Replacing Mercruiser 4.3L Motor

maelstrom · Nov 2, 2011

I would like to be as detailed as possible when I post this so as to get a detailed answer from someone that may know the answers to my questions.

I am replacing my Mercruiser 4.3L "Vortec" motor from a very bad winter freezing from last year. Unfortunately I was not able to winterize the boat prior to my deployment... my bad, I had other things on my mind at the time.

Anyway, the winter freeze killed both heads, and manifolds,.. which I replaced, but still had a.. forget the pun.. boatload of water in the oil system, so I suspect a crack in the block, as any mechanic would.

I went out to the local Pick and Pull, yanked a "4.3L Vortec" from a GMC and tore it down to the block. The thing that puzzles me is... inside the valley, under the intake there is some kind of rotating shaft that is not on the block that I pulled from the truck.

My question is.. What is this mysterious shaft that is geared into the cam cog? Do I really have to have this in the block if I am replacing the block with one that does not have this?

Any assistance with this would be most appreciated,... Thanks in advance

thumpar · Nov 2, 2011

Re: Replacing Mercruiser 4.3L Motor

That is the balance shaft. I am not sure what year they started using them but must have been early 90's.

Bondo · Nov 2, 2011

Re: Replacing Mercruiser 4.3L Motor

Ayuh,... Ya picked, 'n pulled a later vintage motor,...

I'm sure you'll run into other Differences as well...
It's also got the Vortec heads...

maelstrom · Nov 2, 2011

Re: Replacing Mercruiser 4.3L Motor

Is there any dangers or major complications with installing the motor that I picked?

What is the balance shaft used for in the first place?

joewithaboat · Nov 2, 2011

Re: Replacing Mercruiser 4.3L Motor

The balance shaft started in 1992 (in cars & trucks) and is an attempt to counter act vibrations given off by a 90 deg v-6. It shouldn't be a problem to install in your boat, as long as you have some needed components from the newer engine; timing cover i think is different. I'm not sure, but head gaskets you will have on that new motor may need to be replaced. You should replace the core/freeze plugs on all the water jacketed areas with a brass kit. The rest will depend on the year of your old engine. You will need to buy a new "Vortec" intake manifold and maybe a carburetor.

This link might help explain some year to year differences.
http://www.hotsixes.com/index.php?option=com_content&view=article&id=14&Itemid=12
Good luck

thumpar · Nov 2, 2011

Re: Replacing Mercruiser 4.3L Motor

The intake may be the biggest part you have to change. The carb should work if you get an intake that is compatible. Another thing to add to the list is that you may have to plug up where some sensor where in the block. I know some had a crank sensor but it really depends on how late of a model you got.

maelstrom · Nov 2, 2011

Re: Replacing Mercruiser 4.3L Motor

Thanks brotha, that link is great. I know now that I do have the correct block for my needs, even though it does not have the "extra balancer" in it. I will be ordering the brass plugs and install them on this block. I have checked the bore and stroke and are within tolerance as well as the main bearings etc.. It really appears as though this motor was rebuilt recently before they scrapped the truck. I have all of the accessories from the original motor that will bolt right to this block, as well as the new heads/gaskets etc. The true test will be when I pull the original motor out after I get my boat back from the shop (getting lower unit repaired), which I am not worried about time because here in Tx, we haven't gotten much rain for the lakes, and it will be turning too cold to enjoy.

The intake manifold will fit the block because it is the "12 bolt" style for both, not the "8 bolt" I made darn sure of that when I went hunting for the donor block.

