Engine swap, OMC to mercruiser

[s_explore]

So I bought a boat last year. I did restore the transom, stringers, etc and last week I was looking at the engine. An OMC 1987 V8 5.0L. I guess the last owner didn't really care about it, he didn't winterise it and there I am with a cracked block which shoot water everywhere.

So I am looking into the mercruiser engine. Want to go with injector instead of carburator. I found one 2006 mercruiser at 5000$. My question is can I make the swap without too much modification? Are the engine similar? Is it gonna fit in my sterndrive?

Thanks in advance

 

[dubs283]

Best option is to install an entire mercruiser package. Engine, transom assy, drive and controls/cables/wiring

Nothing from mercruiser will match up with OMC

 

[Bondo]

Ayuh,..... Welcome Aboard,...... The only thing they have in common is the GM Long Block,.....
Everything else is different,.....

 

[Scott06]

So I bought a boat last year. I did restore the transom, stringers, etc and last week I was looking at the engine. An OMC 1987 V8 5.0L. I guess the last owner didn't really care about it, he didn't winterise it and there I am with a cracked block which shoot water everywhere.

So I am looking into the mercruiser engine. Want to go with injector instead of carburator. I found one 2006 mercruiser at 5000$. My question is can I make the swap without too much modification? Are the engine similar? Is it gonna fit in my sterndrive?

Thanks in advance

Unless you swap the entire drive, transom housing etc with the engine you will have to do some mix and match of parts. If your existing OMC is a cobra the keyhole in the transom is the same size as merc and volvo sx. Best bet is get pulleys to prop engine, transom housing, drive tear out

I doubt the drive was well kept if the stringers were rotten and block cracked. no sense turd polishing the OMC items to fit a merc long block....

 

[Lou C]

A well maintained Cobra is fine but if you really don’t know if what you have is any good best to go with all Mercruiser. Sell off the OMC stuff and start over.

 

[Scott Danforth]

So I bought a boat last year. I did restore the transom, stringers, etc and last week I was looking at the engine. An OMC 1987 V8 5.0L. I guess the last owner didn't really care about it, he didn't winterise it and there I am with a cracked block which shoot water everywhere.

So I am looking into the mercruiser engine. Want to go with injector instead of carburator. I found one 2006 mercruiser at 5000$. My question is can I make the swap without too much modification? Are the engine similar? Is it gonna fit in my sterndrive?

Thanks in advance

EFI doesnt get you any additional power or economy. if you dont want to spend much money, get a 1987-1995 5.0 or 5.7 liter motor from a low mileage truck for $300 and drop in the longblock. you will need to buy new exhaust manifolds, and maybe an intake. however total cost would be in the $1000 range. otherwise if you want to swap to a late-model mercruiser EFI setup. swap a complete longtail (pulley to prop)

 

[nola mike]

EFI doesnt get you any additional power or economy. if you dont want to spend much money, get a 1987-1995 5.0 or 5.7 liter motor from a low mileage truck for $300 and drop in the longblock. you will need to buy new exhaust manifolds, and maybe an intake. however total cost would be in the $1000 range. otherwise if you want to swap to a late-model mercruiser EFI setup. swap a complete longtail (pulley to prop)

Are you talking keeping the omc setup? If so, yes long block is easiest. Otherwise pulley to prop Merc is the way to go. Anything else and you'll be chasing stupid random parts.

 

[s_explore]

Unless you swap the entire drive, transom housing etc with the engine you will have to do some mix and match of parts. If your existing OMC is a cobra the keyhole in the transom is the same size as merc and volvo sx. Best bet is get pulleys to prop engine, transom housing, drive tear out

I doubt the drive was well kept if the stringers were rotten and block cracked. no sense turd polishing the OMC items to fit a merc long block....

Sorry if I look like a noob here, but what do you mean by pulleys to prop engine?

 

[nola mike]

Whole engine, all accessories, wiring, controls, outdrive, transom bits, everything. You'll need a donor boat with a rotten hull.

