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Balancing motor questions

 
Old 12-19-2018, 06:28 AM
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Default Balancing motor questions

I was going to balance my pistons to within a tenth of a gram but my engine builder says its pointless. He also says most aftermarket rods will be spot on good enough. I am going to run a LS2 crank from a motor that broke a piston from dropping a valve. The crank looks good but I was going to have him check it for trueness. Should the crank need to get balanced?
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Old 12-19-2018, 07:57 AM
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If you’re not using stock LS2 rods and pistons, then it most likely needs to be balanced.
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Old 12-19-2018, 10:02 AM
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I’d find another builder. I’ve NEVER seen a set of rods that didn’t have have something done to them. I just picked up another 6liter build from my machinist Monday, and the brand new h-beam rods I dropped off have been considerably massaged.
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Old 12-19-2018, 10:15 AM
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Even the “pre-balanced” forged Callie’s/Wiseco assembly I bought for my 434” wasn’t right. I had them check it, (of course I did) and it was off.
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Old 12-19-2018, 11:27 AM
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Sounds like "Trust, but Verify" where you figure out just who to trust after enough verification....
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Old 12-19-2018, 03:09 PM
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So all the engines that come stock from gm, all the rods weigh exactly the same? so like in my ls1, are all my rods weight matched within a half a gram? should i pull my engine apart and weight match them?
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Old 12-19-2018, 04:10 PM
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Originally Posted by Kfxguy View Post
So all the engines that come stock from gm, all the rods weigh exactly the same? so like in my ls1, are all my rods weight matched within a half a gram? should i pull my engine apart and weight match them?
I wouldnít bother. Once oil starts to accumulate on the internal parts, all that work to get them all within a gram was basically for nothing.
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Old 12-19-2018, 04:41 PM
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Originally Posted by KCS View Post


I wouldnít bother. Once oil starts to accumulate on the internal parts, all that work to get them all within a gram was basically for nothing.

and this was exactly my point after an above poster said he should find another builder. If the rod weight is off by a little bit, that wonít amount to a hill of bean. Reciprocating weight doesnít notice it near as much as rotating weight. As long as the rod weights and piston weights are close, it donít matter. I agree with the original posters machinist.
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Old 12-19-2018, 04:45 PM
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Originally Posted by Kfxguy View Post



and this was exactly my point after an above poster said he should find another builder. If the rod weight is off by a little bit, that wonít amount to a hill of bean. Reciprocating weight doesnít notice it near as much as rotating weight. As long as the rod weights and piston weights are close, it donít matter. I agree with the original posters machinist.
You can use a machinist that says ďthatís close enoughĒ if youíd like. Itís your money, and your engine.
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Old 12-19-2018, 05:02 PM
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Originally Posted by Che70velle View Post


You can use a machinist that says ďthatís close enoughĒ if youíd like. Itís your money, and your engine.
when it makes zero difference then why worry about it. Thereís many many fast cars around here that didnít worry with that. And most of the fast factory stock cars included. But if you like throwing money away for fake peace of mind, thatís your choice, but why tell someone find another machinist when truly his machinist is correct? Makes zero sense to me.
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Old 12-19-2018, 06:53 PM
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So for my own personal learning and to help the OP, what is considered an acceptable +/- on the balance? At what point is it that itís too far off and parts needs to be shaved?
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Old 12-19-2018, 09:52 PM
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Originally Posted by Kfxguy View Post

when it makes zero difference then why worry about it. Thereís many many fast cars around here that didnít worry with that. And most of the fast factory stock cars included. But if you like throwing money away for fake peace of mind, thatís your choice, but why tell someone find another machinist when truly his machinist is correct? Makes zero sense to me.
LOL...ok.
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Old 12-20-2018, 08:47 AM
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Originally Posted by s30.hybrid View Post
So for my own personal learning and to help the OP, what is considered an acceptable +/- on the balance? At what point is it that itís too far off and parts needs to be shaved?
There isnít really an answer for that. The Hines digital balancer I used to use would stop once you got within 2 grams, however, not all machinists calculate the bobweight the same way.

