Automotive engine into boat

Hi everybody. I’m making a new thread because I can’t find the answer to my question anywhere.
Maybe someone on here can help and point me in the right direction, anyway I have a 1987 four Winns boat I believe it’s an 18 foot
it came with an OMC 5.7L Chevrolet engine. The engine blew and I bought an automotive engine one that came out of a 1977 Chevrolet c10 pick up. I changed the head gaskets as everyone suggested because of the salt water and I used everything else from the marine engine water pump etc. My problem now is that the engine coupler won’t bolt up to the rear main on my automotive engine so with that being said I can’t use the old flywheel and I can’t use the old engine coupler so I can Put the engine back in also I am waiting on the engine alignment tool I am not getting one yet because maybe I’ll have to get another coupler ? What are your guys thoughts ? Any suggestions ? Have you guys ever ran into this problem is there some sort of adapter I can use ?

I believe there are some differences in the engine specs between an automotive 5.7 and a marine 5.7 because the crankshafts are different I’m sure the camshaft is different. Anyway any info is great thank you.

Ok..... in 1986 GM changed the SBC crankshaft flange to accommodate the update to a one piece rear main seal. This change decreased the bolt pattern circle, and is also why your flywheel and drive coupler do not fit the earlier 1977 Engine.
This is a GM change and has nothing to do with Marine -vs- automotive!

Rick E.

RicardoMarine said:

Ok..... in 1986 GM changed the SBC crankshaft flange to accommodate the update to a one piece rear main seal. This change decreased the bolt pattern circle, and is also why your flywheel and drive coupler do not fit the earlier 1977 Engine.
This is a GM change and has nothing to do with Marine -vs- automotive!

Rick E.

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Thank you Ricardo that definitely clears things up for me I will go look for an older model coupler and go step by step from there. And count splines as well.

Dgalicia said:

Thank you Ricardo that definitely clears things up for me I will go look for an older model coupler and go step by step from there. And count splines as well.

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Ayuh,..... I think yer gonna need a newer motor, rather than an older coupler,......

OMC switched from the ole Stringer drives to the Cobra, 'bout the same time Chevy changed the crank bolt pattern,......
The couplers, as well as the drives are very, Very different, 'n I don't believe there was any cross-over,.....

Sierra carries a conversion coupler though,..... late model spline, with the early model bolt pattern,.....