A question for the 'airsmiths'

fat tony

fat tony

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I am making a new cylinder for a cheap underlever airgun. Yes, I wonder myself sometimes.

After several hours of fitting with a file, the seal is almost the right diameter.

This is where I need some input. It seems the pistonhead seal is the same diameter or slightly greater in diameter than the cylinder bore by maybe a few thousandths of an inch.

I know I am getting close, but this is where I need input. with the piston seal dry and the cylinder bore dry, how much resistance should run up against while pushing the piston in and pulling the piston out?


I know this is not worth it, but I want to get this right.


Thanks for any advice from spring piston gun tuners.


Tony :wave:
 
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Well how much resistance? When fitting new piston seals I normally can move the piston by hand. If it is a struggle to do so then it is likely too tight. A few pounds of resistance is not a lot considering you are probably in the 100-200 lbs mark for a cocked spring depending on the energy you are shooting. The seal will also break in some as you shoot it. It is normal to get some velocity increase with a new seal after things wear in and settle. I also use a very light amount if chamber oil. Good luck.
 
I reposted this on an airgun froum I am in and one of the tuners there says the following:

I wouldn't worry about the resistance, I'd be concerned the seal is a tight enough fit that it can work. Oem seals typically have a rather tight fit with the seal being .020" or more larger than the tube, but they form to the tube well and eventually take shape for a looser fit. When I modify a seal it ends up being closer to .005 to .010 larger than the tube. I worry that smaller than .005 will not be enough to maintain a seal, for example if the gun sits horizontal the seal may settle under the weight of the piston and cause the opposite side to have an air gap. I also sand them down in a manor that leaves them round, if not round it may have points where it isn't touching, and the looser the fit the worse it'll be. Leakage not only costs power but is harder on the gun and scope. I was wondering if using a file left it with flat spots or too rough a surface to seal well. You can probably just buy a new seal, maybe a Crosman or similar 25mm seal like from a TX200 will fit.
I'm guessing your new tube has a smaller ID, or did you sleeve the old tube? Either way, how does the piston itself fit?
I'm not sure if I would've replaced the tube or filled in the pits with epoxy, but you're there and I'm not.

Hope this is of use.... I just shoot em and let others work on em.....
 
I got her back together and lo and behold, I still have my fingers lol. I heard all the dramas about fingers being sliced off on Chinese underlever rifles; (never any details, just obscure urban legend style stories).

Time to take it to the range.

This is a nice gun for the price, modest power, shoots smoothly, and without the dreaded soft laminated direct to sear trigger.

Just out of curiousity, has anyone tried the 'teflon engine additive' trick, and if so, did you notice any useful results?
 
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fat tony said:
Just out of curiousity, has anyone tried the 'teflon engine additive' trick, and if so, did you notice any useful results?
Click to expand...
Do you mean to add engine additive to the compression chamber to increase velocity? This is not a good idea as it can cause dieseling or detonation, which is not good for your rifle or your piston seal work.
 
Keyword: teflon.

Some degree of dieseling is a reality with spring piston rifles. The oil is the carrier, the main advantage that I can see is the coating of microscopic teflon particles into the steel of the cylinder after the drama of dieseling is finished lol. I have experienced harsh dieseling before and I agree it is something to be avoided.

I just don't want to shell out $$$$$$ for some special special special special proprietary golly gee whiz celebrity airgunsmith endorsed ####e for nothing.

On the other hand, I might start shopping around for a a spring with a bit more poundage than what it comes with.
 
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