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Discussion Starter #1
I would like to know how critical it is to have the rotative assembly re-balanced when you change pistons. My BBC is currently balanced for 10-1 pistons, and I'm wanting to go to 12.5-1 pistons, which will weigh more...

comments?

PS: yes I know that you should have it re-blanced, I'm just wondering how critical that change would be and if I can get away wit it.

PS2: will be shifting it around 6500rpms, but it spends most of its life bellow 4k...
 

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If the change is more than 2-3 grams, then absolutely get it rebalanced.
If you can get the pistons to weigh within a gram or two of the old ones, then I forsee no problems.
Plus, if you are already pulling the motor apart, then why not. You have to spend a little more now, so that you don't have to spend a bunch more later on.
 

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I'm wondering why put 12.5 pistons in a street motor on pump gas. you are going to have to run 97+ octane to keep knock in check unless you water or alky inject.

If you are tearing down to change pistons, re-balance. If the piston weights are the same, its OK but I doubt that is the case.
 

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In the post, he referenced a BBC. So I assume it is a race motor. Or at least one that is run mostly on the strip.
 

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If you were switching to a piston that was slightly lighter than the original ones then you could get by because you are creating an overbalance, but switching to a heavier piston will create an underbalance which will vibrate. The factory balances motors with a over-balance because they will not vibrate until you make a significant difference in weight. An underbalance is when the counterweights are balanced lighter than the piston/rod assy. Because you said that this motor was balanced for your 10:1 pistons they most likely did it to "0" and any weight you add or subtract will push the balance one way or the other.

Cliffs: Get the pistons weighed and if they are not withen a gram or two get it balanced. Should only be $250-350
 

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Discussion Starter #6
the reason I asked was because I have a very short time to switich pistons over x-mas and my builder is on the other coast of FL of where I'll be.

12.5 on the street is fine on 93 as long as you got a big enough cam. I've had couple friends running 12.3 and 12.7-1 on LT1 stroker with 93, ran fine

I'm wanting to go more cr because after first I started with a 244 dur cam at 50, which peaked at 5400 rpms, then went to a 254 that peaked at 5900 and now I'm running a 269, which I'm hoping will peak at about 6300. Of cours I also went to a different grind (the first 2 where compcams, magnum, then highe energy, now running a Lunati billet).

Anyway, 10-1 on that cam is limit. My buddy is working at Mahle's so he is helping me out with that, probably will end up with 12.5-1 pistons and do some smothing out of the domes to avoid detonation and bring it down to 12-1. Anyway, you all are confirming what I already knew, but I wasn't sure the tolerance on balancing, which you guys answered that, so thanks! :)

ps: it's a street car...doesn't go on the strip, cuz it breaks when I take it there lol -she don' t like le strip lol-
 

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Brazen6.0 said:
ps: it's a street car...doesn't go on the strip, cuz it breaks when I take it there lol -she don' t like le strip lol-
That suchs....
My last project car would always break a day or two before I could take it to the strip. I think it knew abuse was coming...
 

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aslong a 6,500 isn't hit all the time you should be fine. this being true if they are within a few grams
 

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If I remember correctly, for each 1,000 rpm increase the stress factor on all the reciprocating parts SQUARES. As stated above, a slight difference is tolerable, but your piston provider and your machinist should have some nominal weights for you to compare. That much difference in piston dome volume would probably require rebalance unless you are altering the volume by moving the ring land up on the new pistins and decreasing combustion chamber voume by milling the heads or reducing head gasket thickness.Safest bet is to rebalance for the piston/rod combo installed and then you won't have to second guess as to whether or not you made the right decision
 

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Discussion Starter #10
yea, my thoughts exactly, I'm not taking a shortcut, I was mostly wondering the slag within the blancing tolerance. That said, I won't have time to mess with that over x-mas break...puting a 327sbc in my gf's 69 camaro, so that will take most of the time...
 
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