If anyone is using ARS's MASK piston heads, I have a tale of caution for you.
Specifically these ones
http://www.airsoftparts.ca/store2/in...oducts_id=1773 but it is likely a problem with all of them.
The sorbo pad on the piston face is glued into place. I highly recommend taking it out (it is poorly glued into place) and securing it with a better glue. Mine became detatched, stuck over the hole in the cylinder head, and created perfect compression. This caused the piston to not go forward all the way and consequently my gears nom nom nom'd my piston teeth.
Its actually a very good set and provides really good compression but this problem can ruin your day. This failed after less than 1000 bbs.