’70 to early-’78 models, without the large o-ring, require a sealant around the cylinder base at the opening in the engine case. Opinions vary widely about what to use, but favorites include:
o Drei-Bond 1209, aka Yamabond (Cutter)
o Drei-Bond 1216 (Clymer)
o Hylomar, aka Wurth DP300 (Oak)
o Permatex High-Temp RTV Silicone 26B or 27B

Theoretically, later models with the large o-ring don’t require a sealant, but it won’t hurt.