 

[s_explore]

EFI doesnt get you any additional power or economy. if you dont want to spend much money, get a 1987-1995 5.0 or 5.7 liter motor from a low mileage truck for $300 and drop in the longblock. you will need to buy new exhaust manifolds, and maybe an intake. however total cost would be in the $1000 range. otherwise if you want to swap to a late-model mercruiser EFI setup. swap a complete longtail (pulley to prop)

You think that a truck engine would be reliable in a boat? I know some guy did it, but is it worth it

 

[s_explore]

Whole engine, all accessories, wiring, controls, outdrive, transom bits, everything. You'll need a donor boat with a rotten hull.

Oh god ok that seem way more than what I willing to buy. Maybe I should find another omc and stick with that. I was thinking to go with injection but it look like it's not gonna really be better here.

 

[nola mike]

You think that a truck engine would be reliable in a boat? I know some guy did it, but is it worth it

I got one on my boat. Same engine basically as what you have now. My feeling is a truck engine will be better than a used marine block.

 

[JASinIL2006]

Sorry if I look like a noob here, but what do you mean by pulleys to prop engine?

He means that if the OMC is a Cobra, it will have the same keyhole as a Mercruiser or Volvo SX, and the easiest thing will be to replace the entire engine/outdrive assembly (aka, replace everything pulley to prop).

 

[Lou C]

First you need to evaluate what shape your OMC drive system stuff is in before using it with a new to you engine. If it’s a Cobra keep in mind while these were good when maintained by someone who knows OMCs (like me, owned this one about 20 years) but I also have the manuals and LOTS of spare parts because you can’t just pick up what you need whenever you want it like with Mercruiser. If you can find a good deal on a complete Merc take out (engine/transom mount/drive) I’d go for it. It’s no fun searching eBay & NLA Marine parts looking for stuff not made since 1993.
For mine I have a complete spare drive, transom mount & extra set of trim rams, trim pump, power steering actuator, power steering pump, hydraulic hoses for trim & p/s system, etc. With Merc you need a part you go buy it.
Forget EFI in an old boat. Parts are more expensive for little benefit. You can buy brand new Marine Holleys or Edelbrocks and a low cost electric fuel pump set up, and these are 100% supported in the aftermarket. 15-20 year old marine EFI is not, certain parts are NLA.

 

[s_explore]

I got one on my boat. Same engine basically as what you have now. My feeling is a truck engine will be better than a used marine bloc

From which truck did you take it? Any v8 5.0l will do the job? You were able to fit the exhaust from the omc on it?

 

[Lou C]

You could go with a 5.7 even if you originally had a 5.0 in it. The exhaust manifold pattern is the same on all small block Chevrolets the OMC exhaust will bolt up to any 5.0 or 5.7. The main differences:
If your original engine has a one piece rear main seal & flywheel the new engine has to have the same set up…
Marine cam more like a truck or RV grind not high performance car grind…
If your engine is a pre 1996 and you get a post 96 to replace it, that will be a Vortec which requires a different intake manifold & electric fuel pump…
Brass core plugs
Stainless steel head gaskets
All marine accessories like starter, alternator, distributor, fuel pump & carburetor. Do NOT use any auto accessories on a marine gas inboard.

 

[Scott Danforth]

You think that a truck engine would be reliable in a boat? I know some guy did it, but is it worth it

your long block and a truck long block are almost identical.

core plugs, head gaskets and circulating pump are what makes them different.

Oh god ok that seem way more than what I willing to buy. Maybe I should find another omc and stick with that. I was thinking to go with injection but it look like it's not gonna really be better here.

Injection doesnt buy anything that a well maintained carburetor gives you.

