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change out broken Mercruiser 4.3 with good OMC 4.3

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fishndad62

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My 1992 Mercruiser 4.3l has a cracked block. My bad, I went from memory last year when winterizing instead of digging out the book (slaps himself across the back of the head). I have a known good 1990 OMC 4.3l I intend to swap in. Obviously the exhaust manifolds need to be swapped. Since the OMC was mated to a Cobra outdrive, I presume the bell housing will also need to be swapped. I'll guess (and find out soon enough) that the wire harness/plugs are different. Seems like pretty much everything else should stay on the OMC - for example, I think the Mercruiser had electronic ignition and the OMC might have traditional points. Is there anything else on the Mercruiser that needs to be swapped onto the OMC?
 
Swap ALL Mercruiser parts to new long block, intake and carb.
Everything.
If you dont and you go to get parts or have a service place work on it you will be out of luck.

see if you can sell the OMC junk on the web.
 
kghost said:
Swap ALL Mercruiser parts to new long block, intake and carb.
Everything.
If you dont and you go to get parts or have a service place work on it you will be out of luck.

see if you can sell the OMC junk on the web.
Click to expand...

I'm hoping by "new long block" you mean strip down the OMC to the GM long block underneath, not actually "New" or even remanufactured - because if I could have afforded that, I would have.
 
To clarify...... leave the OMC cylinder block, cylinder heads, valve covers, circulating pump, etc as they are.
You may need to look at the flywheel diameter..... use the Merc if they are different.
If they are identical, use the one with the best ring gear teeth.

Relocate the Merc items onto the OMC long block..... I.E., drive coupler, exhaust system, ignition system, carburetor, throttle cable bracket, alternator, starter motor, complete wiring harness, and so on.

As for the "bell housing", this is actually a Flywheel Cover in the Marine world.
And yes, you will be using the Merc flywheel cover.

When swapping ignition systems, first bring the OMC engine around to #1 cylinder at TDC on the Compression Stroke.

Note that you must also go through the drive coupler alignment procedure after installing the OMC engine.
 
RicardoMarine said:
To clarify...... leave the OMC cylinder block, cylinder heads, valve covers, circulating pump, etc as they are.
You may need to look at the flywheel diameter..... use the Merc if they are different.
If they are identical, use the one with the best ring gear teeth.

Relocate the Merc items onto the OMC long block..... I.E., drive coupler, exhaust system, ignition system, carburetor, throttle cable bracket, alternator, starter motor, complete wiring harness, and so on.

As for the "bell housing", this is actually a Flywheel Cover in the Marine world.
And yes, you will be using the Merc flywheel cover.

When swapping ignition systems, first bring the OMC engine around to #1 cylinder at TDC on the Compression Stroke.

Note that you must also go through the drive coupler alignment procedure after installing the OMC engine.
Click to expand...

Thanks, this makes sense and I suspect will take less time in the end than kludging things like wiring, and troubleshooting said kludges.

I expected to need to look at the flywheel and drive coupler, so this makes a great checklist. I've done some amount of engine work, so the #1 TDC was already on my list as well.

Got the alignment tool already, now I need to get my list together of gaskets and get them on order since I live in a pretty rural area and we don't have any marine suppliers around (though the NAPA in town can get me some of the things I'll need).

Thanks again, this is a great forum!
 
Napa use to have a full marine catalouge.
I used it for many items!!
You may want to ask them if its still available.
 
kghost said:
Napa use to have a full marine catalouge.
I used it for many items!!
You may want to ask them if its still available.
Click to expand...

NAPA can indeed get me much of what I need. However it appears that the flywheel cover gasket 27-96274 is no longer available anywhere with no substitute, I checked everywhere including ebay where sometimes I can find new old stock items.

Is this a paper gasket, can I cut one out of a roll of gasket material? Pulling that engine this weekend so I haven't actually looked at that part yet.
 
However it appears that the flywheel cover gasket 27-96274 is no longer available anywhere with no substitute, I checked everywhere including ebay where sometimes I can find new old stock items.
Click to expand...

I would not be concerned about a gasket or any sealant for this area.
The mating surfaces come to within a very close proximity of one another, and are not intended to seal against water or any other substances.
 
Finally have them side by side (I had to make an unplanned trip to help my son move). Looks like I'll need to swap the PS pump an Alternator too, or at least the brackets and location. Hose routing will otherwise be an issue.

