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PostPosted: Mon Jul 06, 2015 1:02 pm 
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Joined: Wed Jul 24, 2013 9:13 am
Posts: 4027
Location: Toronto
This is from a fella on another forum but I bet one of you lads can help:

A question for the 'airsmiths'

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

Anyways, the old one looked good on the outside, but on the inside it was severely pitted, I am not talking pinhead sized areas of minor pitting, but some of the pits might have been approaching .5 to 1 mm deep!

Got a nice piece of mechanical tubing & am in the 'fitting stage'. The original cylinder had a bore size of ~ 1.010", the pistonhead seal was similar in diameter. It was made of polyureathane, which had a slightly amber colour to it.

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 have stopped filing on the piston seal and have been trying out the piston in the bore of the newly made cylinder.

As you can imagine, there is a slight resistance.

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 am thinking that at this point I should stop filing on the piston seal, put the gun together & try shooting it.

Depending on how that goes, I might have to do a tear down & repeat fitting until it is right.

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

As for what gun it is, it is a Roebel .177 underlever. Seems there are many minor variations of the Roebel.

Thanks for any advice from spring piston gun tuners.

Keep you powder dry and your seals oiled.
Shoot straight and safe.

PostPosted: Mon Jul 06, 2015 4:08 pm 

Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1271
Location: United States
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.

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