75 hp OMC powerhead compatible internals?

This topic has 12 replies, 2 voices, and was last updated 3 years, 8 months ago by fleetwin.

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July 22, 2020 at 10:32 am #209432
shoestringmariner
Participant
Hey guys, hope all of you are keeping safe and well.

Question about 1977 75hp OMC internals, will they be interchangeable with 1982?

I have a PH with great compression but had water intrusion. Wondering if I can use block on a 1982 75hp motor with poor compression

Thanks in advance
July 22, 2020 at 12:33 pm #209441
fleetwin
Participant
US Member - 2 Years
Just want to be sure I understand what you want to do…. You want to use the crank/rods/pistons out of a 1977 model on a crankcase from the 1982 75hp, correct?

The crankcase part numbers are different, but the rods/crank/pistons seem to be the same part numbers…. So, I am not sure that answers your question, or if I understand what you are attempting to swap…D

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shoestringmariner
July 22, 2020 at 7:21 pm #209480
shoestringmariner
Participant
fleetwin wrote:

Just want to be sure I understand what you want to do…. You want to use the crank/rods/pistons out of a 1977 model on a crankcase from the 1982 75hp, correct?

The crankcase part numbers are different, but the rods/crank/pistons seem to be the same part numbers…. So, I am not sure that answers your question, or if I understand what you are attempting to swap…D

Actually the opposite. Because of the good compression in the 77, I want to combine the 82 crank and bearings with the 77 slugs and rods and put the 77 powerhead in the 82 motor.

My concern now is does the 77 powerhead match up with the 82 mid-section?

If there’s differences in ignition systems between the two that don’t crossover, I could always keep the 77 ignition with the 77 powerhead ?
July 23, 2020 at 8:14 am #209529
fleetwin
Participant
US Member - 2 Years
OK, well the base gaskets are the same. Looks like both gearcases use the oring on the driveshaft. So, I see no reason why it won’t bolt up. But, I would keep the add on stuff with its original crankcase… So, if you are using the 77 crankcase, I would use its ignition stuff and carbs.
And yes, the 77 models had a bracket that all the ignition stuff bolted onto, the ignition stuff on the 82 models bolted directly to the head and block…

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shoestringmariner
July 23, 2020 at 7:56 pm #209644
shoestringmariner
Participant
Excellent, thank you very much
August 1, 2020 at 1:10 pm #210555
shoestringmariner
Participant
Hey guys, I’m wondering about another possibility here; I have a 1977 70 hp Johnson parts motor with a badly damaged piston.

If the rotating assembly is still good, would that swap into the 75 horse? And are the exhaust plates the same?

If you recall, my good compression 75 horse got water intrusion. Rebuild kits on 75 hp motors are expensive

Or is this somewhat of a foolish endeavour swapping a used crank and bearings? ( like putting a used exhaust system into another car )
August 1, 2020 at 3:38 pm #210575
shoestringmariner
Participant
Edit, i opened one up and internals are toast on the 70 donor. The top rod-crank bearings let go and went through the motor

I would like to know if the exhaust plate is compatible if anyone knows though.
August 1, 2020 at 8:22 pm #210598
fleetwin
Participant
US Member - 2 Years
Looks like there are different part numbers for 15 vs 20″ transom models…So, I’m thinking you should be OK if both motors are the same transom height…

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shoestringmariner
August 1, 2020 at 9:16 pm #210601
shoestringmariner
Participant
fleetwin wrote:

Looks like there are different part numbers for 15 vs 20″ transom models…So, I’m thinking you should be OK if both motors are the same transom height…

Excellent, thanks
August 3, 2020 at 6:53 pm #210798
shoestringmariner
Participant
So now I’m back to thinking about rebuilding the 75.

There’s literally NOTHING for sale in all of southern Ontario except motor/boat packages or questionable motors that are overpriced.

Does anyone know of bottom end rebuild kits for these motors? I can only find full rebuild kits and they are more pricy than 70hp kits.

I only want to do the bearings and gaskets/seals
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