Question on 18 HP cylinder heads

[Steve D]

Checking numbers at MarineEngine the cylinder heads for the 22 cubic inch powerhead with the thermostat went through 4 or 5 number changes. Maybe they were just minor changes that didn’t effect the interchangeability. If I’m not mistaken that powerhead remained pretty much the same until ’73 so I would think you could take one from a ’59 and put it on a ’73. But I told somebody I would check to be sure before he buys the one I have. I have one from an early 60s Evinrude 18 and he needs one for a 1970 Evinrude 18. Thanks in advance for your help!

[frankr]

Where you get into trouble is if swapping between 18-20-25hp. Don’t do it. Otherwise, I’ll stick my neck out and say if it fits, ok. There probably are differences according to what might be hung onto the outside, like coils, etc.

[Steve D]

Ok Frank thanks, we’ll give it a shot. I was kinda leaning that way too.

[fleetwin]

Well, I remember that the early thermostat models had a completely different thermostat cover/gasket, probably routed the water a bit differently as well. But, am guessing you could interchange the complete assemblies. The older ones definitely won’t work on 73 models due to the mounts for the external coils. The head gaskets are the same part number. Don’t know what the situation is surrounding the sale, but expect it to be returned if the thermo cap/gasket is different from the one you have….

[johnyrude200]

Like others have said, 18/20’s are the same until the dreaded low tension magneto. Depending on the production run in ‘73, it could have either head and either the older or new style magneto coil placement.

You can put an old style head onto one of these powerheads right up to ‘76 if you switch back to the universal magneto which, IMHO, is better and easier to deal with anyway. May not throw as much voltage but SO much less hassle dealing with in terms of repairs.

Heck you can put a ‘76 head on a ‘59 just leave the coils off and go with the universal magneto so long as its a thermostat controlled model to begin with.

Check out these pics:

http://runneroutboards.com/motor101/powerhead/powerhead.html#18-20_Differences

You can see independant of the external mount coil bosses, these heads are the same other than the cupping for higher compression numbers used on the 25hp models. The late ‘73 production model 18/20’s with the bosses are the same as pre low tension magneto models in terms of cylinder cups.

[Steve D]

Thanks for the extra info guys. Johnny that’s a great link, good explanations and pictures, and info on the cylinder heads. Will have to save that and read the whole thing over thoroughly. Don, as far as the sale, the guy has a head with a heli-coil and it’s giving him trouble and he just wanted to get a good one and be done with it. Since his doesn’t involve the external coils, from what I’m reading here, it shouldn’t be a problem to use the one I have. Thanks again.

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