I have a 77 360 with a bad crank. I just bought 2 cranks a PN# 4027169 I believe to be a pre magnum crank and a pn#3418640 that may be a magnum crank. My question is are the magnum and pre magnum cranks intechangeable in a ballence respect. The magnum crank is good enough to install with just a cleaning but the early crank will have to be turned. the crank in my motor has unrepairable damage to the trust surface.

pretty sure diff balance

3418640 and 4027169 are both listed as fitting a '71-'86 360.

3418640 most likely being the earlier of the two.

MY buddy bought a B&M flex plate for his duaghters 360 magnum and got an early flex plate. He got the magnum plate anf the engine vibrations went away. I'll call him and find out why he had to change flex plates plates. I'll have an answer this evening.

Forget it both your cranks are pre magnum.

IIRC the crank itself is the same between magnum and LA. the different balance factor comes into play because the pistons and rods on a 360 magnum are lighter than an LA.

Well, whatever the reason the Mag imbalance is quite a bit less than the earlier 360 imbalance. Never the twain shall meet.
R.

I was aware of the differance in ballance between the la and mag. Just was not sure if some of the weight diff was taken out with the crank counter throws. If I had the $ i would have the assy ballanced.

NEITHER of your different cranks are late enough casting numbers to be Magnum. They're both just normal 360LA cranks. No worries.
R.