hi, i have two 65 starfire engines , noticed today that one crank has a 3/4inch bolt holding on the harmonic balancer and the other has a 1/2 inch bolt ,any one know why . thanks will.

 

Most likely, one of the crankshafts is forged steel instead of regular cast iron.
Look at the flexplate bolts. 1 of the bolts is always off center a little bit.
Some have the offset bolt above the centerline, and some below the centerline.

Both 65 motors from the factory should have forged cranks, but maybe one was swapped out a long time ago.
Look up your casting numbers in the oldsmobile wiki in the tech section.