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91 Mercrusier 5.7 replacement (91 638 to 92 LT1 style)

J

joshsopczak

New member
Good afternoon, gentlemen-

I have a 1991 Mercrusier 5.7 with 638 marked on the block's sidewall (motor serial number 0D427250) which did not have a fully successful drain of the block during winterizing due to a mud/rust dam. I've done a THOROUGH JB weld repair and suspect it will be successful, but won't know for another couple weeks until I can run it on muffs for 30-60 minutes in the driveway to get it heated up for a while and then check the oil. As a secondary plan to make sure I'm ready for fishing season, I saw a 1992 LT1 4-bolt 350 block for sale local to me that looks pretty good. My mechanic friend indicates I should have no trouble using that for a swap, since I'm going to strap the heads and all my Mercrusier gear to the new engine, saying the only differences are in the oil pan and exhaust manifold areas, for which we'll be using the existing. However, the seller is concerned that the LT1 may not be a swappable block for the 638 version of the 350 I have.

I'm looking to see if anyone can help clarify for me the differences between them (if any) for the block and pistons, or if this should be fine to use as a replacement for my cracked unit. To clarify again, I'd be using my existing heads, alternator, pumps, starter, and all other parts, other than the lower half (block, cylinders, and related).

I appreciate any guidance.

Josh Sopczak
Racine, WI
 
Bt Doctur said:
brass or SS core plugs
Click to expand...

Good evening, and thank you! You're historical comments in other posts have been a legendary help to me, and likely many others!
I believe my current engine has brass plugs (someone in the past painted over them, but I think I see brass under one where they missed with the paint). We're in freshwater, Lake Michigan, for that matter. For the new engine, I don't have a good view from the advertised pictures.
 
Kicking myself for the communication error, too. The motor block I'm looking at is a 1993 LT1 style. I don't know if that makes any different impact on a prospective replacement.
 
Most 80's and 90's era marine engines (standard, NOT High Performance) are Truck blocks with basically a Truck cam shaft. Moderate power with good torque in the range needed for a boat. 2500-5000 rpm range.

There is no other significant difference. If there was an LT1 in the early 90's I would suspect it was used in a car like the Impala SS.

What you will need to do to an Auto engine to marinize it.

Core plugs for Auto are steel, they have to be replaced with Brass or Stainless steel, Brass is the most common. Steel will rust away regardless of fresh water or not.

Parts from current Marine engine to be swapped over to Auto engine.

Intake and carb. Marine intakes and carbs are different then Automotive

If there is a mechanical fuel pump it must be swapped to new block. Auto and marine mechanical fuel pumps are not the same. Auto's drain to ground if diaphragm ruptures. Marine fuel pumps have a barbed fitting that allows fuel to go to carb.
If the new block was EFI/TBI and the Mechanical fuel pump mounting is un usable (fuel pump push rod hole is not machined to correct diameter) then you have to upgrade your fuel delivery system to a Marine electric fuel pump system ( Low pressure for carb) with Marine approved components only.

Flywheel and coupler assembly and all back of engine parts (motor mount, shields etc)

Starter. Auto and marine starters are the same internally but Marine are spark arrested so swap this part.

Ignition system (complete swap)

All alarm switches

All Senders, Oil, temp,

A marine on engine water circulation water pimp is different than automotive. Automotive have a stamped steel impeller and is one directional. A Marine water pump has a bronze bi-directional impeller so swap the water pump out also.

Drain plugs on block just forward of starter and oil filter need to be swapped out to the brass ones on current engine.

So as you can see, it can be done but it may be costly and for sure it is time consuming.........

Based on your budget it may be easier to find a complete used marine engine that would or could be plug and play. Or purchase a re manufactured long block (with heads) and then do the swap of all other parts. Just remember if you have a mechanical fuel pump you have to specify that when ordering a re manufactured engine.

One last thing, there are two different blocks and are typically identified but the number of intake manifold bolts. Make sure the block you are looking at is the same as the current one in boat. The difference is standard block 12 intake bolts vs Vortec style 8 intake bolts. The intakes are not swappable.
 
Last edited:
KGhost-

Greatly appreciate the guidance. I planned to move everything from my existing engine over to the new block, and would change the plugs if they are steel (likely) to brass. Beyond that, I was hoping to keep all other components from my current motor from heads to everything else you mentioned.

