So I entered in the numbers using a compression ratio calculator. When a Z129 head is installed on a Z120 engine with flat top pistons the compression is raised to about to 8:1. Way too high for a working tractor. You may get by with it on a parade tractor but on a working tractor you would have holes in the pistons in a short period of time from spark knock. Especially if the Z129 camshaft is used because it has an earlier closing intake valve than the Z120 did,therefore the Z129 camshaft builds more cylinder pressure. Dished pistons must be used when installing a Z129 head on a Z120 engine. If a Z120 head is installed on a Z129 engine the compression is lowered to about 5:1. Kind of low but that is better than being too high.