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Balance Question


Well-Known Member
Local time
5:49 AM
Jun 17, 2021
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Mid Michigan
Had my motor built by a shop (Petit Racing, in Mich) that my good friend recommended, they do all his engine work. My question is balancing: the motor is a 400 (external balance), the rotating assy was balanced during the build, do I still need to run an ext-bal flex-plate (the one with a scallop cut)?
I have a ext-bal flex-plate, but I'm concerned if I install it and don't need it, will that make the motor want to jump out of the car, damage the trans, crack the flex-plate, all of the above?
If they balanced it internally (removed/added weight to the rotating assembly), then you should be using a neutral damper and flexplate, spec'd for internally-balanced engines. Best bet is to ask the builder...
If you have an original external balance flywheel from that engine and the crank was not changed out, it would have to be a cast crank. The amount of Mallory metal needed to zero balance the crank would be VERY expensive. I doubt it was zero balanced, because the builder would have also needed a harmonic balancer as well as the flywheel, I believe.
Sounds like you will need to contact the builder for details.
You can call the shop. They can tell you if you need external or internal accessories.
There is a flex plate for use with unweighted converter
Or you can add metal to your flex plate as I had done 45 years ago
Thanks guys, I'll call the shop and ask which flex-plate to use. That's the only thing that didn't come with the motor when I picked it up, hence the question.
Your shop can not do a proper balance job on a motor
Iike that without the balancer and converter in question IN HAND, not just a flexplate. Therefore, this is a question best answered by them, not us. Good luck, they will know.
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