Yes.
Cast to Forged requires a rebalance.

I thought so. Thanks for the info.

Tom, if I were you, I'd just put some stock rods back in and be done with it. I know where you can get a set for cheap $$. In fact, you may have already payed for them.

The stock 3/8" rods should be just fine with what you have there. We ran them in some Hi-Perf engines only we ground the beems, had them shot peened and used the ARP "Wave-Loc" rod bolts in them, but I doubt you'll need to do all of that. Although I would go with the ARP "Wave-Loc" rod bolts and use a rod bolt stretch guage while installing them on your engine---just read the installation directions that come with the new bolts and the lube they recommend.

Assuming you have a mark IV block Formula31, the forged and cast parts call for the same balancer/flywheel part numbers. This implies rebalancing is not needed, albiet a wise thing to do.

BT

Very true, but the cranks are not the same and thats where the balancing come in for the piston and rods weights. IM still having the stock rods worked on to determine their suitability and try and find a cause for those deep scratches.

THanks Kaama, I always use my mics to measure proper rod bolt stretch. It's usually pretty close to the torque rating, amazingly.

Tom,
I would think as fussy as you seem to be that after changing rods, that not rebalancing would be out of the question. Considering the cost of balancing the rotating assembly as compared to the entire job, it is minimal.

Good Luck

ok, here's the deal. the 3/8th and 7/16th(bolt sizes) rods are different weights, requiring a rebalance. The 7/16th bolt-equipped rods only came in forged-crank applications, and putting them in a cast-crank application is going to cost a fair amount of money to balance the cast crank for the big rods. If the assembly was balanced before, and you've still got the balance sheet, just match the rod weights (assuming you're staying with 3/8's equipped rods) to the previous numbers to avoid respinning the crank. Forged or cast, as it relates to rods, has nothing to do with it as all rods are forged. I don't recommend changing between forged and cast pistons without a rebalance either, as all the balancing shops I've dealt with have closer weight tolerances than the difference there will be between the two styles. My two cents and a buck-fifty will buy you a cup of coffee.