FWIW, years ago I balanced a Ford mod motor crank without all of the rotating assembly compnents in hand. I was given the weight of the rod bearings by the owner and used that to calculate the bobweight. The engine was built, installed, and never had a problem. Months later, we had another one and I decided to check the weight of those bearings. Turns out they were heaver and that first crank was balanced almost 40 grams under. The customer never noticed and the engine never gave him any problems.
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Old 12-20-2018, 01:48 PM
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Originally Posted by KCS View Post


There isnít really an answer for that. The Hines digital balancer I used to use would stop once you got within 2 grams, however, not all machinists calculate the bobweight the same way.

FWIW, years ago I balanced a Ford mod motor crank without all of the rotating assembly compnents in hand. I was given the weight of the rod bearings by the owner and used that to calculate the bobweight. The engine was built, installed, and never had a problem. Months later, we had another one and I decided to check the weight of those bearings. Turns out they were heaver and that first crank was balanced almost 40 grams under. The customer never noticed and the engine never gave him any problems.
What is actually done to balance an oem crank when doing a forged rod/piston build? Assuming the forged rods and pistons are heavier than oem does that mean you will have to add weight to the crank?
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Old 12-20-2018, 02:37 PM
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Originally Posted by BCNUL8R View Post


What is actually done to balance an oem crank when doing a forged rod/piston build? Assuming the forged rods and pistons are heavier than oem does that mean you will have to add weight to the crank?
Yeah, usually. It depends how much heavier the bobweight is. If itís a lot, you have to add heavy metal. If its not a lot, you can sometimes get away with grinding on the crank 180 degrees opposite of where on the counterweight the weight needs to be added.
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Old 12-21-2018, 07:43 AM
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Thanks for all the great info. I may just balance my pistons myself just to get closer since it hardly takes any time and zero $$$. Good to hear of everyone's actual experiences. Its crazy to hear how different opinions this topic can have.
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Old 12-21-2018, 08:17 AM
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Originally Posted by KCS View Post


Yeah, usually. It depends how much heavier the bobweight is. If itís a lot, you have to add heavy metal. If its not a lot, you can sometimes get away with grinding on the crank 180 degrees opposite of where on the counterweight the weight needs to be added.
With that in mind, would there be any benefit in using a gen 4 6.0 crank when doing an iron LM7 347 forged rods and pistons build? Since it was balanced to heavier rods/pistons it may reduce the chance of needing to add weight to the crank when using forged rods/pistons? Have you ever checked an oem ls rotating assembly to see how well they are balanced?
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Old 12-22-2018, 08:51 AM
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Originally Posted by TurboTim View Post
I was going to balance my pistons to within a tenth of a gram but my engine builder says its pointless. He also says most aftermarket rods will be spot on good enough. I am going to run a LS2 crank from a motor that broke a piston from dropping a valve. The crank looks good but I was going to have him check it for trueness. Should the crank need to get balanced?
What does he charge to "spin" the crankshaft with your MATCHED items? If it's say $50 to $75, that could get more information to help decide how you want to proceed.

FWIW - My engine builds are money pits

I'm OK with ~ $250 to precision balance to make sure it's right. Long ago had a Callies LS stroker crank "within a couple of grams" for my rods & pistons per the shop selling it. Only minor tweaks would be needed at most. My SAM friend building the engine double checked. The crank needed a lot of heavy metal removed. The rough crank balance was for a heavy 900 hp nitrous piston, not the much lighter trick Wiseco pistons we were using. The aftermarket rods & pistons were very close out of the box with minor tweaks needed. After that experience, I always prefer having it checked.

​​​​I doubt a 1/10 of a gram balance vs 2 gram balance would critical in typical applications. My guess is in endurance racing or road racing or very high rpm, that 1/10 of a gram level of precision balance seems more likely to become critical.

Last edited by 99 Black Bird T/A; 12-22-2018 at 09:00 AM.
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Old 12-25-2018, 01:34 PM
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Originally Posted by KCS View Post


I wouldnít bother. Once oil starts to accumulate on the internal parts, all that work to get them all within a gram was basically for nothing.
This is the correct answer.

FYI, the typical "oil allowance" when determining the bob weight to balance the crank with is 8 grams. How close do you need each component to be?

Match a heavy rod with a light piston, number everything, and be done with it. As long as all the bob weights are the same in the end, it doesn't matter.
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Old 12-25-2018, 01:36 PM
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I used a stock 6.2 crank, stock gen IV rods and SRP forged 4032 4.070" pistons in my engine. I ended up having to drill the crank a little because the bob weights were slightly LIGHTER than stock.
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