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Different Pistons - Does it need rebalanced?

4K views 10 replies 9 participants last post by  cobalt327 
#1 ·
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?
 
#6 ·
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.
 
#7 ·
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.
 
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