GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Share Your Simple Home Projects (TRICKS-N-TIPS) => Topic started by: WyoMan on June 11, 2019, 11:27:32 PM
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Some folks get retained air in their valves beyond (xx) pumps. I’ve never had retained air unless it was by design… might be the thinner air up here ;D
A heavier hammer spring may be necessary, but before you resort to brute force there are other ways to improve the valve action that won’t create the wear and tear on bolt handles, breeches, hammer pins, etc.… and,,, the not-so-fun cocking effort.
First, make sure everything is working 100% per design…
1) Check your rear breech screw to make sure it isn’t too long. If it’s too long (even by 10 thou, it can bind on the hammer spring. The hammer spring gets “fatter” when compressed (cocked) so that’s when the binding is likely to occur… This will reduce some of the hammer energy.
2) If you’re using an over-travel limiting screw, make sure that the trigger can still move about 3/32” after the sear breaks off of the hammer. Some over-travel is necessary to prevent “sear-drag” which will also reduce hammer energy. This problem is due to the hammer o.d. being about 10 thou smaller than the tube i.d. The hammer face / sear angle causes the hammer to tip upward and so the break occurs slightly sooner than if the hammer were sitting level with the tube. The sear needs to clear that additional 10 thou, and with the lever ratios you’ll need closer to 30 thou on the trigger.
3) Polish the hammer and give it a good dry-lube on all surfaces that contact the tube and the sear.
(https://i.imgur.com/lsmxgTO.jpg)
4) Debur and polish the inside of the tube where the hammer travels. Burrs, especially from the hammer pin inletting will reduce some hammer energy. I use an extended buff pad tool but a simple dowel made into a “flapper” will also work. Follow it with a good cleaning… WD-40, compressed air, and push a clean rag thru (just one way to do it).
(https://i.imgur.com/iwkJ4rs.jpg)
With everything deburred and polished, and the hammer dry-lubed, your hammer should slide forward in the tube by raising the back of the tube less than three inches:
(https://i.imgur.com/2y9yjSi.jpg)
Some Mods that will help even more…
1) Use a lighter valve return spring… less force to open the valve (better pump efficiency is another benefit). I use a stainless steel 1-3/8” long x 0.313” diameter x 0.031” wire diameter. It has a spring constant of 5 lb/in and is compressed about 0.4” on assembly. Available from Mcmaster-Carr # 1986K83.
2) Cut some flats on the sides of your hammer. This will reduce the “piston effect” which reduces hammer energy. A grinding stone followed with sandpaper does the trick.
(https://i.imgur.com/jGE6UA6.jpg)
3) Use a hammer spring guide and top hat. This will help keep the hammer spring compression / release in a more linear direction… resulting in more energy transfer to the hammer. The top hat eliminates the coil-end binding issues inside the hammer, and combined… they give you some extra preload on the hammer spring. You’ll have more hammer energy and a smoother cocking and firing cycle.
(https://i.imgur.com/GOy2MJo.jpg)
4) Use an Mrod valve stem… Crosman part number 1700-140. The harder Delrin seal will open faster with a smaller hit from the hammer.
(https://i.imgur.com/DmIKxzr.jpg)
Works for me so I hope this helps you…
Wyo
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Great write-up and pics!
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Great information. thanks for sharing this. 8)
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Great mini tutorial/pictorial for those of us just taking up this hobby, thank you.
Since your advice is to cut some flats on the hammer sides, would a fluted hammer accomplish the same effect?
Thank you,
Ted
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Hi Ted, thanks and yes, it's the same idea... i.e. provide more air passage to reduce the air pressure on the leading edge of the hammer.
There will still be some, but it will be reduced. The flats are a DIY version of the fluted hammer.
Wyo