If you inherit one of these old boat motors and you are not sure of the history, it is a good idea to pull the cylinder head and take a look at what is underneath. Remove the spark plugs. Using a 7/16 wrench, remove the ten bolts that hold on the cylinder head. Gently pry the cylinder head off the crankcase to break the seal of the head gasket.
You should replace the head gasket with a new one.
Head Gasket OMC Part Number 303438 NAPA/Sierra Part Number 18-2885
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Now that the cylinder head is off, wipe out, clean, and inspect the cylinder walls, pistons, and cylinder head. Also, inspect the water passages around the cylinders. Using an air hose, blow and clean the water passages. I used a small wire brush to clean the carbon off the pistons and inside the cylinder head. Do not get carried away cleaning this carbon. If you clean too much off and go down to bare metal, you can create a "hot spot" on the piston. You do not have to get this absolutely clean. Some carbon is normal.
One of the main reasons to remove the cylinder heads to to ch to make sure the head is not warped. Over time, with heating and cooling, especially if the motor was ever hot, the cylinder head may warp. Since I do not have a milling machine, I simply place a sheet of fine grit sandpaper on a piece of glass or something flat and move the cylinder head in a circular pattern until the mating surface is flat. You can tell when the surface is flat because you will have shiny bare metal all the way around the surface of the cylinder head.
Lubricate the new head gasket with 2 cycle oil and bolt the cylinder head back onto the motor block. The holes on the cylinder head are not symmetrical so that the head will not go back on the wrong way. You may need to rotate the head 180 degrees if the bolts do not seem to line up. Be sure not to over tighten the bolts. Everyone seems to think that head bolts need to be really tight. This will only warp the head. Again, only tighten a quarter turn past snug. When you tighten these bolts, you need to snug down the bolts opposite each other for evenness until you have them all snug and then go back around until you have them all tightened a quarter turn past snug. This way the head will be evenly attached to the block.