Who is right when deciding to rebalance the crank?

Currently I am deciding if I need to rebalance my crank! I bought new KB pistons to replace the KB pistons I put in a year ago.

The old pistons/rings/wrist pin weigh ~ 746 grams
The new pistons/rings/wrist pin weigh ~ 733 grams

This makes the new pistons 13 grams lighter than the old ones!

An engine builder told me that the crank would need balanced if they differed by more than 7 or 8 grams!

A rep at KB said I would be ok if they were no more than 10 grams different!

And I believe the GM spec states that they can be up to 10 percent different or somewhere around 40 or 50 grams.


So who is right here? And is 13 gram worth the extra $200 to rebalance?

Well.... you will be farther off perfect for the balance.. you can run it that way since a stock balance is not as good as a aftermarket... basically how well do you want the motor done?

Bret

You can run that, it will not be perfect, but it will be like a factory balance job...because you will be placing the rotating assy into an "overbalance" which will not vibrate unlike an "underbalance"
Did you weigh the old stuff and new parts yourself, or did you get that weight off the balance card.... throw in a 4g oil allowance, that may help or hurt your numbers depending on what was measured when...
Like Bret said, street, strip? How perfect do you want to be?

You can run that, it will not be perfect, but it will be like a factory balance job...because you will be placing the rotating assy into an "overbalance" which will not vibrate unlike an "underbalance"
Did you weigh the old stuff and new parts yourself, or did you get that weight off the balance card.... throw in a 4g oil allowance, that may help or hurt your numbers depending on what was measured when...
Like Bret said, street, strip? How perfect do you want to be?

I took all the weights myself, and the scale I was using was accurate up to +/- .5 grams. It did seem that the motor ran smoother once I balanced last time than it ever did from the factory, and I definatly liked that. I guess I'm to the point where I don't know how much more money I can bare to dump into it. This project is coming close to 6 G's with all the work I did last year and the re-do this year. Although I would like to run the car at the track from time to time, it's mostly just gonna be a street machine.

With that being said... What would you guys do? I mean what another $200, right?. I just hope it runs like I think it should.

Like you said...in a 6K project, $200 is not that much coin to have it done right. ;)

Talk to the guy that did your last balance, he might let you off a lot cheaper because the only difference is the lighter pistons. The rods and such have already been balanced, we talking about establishing the weight of one piston rod combo, not the whole assembly.

Just curious, what happened that necessitated new pistons?

Also, to what does the oil allowance get added to or subtracted from? There's oil on both the crank and the rods. :confused:

Add the 4g as if the piston was heavier...reciprocating