what piston did a 318 use?

My books say the compression ratio on a '72 318 was 8.6:1. Compression ratio is not just a function of the piston, however. The size of the combustion chamber in the cylinder head is the other major determining factor, with head gasket thickness also contributing.

For instance, the '71 318 also has an 8.6:1 compression ratio, but the piston sits deeper in the block, while the combustion chamber is smaller than the '71.

FWIW the 72-86 318s used the same piston.