OK, I think I understand now. You have an empty 18hp crankcase?? So yes, you will need a cylinder head from a johnson 18hp, 57-58. I think your forward side recoil cover brackets from the 15hp will work, they have the rubber bumpers on them. Unfortunately, the 1961 18hp crankcase is set up for a thermostat, so the original style cylinder head will not fit properly. I’m pretty sure the crank/rods/pistons could be used in a different/older 18hp crankcase though. Don’t forget, your original 15hp carburetor will probably have to be changed as well.
Why is it that you want to convert this 15hp to an 18hp??
