When I had to remove my damper last year I found the retaining bolt threads to be damaged. At first I thought a metric bolt had been used as the pitch was not correct and by marking on the head that I thought was metric. It was pointed out to me by someone familiar with Y Blocks that it was an original bolt and the marking was metic. We came to the conclusion that the bolt had been stretch from over tightening and that is why it appeared to be a metric thread. I cleaned the threads in the crank and replaced the bolt with a 9/16UNF grade 8 bolt and torqued the bolt to a spec suitable to that bolt.

Did 312 cranks get a larger bolt or one of a higher grade? The workshop manual indicates 85-95 foot pounds for all engines except 312 which it specifies 130-145 foot pounds? There is also in the manual a general bolt torque chart that indicates 5/8 x 18 bolts 130-145 foot pounds. I suspect previously the bolt was tightened to 312 specifications but was only a grade 5 bolt and not able to be torqued that high.

My car was originally a 312 on the compliance plate, but does this mean I have a 292 crank?