The seller didn't think it would be a compatible swap (his 93 LT1 vs my 91 638), though my local mechanic believed it would work fine. They only item I could find that sound different is that the LT1 appears to have a different water flow configuration (heads to block) versus mine (block to head). I found two articles mentioning that as something of a re-design difference between them. That's why I'm looking for guidance. I may just look to get a remanned 5.7 to be safe.

-Josh Sopczak
Racine, WI



kghost said:
Most 80's and 90's era marine engines (standard, NOT High Performance) are Truck blocks with basically a Truck cam shaft. Moderate power with good torque in the range needed for a boat. 2500-5000 rpm range.

There is no other significant difference. If there was an LT1 in the early 90's I would suspect it was used in a car like the Impala SS.

What you will need to do to an Auto engine to marinize it.

Core plugs for Auto are steel, they have to be replaced with Brass or Stainless steel, Brass is the most common. Steel will rust away regardless of fresh water or not.

Parts from current Marine engine to be swapped over to Auto engine.

Intake and carb. Marine intakes and carbs are different then Automotive

If there is a mechanical fuel pump it must be swapped to new block. Auto and marine mechanical fuel pumps are not the same. Auto's drain to ground if diaphragm ruptures. Marine fuel pumps have a barbed fitting that allows fuel to go to carb.
If the new block was EFI/TBI and the Mechanical fuel pump mounting is un usable (fuel pump push rod hole is not machined to correct diameter) then you have to upgrade your fuel delivery system to a Marine electric fuel pump system ( Low pressure for carb) with Marine approved components only.

Flywheel and coupler assembly and all back of engine parts (motor mount, shields etc)

Starter. Auto and marine starters are the same internally but Marine are spark arrested so swap this part.

Ignition system (complete swap)

All alarm switches

All Senders, Oil, temp,

A marine on engine water circulation water pimp is different than automotive. Automotive have a stamped steel impeller and is one directional. A Marine water pump has a bronze bi-directional impeller so swap the water pump out also.

Drain plugs on block just forward of starter and oil filter need to be swapped out to the brass ones on current engine.

So as you can see, it can be done but it may be costly and for sure it is time consuming.........

Based on your budget it may be easier to find a complete used marine engine that would or could be plug and play. Or purchase a re manufactured long block (with heads) and then do the swap of all other parts. Just remember if you have a mechanical fuel pump you have to specify that when ordering a re manufactured engine.

One last thing, there are two different blocks and are typically identified but the number of intake manifold bolts. Make sure the block you are looking at is the same as the current one in boat. The difference is standard block 12 intake bolts vs Vortec style 8 intake bolts. The intakes are not swappable.
Click to expand...
 
You will want a marine cam so that you don't suck water in thru the exhaust. I recommend a rebuilt unit with a warranty over a junkyard pull out. That's just me tho.
 
joshsopczak said:
KGhost-

Greatly appreciate the guidance. I planned to move everything from my existing engine over to the new block, and would change the plugs if they are steel (likely) to brass. Beyond that, I was hoping to keep all other components from my current motor from heads to everything else you mentioned.

The seller didn't think it would be a compatible swap (his 93 LT1 vs my 91 638), though my local mechanic believed it would work fine. They only item I could find that sound different is that the LT1 appears to have a different water flow configuration (heads to block) versus mine (block to head). I found two articles mentioning that as something of a re-design difference between them. That's why I'm looking for guidance. I may just look to get a remanned 5.7 to be safe.

-Josh Sopczak
Racine, WI
Click to expand...

I cannot speak to reverse water flow. That would need to be better understood with regards to that LT1 engine and what if any affects it has on using it as a standard flow engine.

It may be as simple as the water pump and intake manifold design and hoses which you would have to swap anyway. I ma not sure the engine would know the difference. But if you go this route, You best be sure!!
 
Ghost-
I can confirm that the block I was looking at won't work due to that reverse cooling. I really need to focus on finding another 638 from 1987-1994 from what I've learned. Now it is a matter of the best place to get one, another Mercruiser engine, a Chevy truck (or Corvette), or remanufactured.

Our weather is going to be 47* next weekend, so I'm planning on trying my current one with the JB weld fix on it. If it doesn't work, it will be go-time to get a replacement so that I don't miss out on the spring fishing season!

-Josh Sopczak
Racine, WI


kghost said:
I cannot speak to reverse water flow. That would need to be better understood with regards to that LT1 engine and what if any affects it has on using it as a standard flow engine.

It may be as simple as the water pump and intake manifold design and hoses which you would have to swap anyway. I ma not sure the engine would know the difference. But if you go this route, You best be sure!!
Click to expand...
 
