Johnson 20hp cyl head

[ftruntz]

I am rebuilding a 1968 20hp Johnson. It needs a new cyl head. Will a 1969 25hp cyl head work? Have a NOS head. Without using the head gasket the piston just touches on one corner on the intake side. Would using 25hp pistons make the head usable?

Thank you for any help………..
F. Truntz

[garry-in-michigan]

If I were thinking about changing pistons, I would be sure both upper and lower pistons were the same weight. On the other hand, you could use a Dremel tool to remove a little material from the cylinder head where it touches and leave the pistons alone. The head gasket will give you the extra clearance you need. . . 😉

[frankr]

Mixing 20 and 25hp pistons and head will get you in trouble. If you have 20 pistons, you need a 20 head. Sounds like Garry has done some experimenting with them—successfully, I assume (??) What kind of compression did you wind up with, Garry?

[garry-in-michigan]

I worked on Big Fours. LightFours. BigTwins, V-4s And a Zephyr. We would check clearance by sticking a piece of hollow core solder through the spark plug hole and then turning the engine over. In the days of 30 weight oil we would polish the inside of the combustion chamber to discourage carbon formation. Most ’40s outboards had a 5 to 1 compression ratio. We would take that up the 8 or 9 to one. We used high octane Amoco Premium. It was the only unleaded high test fuel available at that time. The modified Zephyr put out 8 horse power and plained a 17 foot sail boat @ 20 MPH. I raced it in the 7-1/2 horse class on a small Aluma craft with an Oakland Johnson prop against Mercery KE4s, Scott Atwater 483s,. and Marten 60s. A Mercury usually came in second. . . 😆

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