I thought I'd researched this as well as I could! I could not find any info on the pistons except for the listing in my old 2005 P.A.W. catalog (its at my house in Glendive) which said w/ 77cc: 10.4? comp ratio. I'm going off top of my head, so don't quote me, but I know I'm close. Gonna try find their site, look for info and I'll be back. My machinist ordered the pistons for me and pressed them on.