I have done much work on this year engine, and I believe it is actually 1987-1995. [One piece rear main seal (center bolt valve covers)]

The NON Vortec engines (8 intake bolts for Vortec vs 12 intake bolts for Non Vortec).

These engines are readily available, Millions! The Re manufacture industry is loaded with these engines!!

The key is what I pointed out, If you have a mechanical fuel pump now, you need to order or find a engine that has the same.

Personally I would stay away from auto engines and try to acquire a marine engine so all the marine stuff is already done (core plugs...etc...)

Many sources for a 350 CU base marine engine typically 260 hp long blocks (Assembled block with heads but require swapping many old parts from bad engine) can be had for under $3000.

Dont get caught up with the engine casting numbers. Stick with the basics. 350/5.7 base engine, Marinized, with heads that accept a 12 bolt intake and mechanical fuel pump.

If you have a 21 ft or smaller not to heavy boat this basic choice will work just fine

FYI I have done this swap a few times with a remanufactured engine. Total cost NOT including my labor was just under $3000.

But I had to swap, oil pump, 4 crank main bolts to hold oil deflector plate, Oil pan, Fuel pump, timing chain cover, water pump, intake and ignition, exhaust manifolds and possibly a few other things. It takes time but for the cost it was a great option for the person I did this for. I think I had about 14 or so hours in total to do the swap.

Now this was a Ski boat with the engine in the middle of the boat so the logistics were easier.
For a stern drive engine swap this will take a lot longer so be prepared for more difficult work.

Below is a link that has a good listing of marine engines and includes casting numbers....

The 638 is listed several times. Not 100% sure that is important as there are other casting numbers in the same listings...

This is for reference only and not a recommendation. That is up to you.

https://www.perfprotech.com/remanuf...-v8-gm-marine-longblock-engines/category/2586

Do your research!
 
Thanks for all the great info.

Saturday is the day! We've got a projected high of 47 and Sunday 48 with overnight low of 45 between them! I'm going to run it for a couple of hours constant each of those two days and then check the oil each time to see if I have any water intrusion. After the test, I'll drain full system and refill with RV antifreeze again until season starts in another month (I don't want to wait too far out only to find out I need to rebuild).

If I have any issues, I think I'll order a new kit from the site you provided, or one similar, and simply rebuild (as you have done before). If I don't have issues during my test, I'm going to run it, as is. A local shop is able to do metal stitching on the surface if it is only the exterior of the water jacket, for around $1,500 if the JB weld does start to weep. They agreed I should try the JB Weld first to ensure the interior of the engine is not damaged, before dropping that cash on the repair.

Any suggestions on what to look for besides water intrusion into the oil? I've already done a compression test and all cylinders are around 110.

Josh Sopczak
Racine, WI


kghost said:
I have done much work on this year engine, and I believe it is actually 1987-1995. [One piece rear main seal (center bolt valve covers)]

The NON Vortec engines (8 intake bolts for Vortec vs 12 intake bolts for Non Vortec).

These engines are readily available, Millions! The Re manufacture industry is loaded with these engines!!

The key is what I pointed out, If you have a mechanical fuel pump now, you need to order or find a engine that has the same.

Personally I would stay away from auto engines and try to acquire a marine engine so all the marine stuff is already done (core plugs...etc...)

Many sources for a 350 CU base marine engine typically 260 hp long blocks (Assembled block with heads but require swapping many old parts from bad engine) can be had for under $3000.

Dont get caught up with the engine casting numbers. Stick with the basics. 350/5.7 base engine, Marinized, with heads that accept a 12 bolt intake and mechanical fuel pump.

If you have a 21 ft or smaller not to heavy boat this basic choice will work just fine

FYI I have done this swap a few times with a remanufactured engine. Total cost NOT including my labor was just under $3000.

But I had to swap, oil pump, 4 crank main bolts to hold oil deflector plate, Oil pan, Fuel pump, timing chain cover, water pump, intake and ignition, exhaust manifolds and possibly a few other things. It takes time but for the cost it was a great option for the person I did this for. I think I had about 14 or so hours in total to do the swap.

Now this was a Ski boat with the engine in the middle of the boat so the logistics were easier.
For a stern drive engine swap this will take a lot longer so be prepared for more difficult work.

Below is a link that has a good listing of marine engines and includes casting numbers....

The 638 is listed several times. Not 100% sure that is important as there are other casting numbers in the same listings...

