Jlo, I know this will run counter to the info in the above posts, but keep in mind everything has a tolerence! If your new pistons and ring pack are within +/- 8 to 10 grams of your old piston and ring packs original weight ....... your machine shop is right, you will NOT have to re-balance this motor.

If your new pistons and ring pack are up to 10 grams lighter than the OEM setup, your motor will now be slightly "overbalanced", something many NASCAR and performance engine builders do anyway. If your new piston/ring setup weighs no more than 8 to 10 additional grams per rod/cylinder you will still pretty much be within the OEM factory tolerance for GM line built engines.

Finally, from my personal experience, about two(2) years ago we re-built my cousin's '94 Z28 LT1 Camaro engine with stock rods and crank but "upgraded" to forged pistons so he could more safely run a small (50-75 hp) nitrous shot, and, even though the forged pistons and ring pack he used were about 10 grams heavier than stock ........ that motor runs "smooth as silk" right up to 6,500 RPM day in and day out.

Hope this info helps.

I learned a lot more than I knew about balancing recently. Basically, the convention of balancing the whole piston/rings/pin weight and 50% the rod weight is somewhat arbitrary. Different shops have slightly different ways of balancing. If you don't specify exactly what you want, many will deliberately "overbalance" by 1-2%. Overbalance means there is more counterweight than the traditional rod+50%. Some engine builders are convinced that overbalancing makes for smoother running at high rpm with a neglible increase in low rpm vibration.

I had a situation when assembling my race motor this year where this came up. After the shortblock was already assembled, I found a piston to valve clearance issue that necessitated new pistons. The new pistons were 36g lighter than the old ones. My machinist, engine building mentor, and JE all said "no problem, it might even be better". In spite of hearing this from three reliable sources I was skeptical but went ahead as the time and cost for rebalancing was prohibitive. And what do you know, it runs smooth as silk to 7,500rpm. Noticibly smoother above 5,000 than with a 50% balance and it is now ~3% overbalanced! I am going to take a look at a main bearing this weekend and will report any probems. But from the way it runs, I doubt there will be any.

Rich
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