ls1290

iTrader: (20)

Is it necessary to rebalance the crank and rods if one where to move them from a 3.91" bore block to a 4.00" bore block?

Steve - Race Eng

iTrader: (1)

It depends on the weight of the pistons. I would expect a 4" piston to weigh more than a 3.91" piston but that is not always the case. If the pistons weigh within 5 grams I'd say you would be fine for a street engine. If not I would have the crankshaft rebalanced.

I hold a tolerance of half a gram when balancing components for a new engine. A new set of pistons, rods, will need touch up to bring them in to range.

I have found that even the so called balanced kits need balancing. Many times the weights listed on the balance card are way off from the pieces in the kit. It is always best to double check everything before putting an engine together.

Steve Demirjian
Race Engine Development

Boosted LS1

I second the above posting. It's very unlikely your piston pack will weigh the same as your old piston assembly. If you are lucky they will be slightly heavier and you can lighten them to the older weights. You will need to weigh the pins, piston & rings. Then you can sort the heaviest items and lighten them to match the lightest items. Use scales accurate to 1/10th of a gram but settle for 1/3rd of a gram otherwise you will be at it forever.

If the parts are lighter then you need a rebalance and if much heavier, you still need a re-balance. If heavier, you may get in the ball park by buying lighter pins.