[S10 Racer]

Hey guys,
Just got parts ordered for my motor. A little history is the rotating assy. was balanced in the old motor (383 SBC) and the crank and rods are going into the Dart SHP 4.125 block. The old pistons were KB 123 Hyper pistons that had been balanced. The new pistons going into the Dart block are SRP forged dome. My question is do you think I need to be concerned about any weight difference between the two pistons and is it necessary to have the assembly balanced again with the new pistons since the pistons are part of the reciprocating weight and not rotating weight?

[vinniekq2]

a bsolutely.
yes

[Landshark928]

+1 on rebalancing

[spinn]

Yes its a good idea.

Define NEED and I can give you a better answer.

[Dave57210]

Will it need to be all re-balanced?

No - For idle speed. Of course if it might someday be run above minimum idle speed then a complete rebalance is mandatory.

[ap72]

A more accurate answer is- it depends. If there is significant weight difference between the pistons it can lead to a balance issue. If its not a large difference then it may not be noticed at all or only at low rpm. Most 350s on the road are not finely balanced, they just shoot for "good enough" and it works out just fine.

That being said there are cases where balancing has shown to increase power... There are also cases that shows balancing decreasing power so a case can be made for either side. A lot of race engines are intentionally overbalanced and a few that are under. Unfortunately there is no easy rule for deciding your case.

Find the weight difference and if its only a small amount then just run it as is.

[1Gary]

I'm on the side of re-balancing.Many of the high dollar race piston set with floating pins so you can swap out pistons come in a set of ten so as to not effect the balance.I am saying for longevity of bearings balance is critical.It's not only at idle,but at the top end where unbalance is amplify or multiplied.

Back when I pounded balance pins into H/B'ers,the good enough factory spec was not even close to what guys did for their high performance application.I promise you all there are no Super Stockers running unbalanced.

[BOBCRMAN@aol.com]

Find out the weight difference. The new forgings will probably be lighter. Lighter is good. Up to ten grams light would be great. Called overbalancing.

Left turn racers do it all the time with no evil problems. Some engines love it.

[S10 Racer]

Originally Posted by BOBCRMAN@aol.com

Find out the weight difference. The new forgings will probably be lighter. Lighter is good. Up to ten grams light would be great. Called overbalancing.

Left turn racers do it all the time with no evil problems. Some engines love it.

Yeah, we're left turn "dirt" racers for sure.

[1Gary]

My opinion is there is a direct correlation between how well a engine is balanced and how fast it rev's.

The Super Modified's engines we have built are finely balanced.

[cobalt327]

Originally Posted by S10 Racer

Yeah, we're left turn "dirt" racers for sure.

As has been said already, forged pistons are often lighter than cast. You have both piston assemblies- weight them both including rings and pins and see where you are.

See that the new pistons, wrist pins and rings all weigh very close to each other (static weight matched). Then calculate the difference in balance percentage by using the balance card from when it was originally balanced.

As a comparison, the difference of 1% under/over balanced on a reciprocating assembly weighing 1500 grams is 15 grams. So you have some leeway until you get anywhere near too far out.

BTW, often differences in reciprocating weight can be corrected by just using different pins.