OMC Interchange?

[Starboardlean]

1959-76 18hp & 20hp
1969-76 25hp
From my novice research I do believe the cylinder head and cylinder head gasket interchange?

 

[jimmbo]

Do not put the 25 hp cylinder head on a 18/20 hp, it will destroy the engine.

 

[boobie]

Why ??

 

[Starboardlean]

1975 Evinrude 25. Almost zero compression in bottom cylinder. Head removal was a bear. Long story short it did not survive. Head gasket revealed two places where gases had been pushed thru to water jacket. So I need a head and gasket.

 

[flyingscott]

I believe the 25 uses a different head and piston than the 18/20 motors.

 

[F_R]

I believe the 25 uses a different head and piston than the 18/20 motors.

That's right. The compression ratio is higher on a 25, but they accomplished that by different pistons AND a different head. I guess they saw it coming---people figuring they could get 25 hp out of their 18 just by swapping heads. The 25 also has a balanced crankshaft to withstand the higher revs.

 

[jimmbo]

To get the extra 5/7 hp out of the 22 inch block, the engine had to spin faster. That entailed a bigger carb, changes to the intake and reed system, and most importantly different port timing. Instead of porting the block differently, OMC changed the location of the wrist pin in the piston. That effectively changed the port timing in regards to the crank and resulted in a lower compression ratio and a much larger combustion chamber volume when the piston was at TDC. A new/different head resulted in a smaller chamber and raised the compression. Using the 25 head on a 18/20 engine results in too much compression and will lead to destructive detionation