Mid 1950’s 10 to 18 horse lower units

[billy-j]

I need some information on the lower units that OMC used on the 10 horse to 18 horse engines from 1955 to 1958.  I have a 1958 Johnson 18 horse engine with a bad lower unit. From what I understand the lower units of the 1955  10 horse engine is interchangeable and identical with the 1956 and 1957 15 horse and  1958 18 horse. Is this true? Would the drive shafts be the same? Also if they would fit interchangeably are there any internal mechanical upgrades that OMC  made over the years that would make a certain year lower unit more desirable. Let me know. Regards Bill,

[fleetwin]

OK, well the 10hp gearcases from 55-57 will interchange, but 58 and later 10hp models used the infamous smaller gearcase.

18-25hp gearcases will fit from 1955 thru the middle 70s will fit as well.

But, it is important to note that the complete assemblies will fit, but there are many minute changes made to the units through the years, so the innards are not necessarily interchangeable.  So, do your best to determine the year of the gearcase you acquire for your engine to ensure you find proper parts in the future.

I know it is tough, but do your best to ensure the clutch dog is in good shape on any unit you might buy/acquire.  OEM replacement parts for clutch dog issues are a very expensive “kit”.

[Mumbles]

I’m not positive without checking but I think the early ten horse units had a slightly different gear ratio from the eighteen horse ones. I do know the props are slightly different.

[frankr]

Yep, different gear ratios.

[billy-j]

Thanks for the information so far. So to be correct the 10 horse lower units are not to be used because of a different gear ratio. What other year lower units would be correct or do I have to use a 1958 18 horse lower unit only on my 1958 Johnson 18 horse. Bill,

[frankr]

That is a tough question to answer.  Read fleetwin’s reply, above.  The basic gearcases for the 22 cubic inch motors were used for over 20 years, and for the most part, they are interchangeable.  But the surprises come when you go to buy internal parts.  Or even stuff like the shape of the cavitation plate.  Doesn’t make any difference, unless you are going for an authentic restoration for judging.

[fleetwin]

OK, wasn’t aware that gear ratios were different, but perhaps.  So that means you can use the 15-25hp models 22CI gear cases from the around 1956-1976.    You are not limited to using the same model year as you have, 1958.    But again, be sure to note the model year of the gearcase you are using to replace yours, and consult that year’s parts manual for future repair parts…

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