:P Pistons are what have teeth on them, piston heads (are attached to the piston) make a seal with the cylinder, pushing the air out the cylinder head.
As for the top piston head, I believe I have a guarder in mine, but you'd have to look around to see what the overall census is. I'm also pretty sure there's a post on the better ones somewhere..
http://airsoftcanada.com/showthread.php?t=6677
This is the bible of upgrading airsoft parts IMO
And I just realized that you want a type 0 for a longer barrel length. You want a type II for your particular barrel length, though I haven't had any experience with using a type 0 with a shorter barrel