Lower unit interchange - 10 and 18 Johnson

[twocyclemania]

I have a 55 Johnson 10 and 63 Johnson 18; both with lower unit problems. They both look identical. I also found another lower unit in my shed that's also the same in appearance. Are these units interchangeable? If not how do I determine which motor the 'found' unit could replace? thanks!

 

[Outboard Obsession]

I have been working on old Johnson motors (1941-1953) of various Horse Powers for awhile now. I have noticed that there seems to be a lot of cross over on parts from model to model. It would seem that if the flanges, bolt holes, and shaft journals line up that there shouldn't be any reason that you couldn't use parts from one size / year motor on another. The greatest concern it that the moving parts line up perfectly, and that you can securely bolt the lower unit to the mid section.

 

[racerone]

Yes they " look " the same.---It will bolt up.---But they are not the same internally I believe,----The teeth on the 18 hp gears are bigger stronger it seems.

 

[twocyclemania]

I checked on-line diagrams of both motors and spot checked a few parts and the part numbers are different. I believe the clutch dog is the same however. Thanks for the advice as I could have made a big mistake. Now I can fix at least one of these motors since I have a spare lower unit. Now, how do determine if the spare lower unit I have was from a 10 or 18? Any idea? thanks for the replies

 

[twocyclemania]

Thanks racerone! I just compared the internals of both the 10 and 18. There IS a difference in the gears. The 18 gears are bigger/stronger. Found out the spare lower unit I have is for a 10. So I guess I'll focus on the 10 for now. I know the clutch dog is not supposed to have any wear. I did find the edges have a little roundness. How much is tolerable or is there a 'rule of thumb?' thanks!