Flexplate Question
 

I trashed my current flexplate this past Monday, so ordered a new one.

Question is, with externally balanced flexplates, I need to bring the old one and new one to the machine shop to be matched?

I wouldn't think so, as long as they're the same imbalance weight.

Matt, Any 86 and later OPRM 153 tooth flexplate should work as long as the weight hasn't been removed.

So any external balanced flexpate comes with the same weight....?

Probably not but id just pick up a new one of the one u trashed

It's my understanding all one piece rear mains are balanced the same. 168 tooth with the larger bolt pattern or the 153 tooth with the smaller pattern have the same weight. They will only bolt on to a one piece crank.

So how does it take into account the crank imbalance...?

When the crank is balanced, they remove/add weight at the crank, not the external counterweight on the flexplate. The external weight should be the same, within a few grams based on manufacturing tolerances, which won't matter. As long as your new flexplate imbalance weight is the same as the old one, you should be good.

I was under the impression that an internal balanced crank required bob weights to be added, which we did not do and it was left external, which means any discrepancies in balance were taken up by the flexplate as it was cheaper to balance.

So, doesn't that mean my flexplate needs to be adjusted for what the engine shop balanced my rotating assembly too?

Yes

If they didn't drill the crank to lighten or add heavy metal then yes you will need to have the new flex plate matched to the existing one. Call the machine shop tomorrow and ask them. It should be very a simple job. I had my set up balanced so I could switch out flywheels or convert to an auto flex plate and not worry about it.

It would be very odd if they modified the flexplate and not the crank; it's not a standard way to do it and ignores the imbalance issues in the middle of the crank.

You fix the imbalance with weight added/subtracted on flexplate.

heavy metal costs a lot of money so I went with external balancing, regretting daily.

Well, the actual 'correct' way is via modifying the counterweights on the crank. Hell, that's how every balance job I ever had done worked out.

If its internally balanced, yes.

Regardless, machine shop said any external balance flexplate is fine. So I am perplexed and just listening to them.