25hp Powerhead on an Evinrude 18hp, Any OMC guys here?

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noglass170

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Hey Guys,

I am a huge fan of the old OMC 18hp motors. I currently have a 1971 Evinrude 18 hp with a bad crank bearing that I'm messing with. I ran across a 1970 Johnson 25hp powerhead that seems to be in good shape, and it is of the same year range. I know these two motors used the same 22 cubic inch block. Do you guys think this 25 hp powerhead will fit onto the 18 hp?
 
Be badazz if it works, surprise a lot of guys on the water!
Pappy will likely know, he's a wealth of information

Sent from my CLT-L04 using Tapatalk
 
The gasket has the same part number for each but the heads don't. Which would make sense.

https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1970&hp=25&model=25002E&manufacturer=Evinrude&section=Powerhead+Group

https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1971&hp=18&model=18103S&manufacturer=Evinrude&section=Cylinder+%26amp%3B+Crankcase
 
Here are some of the issues you may run into.
Apparently the base gasket will fit as has been mentioned.
The crank splines need to be verified
The length of the driveshaft will need to match up or see if the gearcase from the 25 will bolt up. Very often the driveshaft length or the exhaust housing length will stop you.
The shift shaft needs to be verified as well as the water tube locations and shape (bends).
You may need to do some mixing and matching of other components.
Also, place the powerhead on the midsection and see if the 18hp cowl fits okay before you go much farther.

I did put a 1974 40hp powerhead on a 1957 35hp exhaust housing/split cowling for fun but it was no box of chocolates with all the changes I had to make there.
 
Thanks for the fast replies. I think I will go ahead and pick it up. externally the powerheads are exacly the same. The two even have the same bore. The 25 uses different pistons, head, and carb. I think OMC used that same 22.1 cubic inch block for the 18, 20, 25, and maybe even 30hp. I'm not exactly sure on the 30, but the other three I'm sure of. Worse comes to worse I'll just have to find a early 70's 25 Johnson to put the powerhead on. :lol: :D I'll update this thread with my progress. Love these old TANKS of an outboard!
 

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