1984 3.8L OMC upgrade to 1985 4.3L OMC

[stangclassic66]

Hi All,

Earlier this year I bought a 1984 20' I/O Grady White Overnighter for $1500 with the knowledge that the engine wouldn't start. PO said it could just be the starter but he doesn't know.

Well, long story short, I replaced the starter and got the engine to crank but the carburetor was rusted out and I couldn't get it to spark. So without spending more money on the engine, I pulled out my compression tester and pulled the numbers and I got an average of 105 psi per cylinder with one being 95psi. This whole time I assumed it was a 4.3l because why would grady put a 3.8 in there.....jeez, so dumb of me to assume that.

I get on craigslist and I find this 300 hr 1985 4.3 omc that started and ran perfect. Didn't think twice, I loaded it up and brought it home. I start to look at the engine in my boat and start looking at what needs to be swapped over and the bell housings don't really match up....ugh.....so I start looking at the engine tags in the boat and it turns out it's a 3.8l......@#$%@^$%&%*&

Is there any remote possibility that I can put this 4.3L OMC replacement in place of the 3.8L OMC? Motor mounts and bell housing swappable and would it sit right in the same spot? I believe I have a stringer drive on the boat.

Thanks in advance to you all that can help me on this.

Erik

 

[Howard Sterndrive]

Re: 1984 3.8L OMC upgrade to 1985 4.3L OMC

the 3.8L Chev and 4.3L Chev have the exact same bellhousing pattern, as does every 90 degree vee Chevrolet engine.
Should be a straightforward swap.

the reason Grady put the 3.8 in there is because it was the only 90 degree V6 that Chevrolet made in 1984.
The 4.3 wasn't produced for the marine market until 1985.

 

[stangclassic66]

Re: 1984 3.8L OMC upgrade to 1985 4.3L OMC

the 3.8L Chev and 4.3L Chev have the exact same bellhousing pattern, as does every 90 degree vee Chevrolet engine.
Should be a straightforward swap.

the reason Grady put the 3.8 in there is because it was the only 90 degree V6 that Chevrolet made in 1984.
The 4.3 wasn't produced for the marine market until 1985.

Thanks for the reply Howard! Now there is speculation that this 3.8 could be a Buick. Is there technically a difference? I know that buick is GM but back in the 80's i'm not sure if there were differences.

Also, how about the exhaust manifolds, are they interchangeable?

Thanks in advance!

Erik

 

[Howard Sterndrive]

Re: 1984 3.8L OMC upgrade to 1985 4.3L OMC

1984 3.8 OMC is Chevrolet. Like all Chev V6's and V8's, the distributor is at the rear. Buicks have distributors on the front of the engine. (The last year OMC put a Buick engine in a boat was 1972)

The original factory 3.8 exhaust manifolds will not work on the 4.3 - although they will technically fit the porting on the head, they are curved specifically to fit tight around the 3.8 head and therefore will not fit around the wider head and valve cover of the 4.3
OSCO brand aftermarket manifolds will fit both the 3.8 and 4.3.

It doesn't sound like the 3.8 is that far gone - I might spend some more time with penetrating oil down the cylinders, and checking out the valvetrain for the compression issue... valves can stick in the guides - can rebuild it, or replace the short block too.
But unless this boat is absolutely mint, it's probably not worth putting any money into. The stringer OMC drive system is expensive to maintain and there's a few heartbreaker parts that just aren't available anymore. Shift cables for example are $700 and in limited supply. Stuff like that I would be inspecting very carefully.

The good news about the 4.3, if you put it in there, being a 1985, it is an early 4.3 with a 2 pc rear main seal which means the 800 series OMC drive coupler will bolt up to the engine.
4.3's made after 1986 1/2 to current, are not swap candidates as the crankshaft bolt pattern changed.

 

[stangclassic66]

Re: 1984 3.8L OMC upgrade to 1985 4.3L OMC

1984 3.8 OMC is Chevrolet. Like all Chev V6's and V8's, the distributor is at the rear. Buicks have distributors on the front of the engine. (The last year OMC put a Buick engine in a boat was 1972)

Great, this distributor is in the rear!

The original factory 3.8 exhaust manifolds will not work on the 4.3 - although they will technically fit the porting on the head, they are curved specifically to fit tight around the 3.8 head and therefore will not fit around the wider head and valve cover of the 4.3
OSCO brand aftermarket manifolds will fit both the 3.8 and 4.3.

My 3.8 has OSCO manifolds along with an aftermarket fwc heat exchanger, that's the main reason I asked the question. The 4.3 has the stingray style manifolds but i'll look further into if I can retain these stingray manifolds and keep the heat exchanger. The OSCO's are pretty nasty looking that i'd feel compelled to replace if I did need to switch.

It doesn't sound like the 3.8 is that far gone - I might spend some more time with penetrating oil down the cylinders, and checking out the valvetrain for the compression issue... valves can stick in the guides - can rebuild it, or replace the short block too.
But unless this boat is absolutely mint, it's probably not worth putting any money into. The stringer OMC drive system is expensive to maintain and there's a few heartbreaker parts that just aren't available anymore. Shift cables for example are $700 and in limited supply. Stuff like that I would be inspecting very carefully.

Thanks for the heads up on this stuff! I appreciate it.

The good news about the 4.3, if you put it in there, being a 1985, it is an early 4.3 with a 2 pc rear main seal which means the 800 series OMC drive coupler will bolt up to the engine.
4.3's made after 1986 1/2 to current, are not swap candidates as the crankshaft bolt pattern changed.

Now, the big question, how can I confirm this is indeed a 1985 model? Do you happen to know how to decipher engine block castings? Port side near the bellhousing is stamped:

177NI
4.3L

 

[Howard Sterndrive]

Re: 1984 3.8L OMC upgrade to 1985 4.3L OMC

I don't know those numbers, but all 1985 OMC's were painted silver like this:

 

[stangclassic66]

Re: 1984 3.8L OMC upgrade to 1985 4.3L OMC

The 4.3 I have is painted a darker grey than that.

 

[Howard Sterndrive]

Re: 1984 3.8L OMC upgrade to 1985 4.3L OMC

then the only way to be sure, is to measure the crank bolt pattern, or get the part number off the coupler if it has one on there.

Charcoal grey is Cobra, which was introduced in 1986 model year, and a few did go out early in 86 with the old 4.3 with 2 pc rear main - the other 98% of 4.3 Cobras (charcoal) would have 1 pc rear main seals evident of 1987 and newer motors