What years did Chrysler use the 58 Lb. flywheel? Does it fit older HEMI engines besides the 426? Why such a heavy piece?
TIA, WINGNUT

Most likely some engineer thought that the idle would be too rough with a performance cam so they put a heavy flywheel on there to keep the engine running smooth at idle.

some of the early pro-stocks and maybe s/s tried what was called a 60lb wheel. probably for launch. not a production item. I don't know the exact of production wheels but a good guess would be around 40lb.

Maybe it is a truck flywheel
All the early, pre 1962, 8 bolt crankshaft don't have a centering register for the crankshaft or flex plate to ride on, they center on a machined surface on the outer edge of the crankshaft and they normally have studs, bolts in them with no threads in the crankshaft flange for the bolts to screw into
I have seen that type of flywheel used in Mopar SB with small C.I., less than 400 C.I., in some of the old modified eliminator cars

With 5.13 gears and that flywheel it'd take more power to accelerate the flywheel than to accelerate the automobile's mass!

R.

I think that the heavy flywheel would mean less RPM loss on a shift.

Can you imagine a G/Gas car that weighs 3570 Lbs. + the driver weight with a 3.09 1st gear and a 7.17 ratio Dana 70 in the rear with that weight flywheel
I have seen that car raced in the mid 1970s, the driver was very busy shifting (at 8600 RPM) the first 300 Ft.
I drove a old 1970 Duster Pro bracket car back then (mid 1970s)that had a 30 Lb. flywheel with the 3.09 first gear ratio and a 5.57 rear gear ratio and Dana 60 out of that same G/Gas car shifting it at or near 7000 RPM with semi stock 340 motor with 13x30x15 inch slicks, I was real busy the first 300 feet also