Indians For Guns

Here is a good topic on sizing synthetic seal.

http://www.network54.com/Forum/79537/th ... iston+seal++----

Two pounds of pressure to move the piston down a lubricated tube. That's what Gerald Cardew found. At 6 lbs the velocity was way down, and at 2 lbs, Cardew felt it was about right. If you have a parachute type seal, you can have it loose enough, to just fall by it's own weight, but stop if you place your finger over the transfer port. You can get away with this on a parachute seal, as the lip expands by air pressure on the forward motion of the piston, and it seals the tube. For other types of seals this is way too loose. Cardew got maximum power this way, but the velocities were erratic and not best suited for his purposes. You're livin on the edge here. You didn't mention what type of seal you were intending to use. I prefer the 2 pound range setting myself. If you are maximum velocity or bust kind of guy, have at it. I would have an extra seal available in case it doesn't turn out the way you want. If you ever get the chance to read Cardew's book, "The Airgun From Trigger To Target", do it. It's a wealth of information for the Experimenter.

Even though this is an old topic , yet I felt to contribute with my practical experience.
Synthetic seals are not alway of ideal size that one expect it to be.
Utmost caution to be used while putting it into the receiver.
If it is found difficult then do not push it hard , instead sand the OD with 300+grit very patiently.
It may take time to match with the ID of receiver but that will give you long lasting shooting pleasure.
I regret having destroyed one such seal due to my sheer impatience.

Basu