From which truck did you take it? Any v8 5.0l will do the job? You were able to fit the exhaust from the omc on it?

all SBC engines have the same exhaust bolt pattern. boat motors and heads come from the same Tonawanda NY engine plant as truck motors and are made from the same parts. only the stuff bolted onto that long-block is what makes it an OMC, or Mercruiser, or Volvo Penta

since your motor is a 1987, you need the 1987-1995 motors

You could go with a 5.7 even if you originally had a 5.0 in it. The exhaust manifold pattern is the same on all small block Chevrolets the OMC exhaust will bolt up to any 5.0 or 5.7. The main differences:
If your original engine has a one piece rear main seal & flywheel the new engine has to have the same set up…
Marine cam more like a truck or RV grind not high performance car grind…
If your engine is a pre 1996 and you get a post 96 to replace it, that will be a Vortec which requires a different intake manifold & electric fuel pump…
Brass core plugs
Stainless steel head gaskets
All marine accessories like starter, alternator, distributor, fuel pump & carburetor. Do NOT use any auto accessories on a marine gas inboard.

what Lou said

 

[s_explore]

Well that is good to know, will save me money. With a truck engine, what will happen with the cooling system. Do I need to convert the engine cooling to water? Is that something that can be done? Also for the engine mount, are they the same?

If my understanding is good, I will need to check if the engine has one piece rear seal & flywheel. I'm not sure what do you mean by that, what is the rear seal?
When I found one, I remove the coupler from the omc and put it on the truck engine, put the exhaust, with alternator, fuel pump, etc and that should be it.

One other option, is it faisable and a good idea to solder the cracked block of the omc?

 

[Scott Danforth]

Well that is good to know, will save me money. With a truck engine, what will happen with the cooling system. Do I need to convert the engine cooling to water? Is that something that can be done? Also for the engine mount, are they the same?

If my understanding is good, I will need to check if the engine has one piece rear seal & flywheel. I'm not sure what do you mean by that, what is the rear seal?
When I found one, I remove the coupler from the omc and put it on the truck engine, put the exhaust, with alternator, fuel pump, etc and that should be it.

One other option, is it faisable and a good idea to solder the cracked block of the omc?

sort of

you will need to get a 1988 to 1995 running low mileage truck engine. this will get you a compatible motor. do not go older than 1988, do not go newer than 1995 unless you want to convert to electric fuel pump and get a new intake manifold to support the vortec heads.

you only need the long block, you do not need anything else from the truck engine. you do not need any accessory, starter, ignition, intake, carburetor, etc. This is between $125 and $350 depending on the yard you pull it from

you will take the heads off the motor and switch to marine head gaskets. this is about $80

you will remove the core plugs from the motor and replace with brass core plugs. This is about $15

you will use the marine fuel pump from your motor and transfer it over to the new motor. This is a $3 gasket

you will buy a new marine circulating water pump and use that. This is $50

you will buy new marine exhaust manifolds because the same lack of winterization that broke your block, has also broke your OMC exhaust manifolds. This is $550

you will need to inspect the intake manifold you have for cracks. if there are no cracks, you will swap over the intake manifold and carburetor

you will swap over the starter, flywheel, distributor, wiring, accessories, coupler, etc.

you will then test run the motor before putting it back in the boat

you will then install the motor, then align the motor, then install the outdrive


while you can not solder the block crack, some people have attempted to use epoxy. I wouldnt do it personally as its a fools errand.

 

[nola mike]

sort of

you will need to get a 1988 to 1995 running low mileage truck engine. this will get you a compatible motor. do not go older than 1988, do not go newer than 1995 unless you want to convert to electric fuel pump and get a new intake manifold to support the vortec heads.

(unless you want vortec heads and more power). What keeps the 2 piece RMS from swapping over? Different crank bolt/flywheel pattern?

you will remove the core plugs from the motor and replace with brass core plugs. This is about $15

At least some newer engines already have brass, my 2000 4.3 already did and others have reported the same.

while you can not solder the block crack, some people have attempted to use epoxy. I wouldnt do it personally as its a fools errand.

Agreed. Also, only success stories I've seen have been on the external jackets of the 3.0s.