O-Ring Expansion
Streach O-Ring
This mod explanes how to increase the size of an O-Ring to make it fit better. But be aware that streaching risk damaging the O-Ring, and should only be done if you are willing to risk it. The benefit is better air seal and therefore higher mustle velosity (Speed).
The Process
An O-Ring is made of rubber, and rubber reacts to heat. If heated enough rubber expands under heat, and the property's of rubber is so that i will retain the increased size after heat exposure. So in plane language: Applying heat makes the ring increase in size. What you doo is you want to have the rubber stredget before heating, but only alittle. You could use the metal cylinder from the gun to stredge teh O-Ring around when heating.
Heating
When you are heating apply heat slowly and uniform aorund the hole ring. So that you get an eaven stredge all round the ring. Otherwise it may apply pressure dirfferently from side to side. This can be done by spinning the O-Ring wile heating, and having the heat at a distance. To mutch heat will damage the ring, so be carefull to not heat to long! And usualy you only want a small ammount of increase so better heat and try multriple times to not risk over exposiure.
What Size
What you are looking fore is a ring that when on the piston head, it should push just slightly on the cylender walls. A good way to test the air seal is to push the piston into the cylender while blocking the cylender head output with a finger. Then the cylender should trap air and applying more force should just increase the preasure.
This mod is acturaly a more detailed part of the "Air sealing" guide, that focus on how to seal this specific part.