Anyway if your building a piston using a 80mm tee......80mm dwv pipe fits perfect,it can slide with ease and has enough room for an equalization hole,now just cut off a section of 80mm dwv and sleeve it over a small pringles can,its a perfect fit and the ends of the can hold the pipe on as theyre slightly bigger then the middle of the can..just glue/bolt your sealing face to the bottom of the can and fill the inside of the can with expanding foam or something else light.
this piston will be used on my new cannon that should be finished at the end of the month at the latest.this should work for both BSP(barrel sealing piston) and CSP.just thought i would let you guys in aussie know(as our pipes are the same)so if you ever build a 80mm piston this is a really good way to do it, mind you my 80mm tee(3" for you americans
) cost me $60NZD 
EDIT:if you guys want a picture i can post one if your that desperate
