How much change in piston weight before rebalancing? - Team Camaro Tech
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  #1  
Old Mar 24th, 03, 04:56 PM
travis travis is offline
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The title pretty much says it all. If I wanted to change pistons, and the rotating assembly was already balanced with a particular piston weight, how far from this piston weight could I go before needing to rebalance everything again? The way I understand it, balancing isn't a exact figure...I guess they try and get the balance within a certain range? The pistons I am thinking of changing to are about 12-15 grams heavier than what my assembly is balanced for.
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  #2  
Old Mar 24th, 03, 05:05 PM
pdq67 pdq67 is offline
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Travis, I wouldn't worry about it the way my junk 301 used to run...

But please maybe BillK or somebody else will jump in here. pdq67
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  #3  
Old Mar 25th, 03, 01:43 AM
Oldani Motorsports Oldani Motorsports is offline
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pdq is correct. I have run some mismatched stuff just to throw a motor together at times here and there. I had an interesting tech article on GM engine balancing, I would have to really hunt to find it. The jist of the info was that the same exact crank (in the same balance configuration) being used in a number of piston/rod weight combos. I tried to overbalance my race stuff to about 52%, as engine builders I had spoken to claimed it gave smoother high-rpm operation. I can not tell you it helped, but I know it did not hurt. But, over 15g, I would not worry too much.
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Old Mar 25th, 03, 03:50 AM
Eric68 Eric68 is offline
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Travis, when I went from my heavy KB's to the lighter SRP's the crank balance was 22g out when we checked it with the new bob weight. I would rebalance, but that is just my opinion.
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Old Mar 25th, 03, 11:18 PM
68rs406 68rs406 is offline
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i'm with eric on this one. i would bet you could get by w/o rebalancing, but, imo, just weigh the cost vs possible outcome. 150$ typically to balance, and since the motor is out and apart, there really is no reason to "shadetree" it, imo. imagine what you will do if it has a nasty vibration once its all togethor and back in the car? [img]graemlins/angry.gif[/img] just my .02, good luck, at any rate
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  #6  
Old Mar 26th, 03, 01:45 PM
BillK BillK is offline
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Travis,
Normally, this would be an easy balance job but....since your new pistons are heavier than the old one, you have to add weight to the crankshaft, meaning "Heavy Metal" ... meaning expensive. A possibility would be to try and find some pins that are lighter, or depending on the pistons you are buying and the rods that you have...it might be possible to take 3-4 grams off the pistons, and 5-6 grams off the small end of the rod, and end up very close to your present weights. I would talk to your balance shop about it.
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Old Mar 26th, 03, 03:32 PM
K-Performance K-Performance is offline
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Travis,

There is no need to balance the entire assembly. Just bring your new pistons with one old piston to a machine shop and they can lighten the new ones to be the same weight as the old. Very often when you get a new set of TRW or similar quality brands they are not all of equal weight. Without you realizing it the machinist does this anyway. They will remove material from above the wrist pin bore. This is a easy cheap solution which will put you at ease.

ED
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Old Mar 28th, 03, 01:15 PM
BillK BillK is offline
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Kperf,
Have you actually ever balanced a piston ? It is virtually impossible to take 12 grams of weight out of a piston without seriously weakening it. Aluminum just does not weigh that much. On most TRW pistons, if you take .010 off the pin bosses it redusec the weight about 1 gm. In order to get 12 gms off, you would have to completely cut in to the piston pin Not happening.
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  #9  
Old Mar 29th, 03, 02:38 AM
Oldani Motorsports Oldani Motorsports is offline
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BillK is correct, you'd cringe if you saw the race pistons I use in my SBC stuff. Patterson Racing in Augusta, KS, is who did them, they are Bill Miller Engineering blanks that have the domes left unfinished, then they go to Patterson. They are milled everywhere, and I would not be too keen on running them in any endurance situation. But, they are right at the barely-under-400g area, and this is for a 1.78" compression height, and a hand-fitted massive dome. I agree that you may be able to get some on the rod small end, and a tiny bit on the piston. I still feel that 12 or 15g is pretty inconsequential though. You would not see any bearing issues, or even feel any vibration, at least IMO.
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  #10  
Old Mar 29th, 03, 04:35 AM
K-Performance K-Performance is offline
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BillK,

Yes, I have balanced pistons and crankshafts before. Removing 1-8 grams is a common amount to remove from a piston. Yes, 12 grams may be difficult, especially not knowing what the piston looks like. Usually if you can get the pistons within 3 grams from each other it is acceptable. You mentioned taking .010" off of the pin bosses, not sure where you are removing this from - sounds dangerous. With a special cutter in a Bridgeport and the piston supported upside down I remove material from between the pin boss and bottom side of dome. I wind up cutting out a square on each side of the piston.

ED
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  #11  
Old Mar 29th, 03, 04:27 PM
Ed Rempel Ed Rempel is offline

 
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Balancing is more like $350.00 here in Canada. That's about 235.00 real money.
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