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harmonic balancer and flywheel question for 5.7, whats the differance with the 2 size balancers

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potatoe

New member
hello. i have been speaking with many mechanics, and im getting mixed answers. i was wondering if anyone can give me some solid information. i have a rebuilt gm 5.7 longblock that i purchased to replace with my gm 5.7 engine. both of these blocks have the same # gm 3970010 labled on them. the problem is, the old engine that im swaping all parts over from to new block has a 8 inch harmonic balancer. but the new longblock came with a timing tab for 6 3/4 balancer. is it ok to swap the tab out with my 8 inch timing tab, so all my parts will go on? or do i need to use a 6 3/4 balancer? and the other question is, if i change balancer, do i need to change flywheel? because i also have another parts motor, 1985 mercruiser 5.7 engine with 6 3/4 balancer with smaller flywheel, and the gm engine with 8 inch balancer has a larger flywheel. i know im asking many questions, but, what i want to know is does it matter? can the flywheels and balancers be mixed around? will my rebuilt longblock accept either size balancer without doing damage? some people are telling me it dont matter. some say it does. any info would be much appreciated. again, both my old gm motor and the new gm longblock have the same numbers on them. thanks my boat is a 1985 formula 25pc with mercruiser (GM) 5.7. alpha one
 
and i think if i use smaller flywheel, then i would need a different starter. but not sure. im hoping someone else can explain what the differance is, or if it even matters. thanks
 
Perhaps kghost, ricardomarine and some of the other seasoned posters on here can help and may have a different insight on this than you got on the other forum when the same question was asked. See my last post on the other forum and then prhaps you can make an "informed" decision on which way to go..
 
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If I was doing this, I'd use the larger Harmonic Balancer and timing chain cover for it. These are designed to reduce Harmonic vibrations, and our Marine engines may be more prone to this.
Harmonic vibrations are vibrations of unequal frequencies.

Since this is a used balancer, you may want to consider doing a PPS procedure. (positive piston stop)
With this procedure, you'll be able to verify TDC. ....... of which is very important for Marine Ignition timing.


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is the timing cover different too? because theres a tab on cover that i can replace with 2 bolts so i wont have to replace entire cover. i have tabs for both. thanks a lot for the info.
 
potatoe said:
is the timing cover different too? because theres a tab on cover that i can replace with 2 bolts so i wont have to replace entire cover. i have tabs for both. thanks a lot for the info.
Click to expand...
Yes, the two balancer diameters require a corresponding timing chain cover.

Yes, you sure can use the bolt-on tabs......... however, this becomes all the more reason for doing a PPS procedure, IMO.

Do a search for Positive Piston Stop procedure.
I've posted on this topic several times here.

With your cylinder heads in place, you'll be using this style PPS.



For better accuracy, use a 25* to 30* split... not 10* or 15*.



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kimcrwbr1 said:
  1. Is the piston stop the same as using a dial indicator where you measure the distance the same on both sides of the stroke and mark the balancer then split the difference for true TDC?

  2. I use a dial indicator frequently for outdoards I can see going further down on the stroke would be more accurate.
Click to expand...

  1. Basically yes!
    The indicator can be used while the cylinder head is removed, whereas the PPS is used while the head is in place.

  2. Yep..... this is why I use a split of 30*ish, as apposed to 10* or 15*.
 
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