Need advise on choosing a mercruiser engine coupler

[Chief Will]

Hello everyone,

This is my first post. I need help in choosing the proper engine coupler for a mercruiser.

5.7 mercruiser. I have swaped out a seized OMC 5.7 for a mercruiser 5.7 that I was able to buy at the right price. while prepairing the merc to be droped in i noticed that the flywheel was smaller than the one on the OMC engine and therefore the engine coupler wont bolt. So, i searched the engine over for the serial number and yes I looked in all the places that merc would have pinned the number to the block and the only number I found was 3870010. Now, I have tried to decode that number on several sites and they all say that that number produces no result. I need to know the year of the engine so I can buy the corect coupler. It seems that GM used two different sized flywheels on the 350 . can anyone help?

 

[Bondo]

Re: Need advise on choosing a mercruiser engine coupler

Ayuh,... What year is the OMC,..??

That 010 block is a Chevy, 2 pc rear main seal block...

 

[Chief Will]

Re: Need advise on choosing a mercruiser engine coupler

Bondo, The OMC is a 1986. and yes, the crank looks to be a 2 piece.

 

[Bondo]

Re: Need advise on choosing a mercruiser engine coupler

Bondo, The OMC is a 1986. and yes, the crank looks to be a 2 piece.

Ayuh,... Is this going into a Merc powered boat, or OMC...

If Merc, what drive, 'n what vintage,..??

 

[Glasply]

Re: Need advise on choosing a mercruiser engine coupler

Bond-o says that your block from the merc is a two piece rear main by the number. You say your omc is a two piece also. The flywheel on the merc engine is 12.75", right? Also, the bolt pattern on the crank is 3.5" on center, right?

But the flywheel on the OMC is 14"? So the bolt pattern on the crank is smaller, if I remember right it's something like 3" o/c, but don't quote me. It was my understanding that the larger flywheel and smaller crank bolt spacing was 86 and up on sbc's, and corresponded to the change to a 1 piece rear main. Again, I'm not an expert, but maybe somebody can chime in. I think you've got a one piece 86 and up from your omc, and need the adapter coupler back to the larger crank bolt pattern.

Most people go from older engines to newer, so they make those couplers. You're going the other way, though. I looked around, and found this part; VOL3853962 Give that a look and see what you think...

 

[RogersJetboat454]

Re: Need advise on choosing a mercruiser engine coupler

Hello everyone,

This is my first post. I need help in choosing the proper engine coupler for a mercruiser.

5.7 mercruiser. I have swaped out a seized OMC 5.7 for a mercruiser 5.7 that I was able to buy at the right price. while prepairing the merc to be droped in i noticed that the flywheel was smaller than the one on the OMC engine and therefore the engine coupler wont bolt. So, i searched the engine over for the serial number and yes I looked in all the places that merc would have pinned the number to the block and the only number I found was 3870010. Now, I have tried to decode that number on several sites and they all say that that number produces no result. I need to know the year of the engine so I can buy the corect coupler. It seems that GM used two different sized flywheels on the 350 . can anyone help?

I think you may have misread one of those numbers in you casting number (happens all the time). I believe its 3970010. Production date should be between '69-'79 (2pc for sure). Is the OMC block a 2 piece crank as well? If it is, then the flywheel will interchange between the engines (2pc 350's are not externally balanced). If you are unsure, get the casting number off the OMC block and post it.

If it turns out the OMC block is a 1pc RMS (stopping you from swapping it to the 2pc crank), it may just be easier and cheaper to find a 168 tooth flywheel for a 2pc crank. Just make sure it wasn't balanced for a 400. If you stick with the 153 tooth flywheel, you will also need to find a 153 tooth marine approved starter.

 

[Bondo]

Re: Need advise on choosing a mercruiser engine coupler

Ayuh,... You guys are Sooo much more articulate than myself,...

I'll only add, that the block has to be drilled for Both starters, to swap diameters of the flywheel...

Some blocks are, Many ain't...

I'd still like ta know just what kinda Frankenboat We're workin' on...

 

[RogersJetboat454]

Re: Need advise on choosing a mercruiser engine coupler

I'll only add, that the block has to be drilled for Both starters, to swap diameters of the flywheel...

Some blocks are, Many ain't...

Could use a tilton style starter which uses the in-line (153 tooth) bolt pattern for both 153 and 168 tooth flywheels.



Dunno if there would be any issues clearance wise with the flywheel cover. A marine example would set him back $450 bucks!

I'd still like ta know just what kinda Frankenboat We're workin' on...

Me too...

*To the OP*
Post pic's of what you've got.

 

[Glasply]

Re: Need advise on choosing a mercruiser engine coupler

"Frankenboat" I see the word's catching on....

 

[Chief Will]

Re: Need advise on choosing a mercruiser engine coupler

Ok, you guys are all going way into left field with this. The OMC is the seized engine. The merc is the replacement. thye merc is complete with flywheel, starter, housing and all. The only thing I do not have is the engine coupler. I will have to switch the headers, hoses and oil cooler. The OMC engine has a one piece rear seal, the merc has 2 piece rear seal. I simply need to know what engine coupler fits the smaller merc flywheel as I cannot find any other engine numbers on the block other than what I posted. i will be mating a OMC cobra drive to the merc engine. I have done this swap in the past and I know that I will have to change the yoke on the Cobra drive to the Merc yoke, the shaft is the same lenth. I know this from experience in the past. I simply need to know what coupler goes on the smaller GM flywheel.

 

[Bondo]

Re: Need advise on choosing a mercruiser engine coupler

Well,... Sierra part # 18-2171 is the old style merc coupler, crankshaft mounted, for the large bolt pattern...

 

[Glasply]

Re: Need advise on choosing a mercruiser engine coupler

You ever get the feeling people spend a lot less time reading then they do responding? I understand how some of you guys with umpteen thousand posts can get frustrated...

Chief, did you look at the part I quoted you? I see by your post that I was right about the OMC being a 1-piece rear main. Maybe I was right about the coupler, too.

Also, swapping the yoke? Why? Your OMC outdrive doesn't know that the other block used to say mercruiser on it.

 

[Chief Will]

Re: Need advise on choosing a mercruiser engine coupler

Ok guys, I thought I had the correct coupler. I found a new sierra 2171 coupler on ebay for 45 bucks, so I thought a shot in the dark was acceptable for only 40 bucks. well, the bolt pattern is correct but the coupler wont seat due to a dowl pin in the flywheel. So, back to the drawing board. lol Oh yea, I am sure the yoke will be the same, it was suggested by a marine mechanic to change it to a merc yoke as a kinda better safe than sorry type scenario. I was also wondering, what about the triangle shaped couplers? is it possible i need one of those?

 

[badbowtie]

Re: Need advise on choosing a mercruiser engine coupler

Someone else can confirm this with me but I think you can remove the dowel pin in the crank and be fine. I am pretty sure it is to just help line up the flywheel on for a car.

 

[Bondo]

Re: Need advise on choosing a mercruiser engine coupler

Someone else can confirm this with me but I think you can remove the dowel pin in the crank and be fine. I am pretty sure it is to just help line up the flywheel on for a car.

Ayuh,... Agreed,... The 18-2171 IS the right coupler,...
Just removed the dowel, or grind it down for clearance...