I appreciate everyone's assistance on this issue, I hope to have this puppy back on the water by springtime for my tests!

thumpar · Nov 2, 2011

Re: Replacing Mercruiser 4.3L Motor

You may still have intake issues. 1) most of the later models had fuel injection so your carb might not work. 2) it may not have clearance for the shaft but if it doesn't it might be possible to rework it. Good luck with the project.

joewithaboat · Nov 2, 2011

Re: Replacing Mercruiser 4.3L Motor

I thought the picture in your o/p was your motor... That's why folks were talking about you having vortec heads and needing a new intake. At any rate good luck with your swap.

maelstrom · Nov 2, 2011

Re: Replacing Mercruiser 4.3L Motor

Ah, to clarify in my o/p, the motor with the balancer is my original, and my replacement block is the one without. I have been reading much about interchangeability for these blocks and I must be one lucky dude, cause I have the correct replacement heads/ valve train to get this done with minimal problems or machining. The only parts that I am needing to replace will be the timing chain/gearset, oil pump ( cause I just feel more comfortable with a new one), and a new oil filter. Everything else will swap over from the original to my new one.

Mechanically I am set, but I hope that I have enough skills to set the timing correctly.

Thanks for the help everyone!

Struc · Nov 3, 2011

Re: Replacing Mercruiser 4.3L Motor

Yes, but...

maelstrom said:
The intake manifold will fit the block because it is the "12 bolt" style for both, not the "8 bolt" I made darn sure of that when I went hunting for the donor block.

You clearly have Vortec heads with a 8-bolt intake manifold according to the first picture. The block you picked up would not have had Vortec heads, unless they were added at some point, and as such, would have a 12-bolt intake manifold.

Thus, everyone's confusion. Maybe you could post a picture of the heads that came on the truck engine you bought, or at least explain what's going on there.

As I recently found out, as detailed in my engine replacement thread HERE, you can get an engine that says Vortec on the valve covers, but the heads are not actually Vortec.

joewithaboat · Nov 3, 2011

Re: Replacing Mercruiser 4.3L Motor

I'm thinking those pictures are ones just grabbed off the net to use as example's ??

maelstrom · Nov 3, 2011

Re: Replacing Mercruiser 4.3L Motor

Ah, my bad on that. The pics were just meant to show the "balancing shaft". My boat is still in the repair bay getting the lower unit repaired so I had to find pics on the net to serve my question.

After much reading, and staying up way past my usuall slumber, I found out that the 12 bolt style heads are different as well as the pushrods that went with the several different types of head/valve train assy's.

There is much that I did not know about this particular engine. I am getting my education on thanks to the much needed links shown above.

As stated perfectly above, "you can get an engine that says Vortec on the valve covers, but the heads are not actually Vortec."

I have valve covers that say Vortec, and now I am certain that the motor is not, but the valve covers are. I suppose someone other than me has tinkered with this boat.

Bondo · Nov 3, 2011

Re: Replacing Mercruiser 4.3L Motor

maelstrom said:
Ah, my bad on that. The pics were just meant to show the "balancing shaft". My boat is still in the repair bay getting the lower unit repaired so I had to find pics on the net to serve my question.

After much reading, and staying up way past my usuall slumber, I found out that the 12 bolt style heads are different as well as the pushrods that went with the several different types of head/valve train assy's.

There is much that I did not know about this particular engine. I am getting my education on thanks to the much needed links shown above.

As stated perfectly above, "you can get an engine that says Vortec on the valve covers, but the heads are not actually Vortec."

I have valve covers that say Vortec, and now I am certain that the motor is not, but the valve covers are. I suppose someone other than me has tinkered with this boat.

Naw,... not necessarily,...
The name Vortec was used , as a motor name, Before the fast-burn head came into being, yet since '96,...
The name Vortec only really applies to the Vortec Heads, 'n related intakes....

achris · Nov 3, 2011

Re: Replacing Mercruiser 4.3L Motor

Here's something else to consider. When Merc used the non-balance shaft engines they used fluid engine mounts, not rubber. In a boat with an engine bolted hard to the floor even with high density rubber mounts, the vibrations created by a 90 degree V6 are just too much... Look into the fluid mounts if you are thinking of using the non-balance block.... It might be easier to find the right block, ie WITH a balance shaft....

Chris......

maelstrom · Nov 3, 2011

Re: Replacing Mercruiser 4.3L Motor

haven't thought of that... I will take that into concideration,.. thanks for the tip!

Replacing Mercruiser 4.3L Motor

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