Big Twin Flathead 80" std bore Piston Set. Single piece oil ring. Includes pistons, rings, wrist pins, and keepers. This is the only off the shelf piston   brand I have found. Fits 80" Flathead 3 7/16" bore. Eastern Product 
Eastern PN. E-836-STD
Product Price $209.95
Shipping in the U.S.A $15.00