good then while in a vice I twisted the barrel on it and now it won’t shoot for shit so I might rob the barrel off of it and cut it off at 12-14 inches and put a shroud on it.

OK I have placed a few orders for some parts and will be putting some pics up as soon as I get them installed. In the mean time here is some pics of the disassembly and valve / trigger.

WOK: I would still like to know where you got the barrel extension for yours. It looks cool and it goes all the way back to the scope mount. That’s the look I’m going for.

Walkon: That is what I’m looking for. Also a power mod. Where and whats the cost??

I looked at the web sites that you gave to me Mig and there is a lot to take in so I will keep lookin and come up with something. ( flat top piston and a moded bulk valve looks like a good start). And it is some thing I can do in the garage while I’m haven a beer.

BTW Adam, sweet 1377 man, real nice!!

quote pinguino:

That is some thing like I’m looking for. Does that quiet it down any? Or is it a bull.
(knew I should not have left my self open like that 😳 . To easy)

You here nothing but a “Puff” when it is shot.

I’m not familiar with the trap door older models, but I think that it should be easy to just replace it with the modern breech with the bolt. There are many mods to be done, to replace for a longer barrel you would need a 1377 barrel band, all that can be ordered from crosman. Check out these sites, it talks about different mods you can do yourself and some modders that can do it for you( professionals). This is a forum specifically for crosman guns.
This is airgunartisan, they have a great section on details to the many different mods available.
This is a professional modder, he can modify your guns real good. His name is Roy. http://www.mountainaircustomairguns.com/

Another good modder, does good 1377 power mods.
You should be able to find all the info you need in these sites, good luck. Let me know how it works out.

That is some thing like I’m looking for. Does that quiet it down any? Or is it a bull.
(knew I should not have left my self open like that 😳 . To easy)

Longer barrel like this…lol

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