I had a 383 built a while back. The engine has less than 500 miles on it now. I am tearing it back down due to the bonehead that built it. He used dome pistons and milled the domes off. The compression was too high and they rattled in the bores. I am replacing those with a matched set of Sealed power pistons with a 12 cc dish.

The crank is an internal balance 3.75 one piece rear main seal. It was balanced, and the rods, as well as the (old) pistons. The new pistons are lighter than the old. Do I need to rebalance the crank? I have been told by my new well experienced builder that as long as I am not adding weight, it should be good. I am, however having the mains cut 010. I'm also installing Comp 26918-16 beehive springs for my ZZ4 cam. The compression should be 9.7:1 with the new pistons.

lighter is usually better than heavier on the piston weights,, and only makes up 50% of the actual because of the opposite piston's direction always offsetting weight. but,,,, how much difference in bobweight??? or total piston/pin/ring package weight??? depends on how much as to a re-balance/spinning up the crank again. let us know and maybe we can give you a second opinion.

The old pistons weighed 737 grams with the pin. The new pistons weigh 641 grams with the pin.

I believe he said that balancing the crank will run $150.00.

Personally if I had the engine apart and at the machine shop there is no way I would let $150 stand between me and knowing the engine is done RIGHT.

Sounds like a pretty large difference not to balance. We have a couple of experienced machinists on the board who should be able to tell you one way or the other.

Ya,
At almost 100 grams of imbalance, re-balance the crank.
If it was 20 grams or less lighter on the new pistons I would tell you not to waste your money. However at almost, no doubt I would rebalance.

I'm just going to go ahead and have it done. Besides, they'll just be taking weight out of the crank right?

yes,, they'll remove weight from the counterweights,, not the rod throws. i'd re-balance with that much difference also. good call.

my current setup is within 15 grams of total bobweight from my last rotating assy,, and it's going "as is" with all weight matched,, but still 15 grams difference total. like jake said,, it really won't show any problems under 20 or so. hope it helps.