4.3 mercruiser vs 4.3 vortec truck motor

[thegoat86]

I am in a debate with my buddy who says the 4.3 mercruiser is just a GM vortec truck motor with different seals and freeze plugs. Also says, put the marine seal and freeze plug kit in, then swap intake, exhaust, distributor, flywheel and accessories and you would never know the difference. Says motor would run the same and last just as long.

I on the other hand would think it would be more than that. To have a motor run at high RPM for hours on end would have to have a decent rotating assembly. Plus to have the high compression ratio these motors have, you would need something like a flat top piston or something of that nature. Not being well versed in GM or marine application motors, I can not say either way. I am an old school Mopar/Chrysler guy.

Would love to know the real story. Not just something someone said or overheard. Which is where I think my buddy is coming from. Although he did work as a GM tech for a few years in the early 2000s.

 

[alldodge]

I am in a debate with my buddy who says the 4.3 mercruiser is just a GM vortec truck motor with different seals and freeze plugs. Also says, put the marine seal and freeze plug kit in, then swap intake, exhaust, distributor, flywheel and accessories and you would never know the difference. Says motor would run the same and last just as long.

I on the other hand would think it would be more than that. To have a motor run at high RPM for hours on end would have to have a decent rotating assembly. Plus to have the high compression ratio these motors have, you would need something like a flat top piston or something of that nature. Not being well versed in GM or marine application motors, I can not say either way. I am an old school Mopar/Chrysler guy.

Would love to know the real story. Not just something someone said or overheard. Which is where I think my buddy is coming from. Although he did work as a GM tech for a few years in the early 2000s.

The GM marine motor is the same as the same "year" truck motor. Only difference is the freeze plugs, head gasket and accessories. The marine market is very, very small amount of engines produced by any manufacture. There just is not enough of them being produced to make them a real player in the engine world.

On a side note. GM stopped making the 502 (8.2L)and it was about to go away with the 496 (8.1L), but it was great to hear the 8.1 went away and the 8.2 is back by Merc

 

[proshadetree]

I know when I replaced my original 5.0 baylinner engine with a 1996 truck engine the block,crank, rod and Pistons all had the same casting numbers. That's been 5 years ago plus it has the same head gaskets and freeze plugs it left the factory with.

 

[thegoat86]

I did some online perusing, and found the same info AD.

How would one distinguish a truck motor from a car motor? I knew the 5.7s had an option for 2 or 4 bolt main and steel crank, would that be the same with any truck motor? I read alot of the Truck motor applications had a cam that operated higher torque numbers at lower RPMs.

This all arose because my friend heard what sounds to be an intermittent knock coming from my motor at high RPM. He told me it was a wrist pin. I know of some engine knocks and it does kinda..... sound like that. But when I heard them before they were consistent not intermittent. But it could also be a cylinder misfiring, or not firing at all. That's when the debate started. He told me to find a motor from an old astro van or truck and swap all my parts to it. But first to change the gaskets to marine and brass or copper freeze plugs.

None of which I would do without first ruling out a fouling plug or maybe a timing issue I am currently dealing with. (another post

Thought I would ask. Seems to be a pretty heated debate amongst boaters and marine mechanics. Answers I read are anything from stainless valves, thicker blocks, low RPM cam profiles, coated water jackets, and flat top pistons (which necessitates the need for high or mid grade fuel???) I just buy the non-ethynol at wawa near me. Oh, and someone mentioned marine mechanics/magicians, putting some kind of fairy dust in the marine motors to make them truly "Marine" :target:

 

[thegoat86]

I know when I replaced my original 5.0 baylinner engine with a 1996 truck engine the block,crank, rod and Pistons all had the same casting numbers. That's been 5 years ago plus it has the "same head gaskets and freeze plugs" it left the factory with.

Must be a closed cooling system?

