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Thread: New Crosman 1377 are under power - here's how to fix them.

  1. #1
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    New Crosman 1377 are under power - here's how to fix them.

    I have an early phase III 1377 (bolt action) pistol - I've converted it into a carbine with a QD shoulder stock and it's very good indeed. It's making 5.7 ftlbs

    I bought a brand new 1377 (the one with the black plastics). It was doing 3.2 ftlbs! It felt the same to pump, same effort required but it just made that power. I'd done some reading and seen mention of reduced transfer port sizes. I did some investigation and sure enough the transfer port was much smaller than in the older gun:
    http://www.flickr.com/photos/30750063@N05/8531486545/
    Here you can see the port from the old gun on the left, the small port from the new gun on the right and the new replacement transfer port I got off an internet auction site
    With the bigger port fitted I was disappointed to see that the power had only gone up to 3.4 ftlbs.

    More work needed, I took the valves out of both guns for a look.
    http://www.flickr.com/photos/30750063@N05/8532593980/
    Here's the problem - the valve on the left is from the new gun and you can see that there's a bleed hole drilled in the back of the valve. When the gun is fired some of the air is vented back into the hammer chamber, thus reducing the power.

    Here's the front of the valves, I'd read about the shape being different on restricted guns, but these are the same.
    http://www.flickr.com/photos/3075006...n/photostream/

    This is a plug I made from an M3 screw, I cut it to length and slotted the end so I could drive it in with a screwdriver. The hole in the back of the valve is 2mm I drilled it to 2.5mm and tapped it to 3mm. I stopped tapping just short of completing the thread, that way the screw tightens itself as it bottoms out. I used some Loctite too to hold it in place.
    http://www.flickr.com/photos/3075006...n/photostream/
    http://www.flickr.com/photos/3075006...n/photostream/
    http://www.flickr.com/photos/3075006...n/photostream/

    So, with the gun re-assembled it's now making 5.7 ftlbs! Woohoo!
    If the power proves to be a bit close to the knuckle the fall back position is to refit the smaller transfer port and then tune the diameter. For reference the small restricted port measures 1.9mm and the large unrestricted one is 3.0mm

    From the reading I've done it appears that the new pistol I bought is a Canadian spec one, they have lower power limits than us. Maybe Crosman decided to rationalise the range and make just two variants - full fat and Canadian restricted and since the Canadian version is legal here too we get these weedy pathetic emasculated versions.
    Last edited by Biker_Bob; 05-03-2013 at 09:55 PM.

  2. #2
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    Nice article Bob, very usefull info and pics...... Top man.

    Pete.

  3. #3
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    I agree with Pete. An excellent article, which will be I'm sure useful to anyone buying or
    owning a modern gun.

  4. #4
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    Bob,

    Absolutely brilliant thread and information, thank you.
    I have one of these, to which I've fitted a steel breech conversion and shoulder stock. I don't have my notes to hand, but I seem to remember that it's doing 4.6 ft.lbs with 10 pumps using Hobbies.
    I've not played with it as far as any further tuning to date, but may well do so in the future.

    There's a lot of very useful information in your write-up.

    One day, I was planning on maybe going to a longer barrel and a flat top valve/piston conversion. Depending on who you listen to, with the shoulder stock screwed on it'll be classed as a rifle, and at say, 9ft.lbs it'd make a lovely little light weight, recoilless, self-contained field gun. Now that the 1322's available, may go for that instead.

    With the 1377 have you played with different pellet weights to ascertain power? What I'd be wondering is...generally with a PCP / pump rifle, muzzle energy would be higher with a heavier pellet. Would this still apply when we're talking pistol power levels?

    Looking at your user name and location.....If you're into bikes, and live in Sheffield, feel free to drop in at my place of work....SMC Bikes. I won't be able to talk for TOO long before I get glared at, but it'd be nice to meet and have a chat.

    Not many Airgun fans, especially Crosman fans that I meet face to face!
    THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
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  5. #5
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    Hello, I popped down there (SMC) a few weeks ago as it happens.
    Regarding pellet weight it seems with these 1377s the muzzle energy can be higher with a lightweight pellet. The gun I've just tuned is a bit naughty with H&N lead free pellets (5.5gn). 5.7 ftlbs might be a little high on reflection - I'm in the process of ordering some brass transfer ports from GMAC, these have small holes in but because they're brass they're easy to drill out to tune the power.
    The alternative would be for me to pull my finger out and turn some up, but I use the lathe as a workbench and there's a TS185 engine in the middle of a rebuild.
    PM me an we can meet up to show off our toys.

  6. #6
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    Will do.
    It's nearly tea time and will have to clear inbox a little.
    THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
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  7. #7
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    From memory the Canadian limit is 500 fps irrespective of calibre before classification as a firearm.

  8. #8
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    Quote Originally Posted by harvey_s View Post
    From memory the Canadian limit is 500 fps irrespective of calibre before classification as a firearm.
    They must love .25 cal then

    Matty
    Opportunity is missed by most people, because it is dressed in overalls and looks like work.

  9. #9
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    Quote Originally Posted by TonyL View Post

    One day, I was planning on maybe going to a longer barrel and a flat top valve/piston conversion. Depending on who you listen to, with the shoulder stock screwed on it'll be classed as a rifle!
    I was under the impression that if it was sold as a pistol originally without a stock as standard it remains a pistol as far as the law is concerned and subject to the 6 foot pound limit.Of course I may be wrong (again).

  10. #10
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    Thanks for sharing that.Wish I had the skills to do a bit of engineering such as this I would struggle with lego .

  11. #11
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    I made this discovery last year. I went off on a few wrong tacks and spent a fair bit on custom parts before the penny dropped.
    Thread here

  12. #12
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    Quote Originally Posted by sniper24687066 View Post
    Thanks for sharing that.Wish I had the skills to do a bit of engineering such as this I would struggle with lego .
    These pistols are so easy to work on you wouldnt believe! Reading this thread has made me want to start fiddling with my 1377 again. There's something about making a silk purse out of a sow's ear.

  13. #13
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    Quote Originally Posted by pcusername View Post
    I was under the impression that if it was sold as a pistol originally without a stock as standard it remains a pistol as far as the law is concerned and subject to the 6 foot pound limit.Of course I may be wrong (again).
    But which bit was sold as a pistol?
    The barrel? The breech? - lump of plastic; often replaced for a steel aftermarket jobbie. The grip frame? The valve? The main body which is actually the bit with the serial number on it? A piece of steel tubing with a slot and a few holes! These things are like Lego.

  14. #14
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    Quote Originally Posted by Adam77K View Post
    These pistols are so easy to work on you wouldnt believe! Reading this thread has made me want to start fiddling with my 1377 again. There's something about making a silk purse out of a sow's ear.
    100% agree.
    Very, very easy to work on.
    Some might be wary of working on a beautiful, expensive gun, but these are cheap and dead easy to work on, so nothing to be scared over.

    And the accuracy is STUNNING for such a cheap thing. Again, you'd expect an expensive, top-end gun to perform brilliantly, but to get such stunning performance out of something so cheap and underrated is a lovely bonus.
    THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
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  15. #15
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    Definately, even the trigger (which is the worst bit out of the box) can be easily fixed for no money by squashing the sear spring down in a vice and 5 minutes polishing the sear with 1000 grit wet and dry.
    Everybody seems to be in a rush to replace the plastic breach - no need at all unless you're intending to fit a scope, the standard breach is perfectly fine.

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