Balancing pistons? How much is too much?
10-25-2010, 06:52 PM
#21
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Bob,
Thanks for the reply. I should have a copy of the balance data by tomorrow. I am not sure of the proper terminology, but I was told that the assembly was zero balanced. No under or over balance. Judging by what you said, the addition of 7 grams to the pistons would put me into an under balance condition.
If this underbalance condition is not desirable, I can get a set of tool steel wrist pins that weigh 13 grams less than my current pins. That would put me 6 grams lighter than original, 6 grams over balanced.
Again, for those questioning why I just dont pull the crank and balance the assembly, the idea was to have 2 sets of pistons available, for the same crank. After all, Wiseco made the original pistons, when I asked them if they could duplicate them to a new set with a dish, rather than a dome, they said " No Problem, we can get the weight exact" Obviously, THEY screwed up along with screwing me. If my request was not feasable, then they should have said so. Engine builders beware!!
Thanks for the reply. I should have a copy of the balance data by tomorrow. I am not sure of the proper terminology, but I was told that the assembly was zero balanced. No under or over balance. Judging by what you said, the addition of 7 grams to the pistons would put me into an under balance condition.
If this underbalance condition is not desirable, I can get a set of tool steel wrist pins that weigh 13 grams less than my current pins. That would put me 6 grams lighter than original, 6 grams over balanced.
Again, for those questioning why I just dont pull the crank and balance the assembly, the idea was to have 2 sets of pistons available, for the same crank. After all, Wiseco made the original pistons, when I asked them if they could duplicate them to a new set with a dish, rather than a dome, they said " No Problem, we can get the weight exact" Obviously, THEY screwed up along with screwing me. If my request was not feasable, then they should have said so. Engine builders beware!!
Last edited by Vinny P; 10-25-2010 at 06:58 PM.
10-25-2010, 06:54 PM
#22
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Vinny,
Just to give you an idea, on the older TRW forgings, if you mill .010" off the bottom of the pin bosses, you will lose about a gram. Your Wiseco pistons are probably designed with as much weight as possible already taken out of them, probably not much room to take any more.
Bill
Just to give you an idea, on the older TRW forgings, if you mill .010" off the bottom of the pin bosses, you will lose about a gram. Your Wiseco pistons are probably designed with as much weight as possible already taken out of them, probably not much room to take any more.
Bill
10-26-2010, 04:27 PM
#24
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Ok guys, here is the information from my balance sheet. I dont claim to understand exactly how it works, maybe someone can explain it? I know there is some sort of mathematical equation that these #'s are put through, but I dont know the formula.
Here are the numbers as they were given to me..
Rod total 868
rot 600
rip 268
Piston and pin 778
Rings with spacer 68
Rod bearing 48
oil and locks 14
Bob Weight 2424
Here are the numbers as they were given to me..
Rod total 868
rot 600
rip 268
Piston and pin 778
Rings with spacer 68
Rod bearing 48
oil and locks 14
Bob Weight 2424
10-26-2010, 07:07 PM
#25
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Vinny,
Looks like he used exactly 50% so it is neither "over" or "under" balanced. I did notice that his allowance for "oil and locks" seems to be a bit high. I generally use 4 grams for oil and locks are usually about 4 grams per piston. If I had the same parts my bobweight would have only been 2418. That also shows you how different balance shops can come up with different bobweights for the same assembly.
So just for example, if you had brought your parts to me to balance, with your new pistons, my bobweight would have only been 1 gram more than his original.
By the way, the bobweight calculation for a V-8 is as follows:
Rod rotating weight x 2 = 1200
rod bearing x 2 = 96
oil = 4
piston & pin = 778
rings w/spacer = 68
locks (?) = 4
rod rotating x 1 = 268
----------------------------------------
Total 2418
That is what I would have come up with assuming your locks are 4 grams. If your new pistons are 7 grams heavier, I would have come up with a bobweight of 2425 which is only 1 gram different than what he had with your old pistons
Stop worrying and run it !
Bill Koustenis
Advanced Automotive Machine
Waldorf Md
Looks like he used exactly 50% so it is neither "over" or "under" balanced. I did notice that his allowance for "oil and locks" seems to be a bit high. I generally use 4 grams for oil and locks are usually about 4 grams per piston. If I had the same parts my bobweight would have only been 2418. That also shows you how different balance shops can come up with different bobweights for the same assembly.
So just for example, if you had brought your parts to me to balance, with your new pistons, my bobweight would have only been 1 gram more than his original.
By the way, the bobweight calculation for a V-8 is as follows:
Rod rotating weight x 2 = 1200
rod bearing x 2 = 96
oil = 4
piston & pin = 778
rings w/spacer = 68
locks (?) = 4
rod rotating x 1 = 268
----------------------------------------
Total 2418
That is what I would have come up with assuming your locks are 4 grams. If your new pistons are 7 grams heavier, I would have come up with a bobweight of 2425 which is only 1 gram different than what he had with your old pistons
Stop worrying and run it !
Bill Koustenis
Advanced Automotive Machine
Waldorf Md
10-26-2010, 07:09 PM
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bob weight = recipricating end+ rotating+rotating+ 4 bearing shells+pin+piston+pin+rings and spacers+ locks+oil
or
2,424 gram=268+600+600+24+24+24+24+778+68+14
with a 3% over balance the crank would be balanced to a 2496gram bob weight. Now, can you see how insignificant the 7 grams are?
or
2,424 gram=268+600+600+24+24+24+24+778+68+14
with a 3% over balance the crank would be balanced to a 2496gram bob weight. Now, can you see how insignificant the 7 grams are?
10-26-2010, 07:22 PM
#28
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Bill and Jeremy,
Thanks for the help. I did weigh the locks and they come in at 4 grams. So, your #'s are accurate. Call me over cautious, but I like to take the time to assemble engines as carefully and accurately as I can. I learn something everytime I assemble one. This time will be no different. I have never messed around with any balancing issues. I have always left that to machine shops.
Looks like this is a done deal for me. I feel confident to assemble.
Thanks for the help. I did weigh the locks and they come in at 4 grams. So, your #'s are accurate. Call me over cautious, but I like to take the time to assemble engines as carefully and accurately as I can. I learn something everytime I assemble one. This time will be no different. I have never messed around with any balancing issues. I have always left that to machine shops.
Looks like this is a done deal for me. I feel confident to assemble.
