I've heard several people say that when you use the 3 5/16" sleeves in a z120 block that the only piston you can get now is the dished piston like was used in the z129 and z134 engines. Z120's of course originally had flat tops pistons. I got a set of pistons from Federal Moguls for the z120 with 3 5/16" bore and they are flat top but there is a catch. You have to bore the sleeves .030" to get the proper clearance. Which isn't much because those sleeves are pretty thick. I've had my machine shop look at it and they said it would be easy to do and would work fine. The upside is it raises the compression to 7.0 to 1. So you will have a more fuel efficient and better burning engine. The best part is if you have a z120 that already has those sleeves installed and you were thinking about rebuilding it and buying new sleeves, just have your worn sleeves bored, get these pistons and you are ready to go.