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wrote:

Thanks again for your comments. The air rifle is a $135 Chinese one
imported by Compasseco--it shoots extremely well but needs some
refinement. This is the only model the importer doesn't sell extra
piston sets and springs for. The original machining on the piston
assembly wasn't quite right and gouged the piston sleeve when fired.
The sleeve then roughed up the plastic piston creating a sealing
problem--(a plastic piston sits on a metal assembly.) It still shoots
fine (dime sized groups at 35-40 yds) but the cronograph says the exit
pellet velocity has fallen to around 815 fps--down from 1100 fps. My
plan was to hone out the sleeve and then replace the plastic piston
with a tight fitting nylon one.

Tut

That plastic piston is likely urethane. To make this work you really
need a lathe, or you can hope to get lucky on the stock sizes of seals
as suggested by another poster. I put your chances of seeing ANY
improvement at about zero, using nylon and abrasives to size the seal,
and the chances of it working at all only slightly higher.

One other thing you could do while you are in there is to "button" the
piston. Buttoning is adding sliders to the piston to take up the space
in the cylinder and hold the piston centered. By placing three buttons
around the front of the piston, and trhee more around the back, you can
decrease drag an make the firing cycle much smoother. Normal practice is
to ensure one set of buttons is opposite the cocking lever, with the
others evenly spaced around.

Check out Jim Maccari's website and see if he has a seal kit for your
model. Google "JM's spring tar". He may be able to upply an oversize
seal, or , depending on how large you have to go to hone out the damage,
you may be able to use one of his stock seals as is or with a small
amount of adjustment.

Cheers
Trevor Jones