Re: 1956 15HP and 1959 18HP interchangability
You can put the '56 15hp powerhead on the '59, but not the other way around. The lower units, ignition, etc. will swap in either direction. Lots of other bits and pieces will go either way, too. The biggest difference is in the cylinder head and block, as the '59 has a thermostat and lacks the cowling mount ears.
- Scott