If pistons and rods are balanced within +- 1 gram, is that sufficient for non-race applications? (327 SB)
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Re: Engine balancing
In a cruciform crankshaft 90 degree V-8, the reciprocating mass (pistons, rings, pins, and top of the rods) generates a first order rocking couple. "First order" means that the frequency is the same as crankshaft speed.
This couple is balanced by the large masses at either end of the crankshaft, so once the reciprocating mass is equalized and known, the large end counterweights must be adjusted to assure that the first order rocking couple is eliminated.
Duke -
The short answer is, yes. *NM*
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The short answer is actually no!!!
Unless you are certain that the counterweights are in reasonable tolerance for the reciprocating mass you have, you should have the entire rotating/reciprocating assembly balanced.
DukeComment
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Re: The short answer is actually no!!!
Steve, as Duke said, if you are rebuilding an engine, the best plan is have the complete engine balanced. If done properly, there is going to be a fair amount of machine shop work anyway, and balancing is not much of the total cost.
If the pistons and rods are not perfectly equal in mass (weight), the machine shop will equalize the mass of all the pistons and all the rods to the accuracy standard they use for street engines. I had my engine balanced...I don't know the tolerance they used, but I consider the information only of academic interest to me anyway.
If you are thinking of ordering a set of standard bore pistons which are specified as being matched within 1 gram, and sticking them back in the same old holes, I am not so sure that's a good plan. Usually, any condition that warrants changing the pistons will mean that the bores are pretty beat up too; professionals take that opportunity to clean up the cylinder bores and optimize clearances by using the smallest oversize piston that gets the job done.Comment
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The question was about the pistons and rods,
not the crank. 1 gram is fine.Comment
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PS
I've never heard of anyone balancing the rods and pistons to some given weight and then instaloling them in the block without matching the crank to the weith of the rods and pistons. Maybe they do that out there where you live. I don't know.Comment
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Re: The question was about the pistons and rods,
I should have added the comment that I am going to balance the pistons and rods and then take the whole rotating assembly to the shop to be dynamically balanced. I was trying to determine if a balance that measures to 1 gram is sufficient. I thought a tenth of a gram was probably overkill for a street engine.
SteveComment
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Re: perspective
There are 28 grams in one ounce. Therefore, 1 gram is only 1/28th of an ounce. 1 tenth of a gram is 1/280th of an ounce. I will garentee that the rest of your engine is not all weighing in within a 1/280th of an ounce tolerance.Comment
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Re: The question was about the pistons and rods,
Steve, dynamic unbalance is specified in (mass)X(distance) units. Specifying the mass without defining it's distance from the rotational centerline is meaningless. I'm not sure what units are used by machine shops, but it may be gram-inches. An unbalance of 1 gram at a radius of five inches is much worse than 1 gram at a radius of 1 inch.
Unless you own a precision mass balance, it would be a better idea to let the machine shop equalize the mass of the pistons and rods. Here's an old contracting axiom to explain why: Whenever possible, avoid multiple contractors sharing responsibility on the same project task. Reason: If the result is unsatisfactory, then either (any) contractor can refuse to take responsiblility by claiming the other contractor(s) created conditions that resulted in failure. You will be left holding the bag.
If the machine shop does the total balancing job, then he and he alone is responsible if the result is unsatisfactory. If he balances only the rotating parts, and the total balance job is unsatisfactory, then he can claim that (your) reciprocating unbalance is the problem.Comment
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