350 GM Engine swap question

Current setup: 1987 OMC Cobra with a 5.7L 350 GM 260 HP (engine has 1 piece RMS)
Replacement Motor: Rebuilt 1985 Mercruiser 5.7L 350 GM 400ish HP (or thats what i was told by the shop I had rebuild it)
This motor has the older 2 piece rear main seal

The crankshaft on my blown engine is a circle with the bolt studs coming off the crank. The rebuilt motor has the crank with the "notch" taken out of it. I am having difficulty locating the engine coupler and flywheel to make this motor work. Does this coupler even exist? or will this motor not work with my OMC setup? If I were to swap over to this motor will i have to replace the transom shield and outdrive with a mercruiser? If I had to swap the drive and shield will a bravo unit fit in my cutout or would i have to use an alpha?

I have been searching for days trying to figure this out and I am at about witts end:facepalm:
Any help is greatly appriciated and I would like to say thank you in advance to anyone who replies


Edit: So after doing some more research it seems that the OMC and Mercruiser transom cutouts are essentially the same. Might have to drill a couple extra holes for the new transom shield. Is this a true statement?

Re: 350 GM Engine swap question

d00bs24 said:

Current setup: 1987 OMC Cobra with a 5.7L 350 GM 260 HP (engine has 1 piece RMS)
Replacement Motor: Rebuilt 1985 Mercruiser 5.7L 350 GM 400ish HP (or thats what i was told by the shop I had rebuild it)
This motor has the older 2 piece rear main seal

The crankshaft on my blown engine is a circle with the bolt studs coming off the crank. The rebuilt motor has the crank with the "notch" taken out of it. I am having difficulty locating the engine coupler and flywheel to make this motor work. Does this coupler even exist? or will this motor not work with my OMC setup? If I were to swap over to this motor will i have to replace the transom shield and outdrive with a mercruiser? If I had to swap the drive and shield will a bravo unit fit in my cutout or would i have to use an alpha?

I have been searching for days trying to figure this out and I am at about witts end:facepalm:
Any help is greatly appriciated and I would like to say thank you in advance to anyone who replies


Edit: So after doing some more research it seems that the OMC and Mercruiser transom cutouts are essentially the same. Might have to drill a couple extra holes for the new transom shield. Is this a true statement?

Click to expand...

Ayuh,.... Welcome Aboard,.... True as daylight....

The Cobra was used with SBC in '86, so the parts are out there,...

Ya need both the flywheel, 'n coupler.... for an '86 OMC Cobra,...

Yer old crankshaft is a 3" bolt pattern, 'n the new older motor is 3, 1/4" I believe....

Is the new older block drilled for 1 starter pattern, or both patterns,..??
That'll determine Which diameter flywheel ya need,...
The straight across pattern is the 12" flywheel, 'n the staggered pattern is the 14" flywheel...
Btw,... a pickup truck standard tranny flywheel is the same as a Marine flywheel,...

I donno what was used with the Cobra drives that year,...

Re: 350 GM Engine swap question

Bondo thanks for the reply coupler has been found and ordered.

Question about the flywheel: it is my understanding that the flywheel should be the 14 inch but I am confused on the "balancing". Did chevy use a balanced flywheel with these engines with the 2 piece rear or just any old flywheel off a truck like Bondo said?

From what I have read GM internally balanced the engines with the 2 piece and externally balanced with the 1 piece. The casting # off the engine is 3970010 which puts its manufactured date somewhere between 1968 and 1979 i believe. Unfortunately I cant find the engine suffix code which would give me more info on it...

Re: 350 GM Engine swap question

d00bs24 said:

Bondo thanks for the reply coupler has been found and ordered.

Question about the flywheel: it is my understanding that the flywheel should be the 14 inch but I am confused on the "balancing". Did chevy use a balanced flywheel with these engines with the 2 piece rear or just any old flywheel off a truck like Bondo said?

From what I have read GM internally balanced the engines with the 2 piece and externally balanced with the 1 piece. The casting # off the engine is 3970010 which puts its manufactured date somewhere between 1968 and 1979 i believe. Unfortunately I cant find the engine suffix code which would give me more info on it...

Click to expand...

Ayuh,... Yer over-thinkin' this,...

With the exception of the 400cid, All the SBCs are known as internally balanced...

The flywheels won't interchange 'tween the 2 pc. seal motors, 'n the 1 pc. seal motors,...
The Bolt Pattern is Different...

Whether ya need the 12" flywheel, or 14", is determined by the starter bolt pattern,..
The straight pattern is for the 12" flywheel, 'n the staggered pattern is for the 14" flywheel...

Yer block, may, or may not be drilled for both starters..

Re: 350 GM Engine swap question

ok. I didn't even think about the bolt pattern not lining up... Definitely over thinking, especially after ordering a coupler because of the bolt pattern. I should have known that... Pretty sure I have the staggered bolt pattern for my starter I'll have to verify that tonite. Looks like I will be off to the junkyard to find a flywheel and putting this thing back together.
Thanks again for the help Bondo