This is for reference only and not a recommendation. That is up to you.

https://www.perfprotech.com/remanuf...-v8-gm-marine-longblock-engines/category/2586

Do your research!
Click to expand...
 
joshsopczak said:
Thanks for all the great info.

Saturday is the day! We've got a projected high of 47 and Sunday 48 with overnight low of 45 between them! I'm going to run it for a couple of hours constant each of those two days and then check the oil each time to see if I have any water intrusion. After the test, I'll drain full system and refill with RV antifreeze again until season starts in another month (I don't want to wait too far out only to find out I need to rebuild).

If I have any issues, I think I'll order a new kit from the site you provided, or one similar, and simply rebuild (as you have done before). If I don't have issues during my test, I'm going to run it, as is. A local shop is able to do metal stitching on the surface if it is only the exterior of the water jacket, for around $1,500 if the JB weld does start to weep. They agreed I should try the JB Weld first to ensure the interior of the engine is not damaged, before dropping that cash on the repair.

Any suggestions on what to look for besides water intrusion into the oil? I've already done a compression test and all cylinders are around 110.

Josh Sopczak
Racine, WI
Click to expand...

Ayuh,..... If yer block is cracked, buy a reman long block,.....
For the $1500. they want to stitch up a common block, you can buy a reman, without any cracks in the block,......
Iron stitching is only for cast iron parts, that are not common, 'n not replaceable,.....
There's literally millions of SBCs out there,.....
'ell,.... for the price of an updated flywheel, coupler, intake manifold, 'n electric fuel pump, you can easily bolt in a late model Vortec motor,....

Btw,.... A LT1 is not a boat motor,..... neither is any other Corvette motor,.....
 
Last edited:
Good morning, Bondo-

Where would you suggest I pick up a block for $1,500?... and what model do you suggest? Right now, I have that 86-94 638 block and would want to keep all my other components to swap. I agree that for $1,500 is a lot, but I've not seem many Mercrusier 5.7's for under $2,000... more-so around $2,500.

I'm all ears with your guys' experience.

-Josh


bondo said:
Ayuh,..... If yer block is cracked, buy a reman long block,.....
For the $1500. they want to stitch up a common block, you can buy a reman, without any cracks in the block,......
Iron stitching is only for cast iron parts, that are not common, 'n not replaceable,.....
There's literally millions of SBCs out there,.....
'ell,.... for the price of an updated flywheel, coupler, intake manifold, 'n electric fuel pump, you can easily bolt in a late model Vortec motor,....

Btw,.... A LT1 is not a boat motor,..... neither is any other Corvette motor,.....
Click to expand...
 
Just an update.... We had 55* weather today and expecting upper 40's tomorrow (overnight in the 50's!!!). I ran the engine for 45 minutes, mostly at idle, but some at 2500-3000 RPMs. All temps maintained normal levels. I used a laser thermometer and observed the manifolds at 180-190, thermostat and water lines all running around 150-160, and the risers in the 130's (water jacket area). It appears the repair did just fine and most importantly, THE OIL WAS PERFECTLY CLEAN!

We're still 3 weeks away from the marina opening up slips, so I've still got some time to test again closer to season. However, for now, I'm thrilled to say I'm going to run with it. I think I will still keep an eye out for a replacement 638 block over this year in which I can do a rebuild in my garage (including new piston seals and a full cleanout of the oil pan system. If I find one, it will probably be my next-winter project and then I'll drop it in to replace the current motor.

Thanks again for all the help, gentlemen.

Josh Sopczak
Racine, WI



QUOTE=joshsopczak;729606]With shipping, apx $2,200... still better than most I've been seeing, and comes with a bit of a warranty!<br>
<br>
-Josh[/QUOTE]<br>
<br>
 
I’m not sure how to start my own post I’m brand new on here but looks like this one is resolved until next winter? I’m on engine number 2 both ruined within a year of each other, both has water in oil. On the first engine oil was extremely milky on the second engine I had closed cooling system installed and oil look perfect except on tip of dip stick and then when draining oil looked perfect except the last quart or so was really milky. Any idea how that could happen? 2 fresh 5.7 rebuilds . Everything minus exhaust elbows and risers. Barely starting to realize it could be exhausts?

sorry if I’m using forum incorrectly. Accepting any guidance to correct. Thanks in advance.

bondo said:
Ayuh,..... I guess inflation has driven the prices up abit,.....

Here's a duck duck go search, 2 of the pictured long blocks at the top of the page are $1900., 'n $1800., on ebay,....
Click to expand...
 
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