 

[Scott Danforth]

yes, the marine motors come out of the same parts bins as the trucks. the cam is slightly different, however the long block, flywheel, sheet metal, and balancer are the same. From there Mercruiser or Volvo marinize the block, providing their own intake, exhaust maniolds, electrics, and induction.

the 8.2 liter (502 cubic inch) is a GM performance block, never a main-stream production block. Mercuiser took their 525hp and detuned it to 425hp to replace the 8.1 where Volvo tweeked the GM 6.0 and 6.2 LS motors for the 8.1 replacements. Prior to the 8.1 (496), was the 7.4 (454)

 

[Silvertip]

Before you do anything, do lots of reading on the issue regarding balance shaft noise due to the bearings it runs in. This noise was very common on the balance shaft motors but had nothing to do with longevity of the motor. If yours is not a balance shaft motor then further investigation as to the noise is obviously necessary. My experiences with this noise compares it to pre-ignition (spark knock for old-timers) at various rpm. GM had a kit to take care of this but other than being an annoyance, most folks chose to live with it. I once bought a '92 AWD Chevy Astro van with about 50K miles on it. At about 60K the slight balance shaft noise appeared and was the same at 150K when I sold it.

 

[H20Rat]

That's when the debate started. He told me to find a motor from an old astro van or truck and swap all my parts to it.

Worst advice ever! Especially for a 4.3! You can get reman short blocks for well under $1000, even down to $600 or so. Long blocks w/heads can be had for $1200 - $1600. These are for essentially new motors with a warranty. Or you can pay $200-$500 at a u-pick-it for a motor that may or may not even run, with a lot of hard miles and unknown maintenance.

 

[thegoat86]

Before you do anything, do lots of reading on the issue regarding balance shaft noise due to the bearings it runs in. This noise was very common on the balance shaft motors but had nothing to do with longevity of the motor. If yours is not a balance shaft motor then further investigation as to the noise is obviously necessary. My experiences with this noise compares it to pre-ignition (spark knock for old-timers) at various rpm. GM had a kit to take care of this but other than being an annoyance, most folks chose to live with it. I once bought a '92 AWD Chevy Astro van with about 50K miles on it. At about 60K the slight balance shaft noise appeared and was the same at 150K when I sold it.

How would I go about finding this out? I have personally never heard of this kind of thing before. But that doesn't mean much...

 

[thegoat86]

Silvertip, I am going to try to not cross threads, Please go to my other thread and read my comments. I would like to hear more about your balancer issue. I may be having the same issue. 91' larson, 4.3 timing issue

 

[thegoat86]

Worst advice ever! Especially for a 4.3! You can get reman short blocks for well under $1000, even down to $600 or so. Long blocks w/heads can be had for $1200 - $1600. These are for essentially new motors with a warranty. Or you can pay $200-$500 at a u-pick-it for a motor that may or may not even run, with a lot of hard miles and unknown maintenance.

I hear you. I only anticipate keeping my boat for another year or so. At $200 a whack, It would be like changing fuses to my pocket book. My buddy makes a great argument. Although when I would be in the changing seals process, the block would be open for inspection. Barring severe wear, and obviously worn parts, it will run. And when it goes, no crying. Just slap in another. I have heard several opinions both ways. I am not swayed at all that the blocks and internals are different. I believe the cam profiles are unchanged as well. Every bolt on, yes. For safety and resistance to corrosive elements. And there may be a few manufacturers who made their own special engines. But overall the jist is GM ships Mercruiser blocks, and they outfit them as necessary.

Please tell me I am wrong.
Gracious

 

[Bondo]

I hear you. I only anticipate keeping my boat for another year or so. At $200 a whack, It would be like changing fuses to my pocket book. My buddy makes a great argument. Although when I would be in the changing seals process, the block would be open for inspection. Barring severe wear, and obviously worn parts, it will run. And when it goes, no crying. Just slap in another. I have heard several opinions both ways. I am not swayed at all that the blocks and internals are different. I believe the cam profiles are unchanged as well. Every bolt on, yes. For safety and resistance to corrosive elements. And there may be a few manufacturers who made their own special engines. But overall the jist is GM ships Mercruiser blocks, and they outfit them as necessary.

