Damper retaining bolt size and grade


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By ian57tbird - 12 Years Ago
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?
By lyonroad - 12 Years Ago
Ian, I recently reinstalled the damper on my 292 and noted that the Eickman book specified a torque range of 130-145 ft lbs. After reaching 130 ft lbs I concluded that this was way too high. I consulted the shop manual and it says 85 - 95. Still being unsure I searched this site and found this post by Ted.

         
85-95 lbs. for the damper bolt torque is more than sufficient. Both the 292 and 312 engines use the same damper bolt so the torque is expected to be the same for both. That 130-145 number found in the footnotes in the ’57 service manual would appear to be just one more typo in that book to add to the list.                           


I went with his recommendation.
By ian57tbird - 12 Years Ago
Thanks. Maybe it is still a 312. Seems to go OK for a stock motor. If I recall correctly I torqued my bolt to 120 foot pounds but that's not high for a grade 8 bolt.