I have taken to making my own out of plastic. but in the past I have used solid rivets of the proper diameter, cut to the proper length. if properly done you can use the hex cap under the piston to to any necessary fine adjustment on piston height. the spring is also critical. I got my best results not using any spring (as far as rate of fire and no double fires) but it would occasionally jam.. making it useless. it does not take much of a spring, so long as the piston is mounted roughly vertical.
I have used a stainless steel #8 wood screw that is 2 1/2" long. They have a smooth end I just cut them off where the threads start. you can get them at any hardware store. DON'T get the Zink coated ones. Make sure they are stainless steel.
The piston ones are more forgiving if you are making your own the ball bearing ones are easy to mess up if not made to correct size and depth. Just my 2 cents
Use stainless steel screws not galvanized steel (they will rust and jam your gun) don't use aluminum they wear down fast hitting brass fittings and steel balls all the time. [][]
we use tinners rivets, stainless steel and no need for cutting, the only problem is where to find them. the springs are from smallparts.com., berillium copper ones.