Please tell me I am wrong.
Gracious

Nope,.... Yer not wrong,.... Truck motors, 'n marine motors are the Same basic long blocks,....

 

[proshadetree]

My 5.0 is fresh water cooled. Same everything except intake, dist, carb, and exhaust. It had 150k on it when I put it in

 

[proshadetree]

If you go the used route check the compression and oil pressure. Mine was 170 or better on all with 80 psi oil pressure while cranking.

 

[thegoat86]

Figured out what was wrong in another thread I started here. so no need. "91 Larson timing issue" Silvertip pointed me in the right direction although he never chimed back in. Gratis silvertip.

Opting to stay with my motor. But it is nice to know I can swap blocks without major out of pocket expense. Let someone else pay for the mercruiser long block LOL.

Newer 4.3 blocks have an internal balancer. and they get loose, and annoy the **** out of you trying to find the culprit. Then when you do find it, there isn't much you want to do as it will not affect performance or hurt the motor. Just a loud knock that never goes away......

 

[thegoat86]

And on a side note, I regret not starting this thread months ago. I almost made a deal on a 32' cruiser boat that needed two motors that the owner said one threw a rod and the other had water intrusion because he left the motor hatch off and the breather off. Then it rained before he got back to it three weeks later. Had dual prop Volvo penta drives, and all the luxuries, onboard gen, a/c, head, ect. But when I started pricing the motors it scared me away. as I was looking at 6K for two remanned 5.7 mercruisers. If that deal were still on the table today, I would totally do it. Used blocks all over craigslist, I am kinda handy with a wrench. And if I got stuck this forum is gold.

 

[juhasev]

5.0 MPI V8 has a different cam which is optimized for Marine use.... Therefore if you just put in a truck motor the Mercruiser computer will not run it well. Also the Mercuiser heads are supposed to have steel reinforced valve seats, where some truck motors won't (which apparently caused my heads to crack).

 

[thegoat86]

5.0 MPI V8 has a different cam which is optimized for Marine use.... Therefore if you just put in a truck motor the Mercruiser computer will not run it well. Also the Mercuiser heads are supposed to have steel reinforced valve seats, where some truck motors won't (which apparently caused my heads to crack).

No such thing as optimized for marine use. Its just a glorified RV cam. Creating the most torque at a low to mid level RPM range. Unless the cam is stainless so it wont rust in seawater, this is a myth. Steel reinforced valve seats? Did they start making plastic heads already?? Its called hardened valve seats. Most truck motors come with these anyways as they are rumored to work better with unleaded fuels. The main reason for these seats is the potential for a well used motor to wear the valve guides quickly. Kinda keeps the valves from walking all over the sides of the seat. But for no other reason. Some builders still run the softer seats and just install brass guides. And for the computer issue, yes if using a stock motor without the performance cam, you will need to have the computer programmed for a leaner fuel curve. But that is true with any motor combo out there when changing, tuning, or upgrading parts.

As for your particular cracked head issue, unless factories start using better more expensive raw materials to cast parts, and use a better quality check system, it is impossible to send out flawless parts. Your head could have been slightly warped or cast a fraction off. Or could have had small hairline fractures. And based on use and such could have gotten worse. Or for any slew of reasons. (ford motor)

"Depending on their locations, crack severity will vary. They tend to form, spread and get worse as heat, thermal stress, heavy loads, repeated bending and flexing, metal fatigue, pounding and vibration take their toll on a part. Cracking is an indication that an area is experiencing more stress than it can handle." (excerpt from hot rod magazine)

So in summary, barring computer programming, seals, and cam profile, no change in my original statement. You can put any long block or junkyard motor or any of the same make model in your boat and run it without issue. Using all of your marine bolt ons.