GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Topic started by: walrus on April 18, 2018, 05:04:24 PM
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I have a Stoeger A30 and have not been happy with the performance. I decided to tear it down to investigate the problem and perform some tuning. Immediately I noticed that the piston seal is broken. Then I noticed that there is some black glue protruding from the bottom of the cylinder. There is a seam around the bottom of the cylinder which has been glued with this stuff, and they have used too much of it so it sticks out from the bottom of the cylinder in one spot. Clearly that blob of glue is the cause of the piston seal failure. I tried to sand it down, but it's really difficult to do it without damaging the cylinder wall.
I got an idea: what if I plug the transfer port and carefully pour some epoxy to the bottom of the cylinder. After it hardens, I can drill the transfer port open. This would reduce the piston stroke about 1-2mm. I don't mind if the power reduces a bit due to this mod.
What do you think about this idea? Does it cause problems if I shorten the stroke a little. Do you think that the epoxy will be strong enough? Any better ideas of handling the issue?
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You
wouldn't be shortening the stroke, you would be shortening the cylinder (and preload), and therefore increasing the compression ratio, and the likelihood of the piston slamming into the end of the cylinder on firing.... Since the end of the cylinder would now be epoxy instead of metal, I think it would disintegrate in short order....
Bob
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I have a Stoeger A30 and have not been happy with the performance. I decided to tear it down to investigate the problem and perform some tuning. Immediately I noticed that the piston seal is broken. Then I noticed that there is some black glue protruding from the bottom of the cylinder. There is a seam around the bottom of the cylinder which has been glued with this stuff, and they have used too much of it so it sticks out from the bottom of the cylinder in one spot. Clearly that blob of glue is the cause of the piston seal failure. I tried to sand it down, but it's really difficult to do it without damaging the cylinder wall.
I got an idea: what if I plug the transfer port and carefully pour some epoxy to the bottom of the cylinder. After it hardens, I can drill the transfer port open. This would reduce the piston stroke about 1-2mm. I don't mind if the power reduces a bit due to this mod.
What do you think about this idea? Does it cause problems if I shorten the stroke a little. Do you think that the epoxy will be strong enough? Any better ideas of handling the issue?
I don’t know how well the epoxy would hold up but I would bet that a really great grade of marine epoxy in a properly decreased compression tube would hold up just fine. And yes Virginia, you would be shortening the stroke as surely as if you were using a longer piston or a short stroke kit such as you see sold by Tin bum Tuning and others. 1 or 2 mm would have a very small effect on output considering that it’s likely about 2 to 4 % of the stock stroke... that and the fact that the length of stroke is no the factor which prevents the piston from “slamming into the end of the cylinder “ rather it’s a cushion of compressed air held captive by the as yet motionless pellet in the breech...
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I apologize, because there is preload on the spring you would indeed be shortening the stroke.... but I still think the epoxy would fail in short order....
Let us know how it works out....
Bob
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Thanks for the answers. I think that you are right: there is a big risk that the epoxy would not last long in those harsh conditions. I actually made a special tool for my drill and managed to remove most of the junk from the cylinder. Not an easy task, as there's not much room to operate and almost zero visibility. Hopefully I did not damage the piston walls too much in the process. I'm still waiting for a new piston seal. Really interesting to see how it shoots after this, as I made many other improvements to the rifle too.