crankshaft balance tolerance

[Belgian1979]

Hi, question: how much gr difference is allowed from one piston rod combo to another in terms of balance?

[Baprace]

Hi, question: how much gr difference is allowed from one piston rod combo to another in terms of balance?

Hi Belgian , this is just my opinion , I have a balance machine so that issue will not show up in my shop , I don't have an actual engine to relate any facts , but if the piston balance weight is just different by a 10 - 15 gram amount , I don't see an issue , just think about the total imbalance being off by 1 or 2 % , so you will be a little over balance or a little underbalance , remember on a Chevy V8 the formula is using 50% of the piston weight , so if you have pistons that are 18 grams heavy or light you are only changing the bob weight by 9 grams , as long as all of the weights are the same over or under weight I don't see an issue.

[Belgian1979]

Thanks Baprace. I was thinking something in that sense. I was looking at difference between rods, pistons and such. All seem within 1 gr. So that is good here.

[rebelyell]

Hi, question: how much gr difference is allowed from one piston rod combo to another in terms of balance?

how much gram difference (between assemblies) are you seeing now ?

[Belgian1979]

conrods (oliver) from 605,60 gr to 606,70 gr
piston wrist pin: 116,8 to 117,10 gr
pistons: 446,5 gr to 447,5 gr.
Oliver bolts (going to be exchanged) all weighed 34 gr. The new ones 33,8 gr.

Total assemblies (optimal combinations) vary from 1170,6 gr on the high side to 1170,00 gr on the low side.

[rebelyell]

conrods (oliver) from 605,60 gr to 606,70 gr
piston wrist pin: 116,8 to 117,10 gr
pistons: 446,5 gr to 447,5 gr.
Oliver bolts (going to be exchanged) all weighed 34 gr. The new ones 33,8 gr.

Total assemblies (optimal combinations) vary from 1170,6 gr on the high side to 1170,00 gr on the low side.

I understand this is Not a professional race motor Nor for a professional team.
0.1 gram difference among rods.
0.2 gram difference among pins.
1.0 gram difference among pistons.
0.6 gram MAX difference among combos
JME, but each and all look good. I wouldn not lose any sleep over it and I would have shop balance rotating assembly as is --- and Not look back.

*if shop is Not going to adjust weights of individual parts/pieces, shop will likely require you to bring Representative/Exemplar parts: One Rod, One Pin, One Piston, One Ringpak (for One piston) and One Pair of Rod Bearing Shells --- and, crank.
* I would also Verify the damper itself is truly correctly balanced as well.

[Baprace]

conrods (oliver) from 605,60 gr to 606,70 gr
piston wrist pin: 116,8 to 117,10 gr
pistons: 446,5 gr to 447,5 gr.
Oliver bolts (going to be exchanged) all weighed 34 gr. The new ones 33,8 gr.

Total assemblies (optimal combinations) vary from 1170,6 gr on the high side to 1170,00 gr on the low side.

Belgian , your connecting rod will have TWO weights -- > this is just an example not your actual weights ( Rotating end is 100% and reciprocating end is 50% ) an average old school Chevy connecting rod vintage 1975 will have an approximent total 598 grams with rotating about 415 grams and reciprocating 183 grams. I weigh total weight first = 598 grams , I hang the rod horizontally not attached to the scale platform by the wrist pin end and weight the big end = 415 grams , subtract the big end weight from the total weight and you will have the wrist pin end weight = 183 grams.

one bob weight set will be approximent because I don't have the actual parts in front of me.
Piston = 446.5
wrist pin = 116.8
rings = 88
locks = 28
small end of rod = 183 ( use only one small end weight )
big end = 415 ( use two big end weights ) = 830 grams
rod brngs = 90 ( use four bearing halves )
oil in journal = 4 grams

TOTAL BW = 1796.3 all of the weights are just a guess NOT actual weights , I think my math is correct

My son does all of our balance work now I haven't set up a bobweight in years so I could be wrong on how many parts and pieces are used but I think I have it correct.

[Belgian1979]

Thanks, I’m not balancing though. The shop used a set of piston + rod,… to calculate it.
I was just wondering about some of the differences I was seeing between combos.

[jcisworthy]

I've seen big-block Chrysler rods 20-25 grams different on the big end and in general driving it seemed fine

Nothing I would be ok with. I balance to two grams or less depending on the application

A while ago I asked Warren Turner how much he thought an engine can be out and not really notice it in a typical street engine and he estimated around thirty grams

[Dan Timberlake]

As others said, in the olden days the total R & P assembly weight or the con rod total weight potentially hide some pretty big differences is recip and rotating weight.

[rebelyell]

Thanks, I’m not balancing though. The shop used a set of piston + rod,… to calculate it.
I was just wondering about some of the differences I was seeing between combos.

? Do either you or the shop KNOW the crank's ACTUAL bobweight? --- I'm Not referring to any bobweight that was Calculated via weighing a Prior Piston-Rod combo/assembly. Each crank is manufactured with a tolerance within a small range of bobweights --- or the pricier cranks with a specific bobweight.
If you or shop don't KNOW, suggest have a shop balance Your Current rotating assembly.
If you don't KNOW for certain, seems that's no better than an educated guess --- difference Maybe Zero grams or Maybe 5 grams or Maybe 50 grams?

[Belgian1979]

Thanks, I’m not balancing though. The shop used a set of piston + rod,… to calculate it.
I was just wondering about some of the differences I was seeing between combos.

? Do either you or the shop KNOW the crank's ACTUAL bobweight? --- I'm Not referring to any bobweight that was Calculated via weighing a Prior Piston-Rod combo/assembly. Each crank is manufactured with a tolerance within a small range of bobweights --- or the pricier cranks with a specific bobweight.
If you or shop don't KNOW, suggest have a shop balance Your Current rotating assembly.
If you don't KNOW for certain, seems that's no better than an educated guess --- difference Maybe Zero grams or Maybe 5 grams or Maybe 50 grams?

I got the sheet by which they calculated the bob weight. Don't know a thing about balancing except that the crank needs to balanced. They had to redo balance as my current pistons were lighter than the previous ones.

[modok]

I usually say individual part weights should be within 1% for stock and within .5% for high performance

Balance factor on a 90 v8 should be 50-51%

[scotts439]

Just my opinion, when balancing for race or high perf, I want everything to be weight matched within 1 gram. If I am doing a "Street" balance that will never see above stock RPM, i will be a little looser, and get everything within 2-3 grams. I am not a fan of grabbing one rod and one piston to calculate bobweight, I have seen stock rods with 10 grams difference in same motor from factory.

[PackardV8]

Balance factor on a 90 v8 should be 50-51%

Agree, as a general rule. However, if one spins up a crank using an old Stewart Warner balancer, it's obvious the 50% is more "in balance" at some RPMs than others.

So yes, get all the pistons and rods within a couple of grams of each other, but once the crankshaft is balanced to a bobweight, if that bobweight changes at some future time, within unknowable limits, depending upon stroke and weights, a few grams plus or minus does not necessitate rebalancing the crankshaft.