1990 vs 1992 4.3 block swap

[Chris51280]

Will changing my flex plate throw my balance off?

 

[achris]

Don't throw away the flex plate. They crack more often than you might think and folks need replacements. You may get a few bucks for it. Well,,, maybe not for a V6, but what do I know ;-)

Flex plate for 86 to 94 will fit 4.3 V6 and 305 and 350 V8.... Same plate...

 

[achris]

Will changing my flex plate throw my balance off?

You have no option... You can't use a flex plate on a Mercruiser, you have to use the flywheel. As I mentioned earlier, take all the rotating bits to a machine shop and have them balanced as an assembly. I would be doing that anyway.. You want a smooth running engine or not?

Chris......

 

[Chris51280]

So i would have to assemble the engine and then test it? How do you balance rods and pistons? I thought gm were hot testing the engines and then balanced them with drilling the harmonic balancer.

 

[achris]

Forget what GM do (or don't do)... You take the crank, flywheel, pistons and rods, harmonic balancer to the machine shop. What they do is balance the pistons by checking their mass, then make them all the same as the lightest. The rods are done by checking the masses of each end, and the total masses, and all made the same. The crank is dynamically balanced, then the flywheel is dynamically balanced, then the balancer.... Together they all are balanced. The counter-weighs on the crank aren't part of it, as they are part of the design stage.

When the machine shop calls you up and says it's all done, THEN you assemble the engine....

Chris.......

 

[DouglasW]

There you go.....

 

[Scott Danforth]

So i would have to assemble the engine and then test it? How do you balance rods and pistons? I thought gm were hot testing the engines and then balanced them with drilling the harmonic balancer.

To add to what Chris wrote

to properly balance a motor:
you weigh each piston, should all be within 1-2 grams each
you weigh each rod end, should all be within 1-2 grams each
you then spin-balance the crank with bob-weights added and you add or remove metal from the crank to get the amount of "balance" that you need. a performance motor that will be turning serious RPM's will be zero balanced or under-balanced
you assemble the engine, install the engine and then run the engines

GM builds their motors to a spec. pistons weigh +/- about 4 grams, Rods +/- about 20 grams and the cranks +/- about 50 grams.... because its "good enough" for 99.99% of the motors as they only spin to about 5k

GM, nor any builder ever "hot-tests" a motor and does anything after the fact. it just doesnt work that way.

 

[Chris51280]

I have notices that I have the light crank. Is this a problem?

 

[Chris51280]

The engine was already rebuild. It had a plate it had a plate
on that it was certified. So it wasn't a backyard rebuild. I assume that it was properly done. I can weigh all the components

 

[Chris51280]

Will the light crank be too light for the boat application? Not sure if I need the extra mass for the low rpm

 

[Scott Danforth]

unbolt flex plate, throw away. get manual transmission flywheel and bolt on. they are spec'd by GM to bolt on the flex plate and the flywheel have the same balancing. just get proper one for your generation of motor. that would be a 1989-1992 non-balance shaft motor.

since your motor was already running, all youre doing is swapping out the automatic flex plate for a flywheel.

 

[Chris51280]

Yup. Got that one from the mercruiser. Wasn't sure if,I can just replace the auto flex plate with the mercruiser one. From what I have read, they are balanced by themselfs.

 

[Scott Danforth]

you could have saved money and bought a flywheel from any auto parts store for a 1989-1992 GM vehicle with a 4.3

 

[Chris51280]

The flywheel is from the mercruiser. The stripped block will be my new one. That one had the auto flex plate

 

[Scott Danforth]

the flywheel is from one of GM's suppliers, sold to GM, sold to Mercruiser who in turn sold it back to you. its a $79 flywheel for a manual transmission application.

 

[Chris51280]

So I just got my camshaft out from the old mercruiser. Found two lobes damaged. Engine was still running good. would you say this one is toast? Look for another one with fuel pump lobe

 

[Chris51280]

I found a used one with the lobes to be still good. $50 with 15 shipping. Hard to find the older style camshaft with the mechanical fiel pump. And i don't want to pay at least 300-400 for a new one.

 

[achris]

I found a used one with the lobes to be still good. $50 with 15 shipping. Hard to find the older style camshaft with the mechanical fiel pump. And i don't want to pay at least 300-400 for a new one.

Make sure you know what the engine that that camshaft came out of was used for.. Needs to be a truck or RV... 'Normal' car is no good.

 

[Chris51280]

It actually came from a 1988 4.3 mercruiser.

Chris, can I swap dampers from one engine to the other? It apears the mercruiser one is a bit heavier. Dont think it matters.

Whats your take on the crankshaft. Does it matter if its a light crank?
I'll take it that I can't use the old mercruiser crank?! It would be original with standard bearings. I would buy new bearings but not sure if it needs to stay together.

Both engines came out of Tonawanda if that matters

 

[achris]

The only things that need to be 'matched' are the pistons to the bores they were in, lifters to the cam lobe they were running on and rockers to the valves they were opening (unless you have the rockers 'faced', then put them anywhere you like). Everything else should be ok to swap around. If you prefer the Merc crank in the new block, as long as you check the bearing journal sizes and are putting new bearings in, that should be fine. Use the flywheel in place of the flex-plate, and the balancer that matches the crank (Merc crank, Merc balancer.) Remember, you're building up an an engine that will not turn any faster than 5,000rpm and produce no more than about 210hp. It's not stressing anything, you have lots of tolerance... If it was going to make 400hp and spin at 7,000rpm, different story.... That said, you can't be 'sloppy', do it all right and the engine will last longer than you.

That's my take. Others, like Scott and friends, have a little more experience building up engines, but I'm sure you'll be fine to do as above.

Chris.......