Reply To: 50’s 18hp OMC retrofit question

[foot_doctor]

I should jump in here to explain the parts needed to convert your 18 to a 25. The cylinder block/crankcase assembly is the same for either model. You”ll need a 25hp crankshaft, a pair of 25hp pistons, a 25hp head and a 25hp carb. Everything else can be used. The crank has the same stroke as the 18, but it has weights riveted onto the balance weights. The pistons have their piston pin holes located closer to the top. That advances the port timing. The 25hp head has combustion chambers with lower ccs. That restores the compression. lost with the piston change. The carb has a slightly larger venturi, and I beleive a larger main jet. That’s it. Install that stuff in your block, and you will seriouslly increase you performance. I built a lot of thos powerheads when we raced them in the early versions of 25SS. My engine was unbeatable, and I could start last and pass everybody on the course. It was 1 mph slower than my MK20H. Have fun boating. R.T.