Vinny P

Bill,
This engine runs 6100 rpm at WOT. I was also thinking that 7 grams is alot of material to remove on a mill. I was going to experiment with an old damaged piston by removing material until the piston got 7 grams lighter. Then see how much material was covering the floor!
As always, thanks for the advice.

fireboatpilot

My head is spinning now. Please correct me if I'm way off base here but it's my understanding that so long as all the pistons are within .5gms of each other and the rods and bins are balanced to the same tolerances is the crankshaft really going to be balanced any differently for an additional 7gms of weight? I understood it to be mismatched weights that would cause a problem with harmonics not a different total weight. So long as the pin placement, use the same rods and ring set up what would be the problem?

pachanga

Voice of reason?? Why don't you just have your rotating assembly rebalanced with the new pistons? I assume the motor is apart!

Young Performance

I can't see where you would have any trouble with it. 7 grams isn't that much, plus it puts you in an overbalance. I don't, but I have heard of several guys that will intentionally overbalance an engine. Some as much as 5-10 grams. Like others, I shoot for less than 1 gram. I understand what you are trying to do and would just go with it. You will not notice the slight overbalance.
Eddie

zt260

I think the real issue here is Wiseco. They were paid to deliver a specific weight and failed. Now if they were paying for the work to rebalance that’s not so bad, but I doubt they would consider it.

I think we agree the weight “should” not be a big deal, but if we’re wrong, who pays for the damage? This is Vinny’s dilemma.

wjb21ndtown

He stated earlier that he wanted to have two motor setups ready to go. If he balances to this new set of pistons it won't be in balance with his old set of pistons - at least with his current line of thinking.

FBP - that's exactly what I was thinking. I really don't think that adding 7gs across the board will throw it out of balance.


What you could do is build the motor the way it is, throw it on the balancing machine and see if it needs any weight added or removed... That should tell you something, and may be worth the piece of mind.

mike tkach

iTrader: (3)

i agree with eddie, put it togeather,you would not beleive how unbalanced gm engines come off the assembly line,nor would you ever know it.

rmbuilder

Vinnie,
The first measure would to be obtaining a copy of your balance sheet to verify the original bobweight. At that point you can determine if the original balance was 50%, or if the shop overbalanced the assembly. An overbalance condition, up to ~4% is acceptable (possibly desirable). However, in your case, when adding weight to the piston side it puts you into/toward and under balance condition, which most builders try to avoid in the majority of situations.
Many shops will intentionally overbalance to accommodate a future overbore and the resulting marginally heavier piston. If your original balance was to ~52%, for example, your +7 gm piston gain would bring your assembly closer to underbalanced/neutral condition while still remaining on the OB side. If your original balance was @ 50%, or marginally less, (neutral or underbalanced) the +7gm gain might bring you further into an underbalanced condition which you may choose to avoid. Your build sheet data will assist you in that decision.

Bob

BillK

FB,
This is kind of basic but, when a crankshaft is balanced, it is made to match the weights of the parts that are attached to it. Most of the time, new performance parts are lighter, so you have to remove weight from the crankshaft counterweights to compensate. In this case the new parts are heavier so to really balance the crankshaft exactly, you would have to put weight back into the counterweights.

Making all of the parts the same weights is a very important step in the balancing procedure, but the most important part is balancing the crankshaft to match those weights.

Hope this helps,

Bill Koustenis
Advanced Automotive Machine
Waldorf Md

BillK

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