on a piston you want a good seal between the piston and the housing so it can pilot/trigger. if it is too loose fitting it may not do anything att all.
a piston can be built up on many ways i can explain some of them.
one is for people that wants to get the job done properly the first time.
like the pic, there is: a sealing face, something behind it to support the thin rubber face, something to seal in the housing so it can pilot, o-rings are best, a guide rod to prevent it to flip, a bumper to soften the impact of the piston as it smacks into the housing and the pilot valve.
for newbies:
cut a wood dowel that is so long it cant flip in the housing so you don't need a guide rod, a sealing face and a pilot valve.
you do like this (simple design):
secure the neoprene (or other rubber sealings) to the front of the piston. the best way to do this is with a screw. make sure you get the stuff air tight.
then you get someway to attach a pilot valve on the back of the piston housing. you have to make a "pass through" barrel to get a seat that is what the piston seals against. the way to do this is to grind down the "stopper" that prevents a pipe to go all the way through your fitting. the seat should be so far back that when the piston is in there there is no gap where the chamber is leading in to the housing and the closed piston position.
the "advanced":
you need a sealing face, a pretty long screw, a fitting to attach a pilot valve, some steel washers, a nut and some air. (mabe o-rings and rubber hose)
you do like this: first you put a steel washer onto your screw that is a tiny bit smaller than your seat.
then you put your sealing face on.
after that you put on a steel washer in the back of your sealing face that is the same size as the housing.
on the back of the big washer you put a smaller one. if the hole in the big washer is to big for your nut.
then you screw the nut on until you can notice your sealing face start to bend a bit. the last touch is to close your pilot valve and screw it on the way you want it into to your fitting that connects the pilot valve to the housing and drop the bolt in to make sure that it not touches anything in there. if it does cut small pieces of it until it doesn't.
bumper for advanced pistons:
you can find a small washer that fits on you guiderod (the unused part of the screw) and put it on.
then you find a rubber hose that fits your guidingrod and drop it on, when you have done that you drop one more washer on the other side of the hose and put a tight o-ring or something on the back of your last washer to stop it from gliding of you guiderod and there you go!
and don't forget the rule that d/4 = piston travel. d/4 is diameter of barrel/4 so for a 1" barrel you need 1/4" of piston travel and on a 2" barrel you need 1/2" and so on. if you use longer travel than that you waste power on bigger pilot volume.
pilot volume is the volume of gass your pilot valve has to empty to get your piston moving.
the final touch is to put a light coat of oil/grease on your piston and you can put it in your housing. from there on it is your choice how to handle your new piston gun.
if you think i am missing something here you can notice me.
still not 100% done i am going to put the coaxial and chamber sealer on aswell
animation (photobucket removed the last two frames), my first so don't complain
another animation how to assemble the easy piston