OMC had a mechanical fuel pump, the mercruiser has an electric one. I'm half tempted to leave the mechanical pump in part because the OMC setup as a better location for the fuel filter/water separator. I can guess what you all will tell me though (chuckle). If I swap the pump and filter from the mercruiser, I may not have to make a new fuel line, but I'll have to find a cover for the mechanical pump comes out of. I presume this is an available part?
 
i have the same problem too 4.3 lx v6 cracked head i was wondering if i cold put my gm atlas i6 in there to replace the mercruiser but the i6 is a 4.2 vortec coming out of a 03 trailblazer with specs of [253.9 cu in; 275hp@6000 rpm and torque 275lb-ft@ 3600rpm
 
Finally got it all put together and ran sea trials yesterday (had a bunch of family things taking me out of town, plus a few amateur mistakes like breaking a starter bolt that took longer). The changeover was pretty extensive, once I got into it. Here's a summary:

I changed over everything as RicardoMarine suggested. The alternator and PS pump and brackets definitely needed to be swapped due to the Mercruiser cooling hose routing to the exhaust manifolds. The flywheel and drive coupler did need to be changed over as well. The pulleys o the harmonic balancer and circulation pump also needed to be changed due to the alternator and PS pump swap.

I did not change the fuel pump or fuel filter/separator, as discussed with kghost. The original fuel filter location was over an engine mount stringer and was almost impossible to change each year, the OMC location solved that problem. This meant that I had to bend a new fuel line, no biggie.

I did not change the dipstick. I wish I could have easily, because the OMC dipstick tube goes into the oil drain plug, On my Mercruiser I had replaced the plug with a hose so I could easily drain the oil. However I could not pop the block plug for the other dipstick easily and didn't want to drop the oil pan to punch it out from the backside. I guess it's back to sucking oil...

At first I did not change the engine mounts - they sure seemed the same. But I quickly realized as I tried to install the engine that the mounts were ever so slightly different in spread and hole diameter. Once I swapped those it dropped in easily. Alignment did need to be adjusted slightly.

The replacement engine had a chrome timing cover with a single bump for a timing mark (not sure if this was an aftermarket item). I had to transfer some timing marks from the original engine, which was easy enough. I probably could have/should have printed out a timing strip, but I'm pretty confident I'm where I need to be and it runs as expected.

By leaving the valve covers, the breather connectors were a different hose size. Fortunately the OMC had simple brackets I transferred over.

All in all it certainly took for too much time, but it's operational. Of course as I got back to the marina yesterday after sea trials, One of the slip owners told me our local mechanic recently pulled a nice low hour 5.0L Mercruiser out of a junked boat and was looking to sell reasonable. I'm told this is an easy fit and no changes would be needed gearwise in the outdrive, which surprised me. Oh well, timing is everything!

Thanks for all the help. I can post a video of the engine running if anyone is interested. Now to figure out how to sell all that OMC stuff on eBay...
 
CASSR said:
i have the same problem too 4.3 lx v6 cracked head i was wondering if i cold put my gm atlas i6 in there to replace the mercruiser but the i6 is a 4.2 vortec coming out of a 03 trailblazer with specs of [253.9 cu in; 275hp@6000 rpm and torque 275lb-ft@ 3600rpm
Click to expand...

Ayuh,..... You should start a thread of yer own, rather than tryin' to hy-jack fishndad's thread,.....

But anyways, to answer yer question,.....

No way it'll work,.....
 
our local mechanic recently pulled a nice low hour 5.0L Mercruiser out of a junked boat and was looking to sell reasonable. I'm told this is an easy fit and no changes would be needed gearwise in the outdrive, which surprised me.
Click to expand...

In a pinch, the 5.0L would be OK with the 4.3L gear reduction. You will be increasing torque, not reducing it!


The replacement engine had a chrome timing cover with a single bump for a timing mark (not sure if this was an aftermarket item). I had to transfer some timing marks from the original engine, which was easy enough. I probably could have/should have printed out a timing strip, but I'm pretty confident I'm where I need to be and it runs as expected.
Click to expand...

If this was my engine, I would verify the #1 cylinder TDC and corresponding TDC markings.


.
 
I repowered my boat from a 4.3 to 5.0 and kept the 4.3 gears in the drive, just repitched from a 19” to a 21”. I wouldn’t be afraid of proping around one step in gear ratio
 
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