Contact Venolia with the piston part number or job number and get the exact CC for your pistons and the go to United(KB) Machine web site and click on the technical help or on line calculator and then compression ratio, plug your specs in, use + for the dish CC on your piston dome or valve relifs and it will give you a real close compression ratio My Ross custom dish pistons for 9.25 to 1 comp. ratio with 86 CC heads are 22.0 CC dish 4.375 bore with a 4.25 stroke, pistons are -.025 from the top of the deck