New pistons 3 grams lighter, rebalance?

[Brummmie]

Hi guys, I’ve got to change manufacturer of piston due to going up on size, I can get to 3 grams lighter if I spec a heavier pin.
It’s on a 419ci V8 LS, all the pistons will be within half a gram individually, 527 grams opposed to 530.
Would a rebalance still be needed?

[Boosted LS1]

Technically yes, I'd rebalance. Real world, probably not. People often swop parts and the engines survive. What does the ring pack weigh? If you decide to go for the lighter pistons could you use lighter pins and then rebalance?

[Brummmie]

I have to use their heaviest pin to get to 3 grams too light. But the lighter pin is around 20 grams lighter, i dont think its worth hacking the crank more than a have to.

Edited by Brummmie on Wednesday 22 November 09:20

[tapkaJohnD]

It's not possible to balance a crank with the conrods and pistons attached. (Usually - no doubt someone's done it) The usual convention is that conrods are two-thirds rotating mass and that equalising the big ends' masses will preserve crank balance. The other third and the pistons are reciprocating mass, and again, equalising their masses will avoid dynamic imbalance, but neither can do anything about primary or secondary vibration due to reciprocating parts.

So if your new set of pistons are all equal, why rebalance the crank?

John

[stevieturbo]

Is that mass of pistons, rings, pins, clips, etc ?

3 grams is such a small amount....maybe not at 7000rpm though.

Although I'm sure when I replaced my pistons last time, despite same manufacturer etc I'm sure they ended up around 2 grams lighter. No chance was I paying a fortune to re-work the crank for that !!

I'm sure there would be more than 2 grams of oil sitting on the piston at any given time, or maybe some carbon on the crown.

Hard to know how anal to be about that.

[Brummmie]

stevieturbo said:

Is that mass of pistons, rings, pins, clips, etc ?

3 grams is such a small amount....maybe not at 7000rpm though.

Although I'm sure when I replaced my pistons last time, despite same manufacturer etc I'm sure they ended up around 2 grams lighter. No chance was I paying a fortune to re-work the crank for that !!

I'm sure there would be more than 2 grams of oil sitting on the piston at any given time, or maybe some carbon on the crown.

Hard to know how anal to be about that.

Hi Stevie,

Thats the mass of the pin and the piston, no ring pack.
I guess i am going to have to order and hope the JE piston rings are heavier than the Mahle, the ring land for the oil control rings weighs 6 grams alone! I know that because i left that on by mistake and was alarmed the piston was 6 grams heavier than the other!

[stevieturbo]

Speak to Dave at Performance unlimited, he might be able to tell you weights of Total Seal rings that might be suitable.

Or if there is maybe a heavier pin option again ?

Are yours thin, thick ? tapered or straight ?

Not sure how much that 3gram might matter vs the entire rod/bearings/piston combo ? ie maybe heavier or lighter bearings too could counter it ?

Probably quite complicated to work it all out in that manner ?

[Brummmie]

stevieturbo said:

Speak to Dave at Performance unlimited, he might be able to tell you weights of Total Seal rings that might be suitable.

Or if there is maybe a heavier pin option again ?

Are yours thin, thick ? tapered or straight ?

Not sure how much that 3gram might matter vs the entire rod/bearings/piston combo ? ie maybe heavier or lighter bearings too could counter it ?

Probably quite complicated to work it all out in that manner ?

I've just spoken to Chris Schirle of TVR factory fame, he's built alot of yank V8's over alot of years, he says not to worry until 10-12 grams.
I've been baffling myself reading up and some balancers over balance, which this effectively will be by upto 4-5 grams to allow for oil, i think i am going with leave alone.

[99hjhm]

stevieturbo said:

Speak to Dave at Performance unlimited, he might be able to tell you weights of Total Seal rings that might be suitable.

Might be just me, but the last V8 I did, weights of the Total Seal rings quoted were nothing like the actual weights measured!

OP.... Chris Schirle knows his stuff so go with that, (depending on if the assembly was well balanced before?), he used Trevor Wilkinson, now trading as Andrews Precission for balancing who is excellent.