crank turn limit


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By DANIEL TINDER - 18 Years Ago
I mentioned to one of my mechanics my interest in a 312 crank that had the rod journals turned .040", and could tell by his concerned expression that he would have taken a pass. I had assumed that since bearings were available in that size, it must be OK. What IS the practical turn limit? Does a large cut risk more machining error? Is the crank surface-hardened and thus will score more readily? Is there a problem with thicker bearings? What's the story?
By charliemccraney - 18 Years Ago
My 318 stroker crank has 2.00" journals. It is offset ground. I don't see why you couldn't go .040". I suppose the journals can be hardened if that's an issue.
By Hoosier Hurricane - 18 Years Ago
Charlie:

Y crank journals are not hardened, so that is not an issue.  .040 bearings are available, so that is not an issue.  Go for it.  How about all the 312 cranks that have been turned .125?  That doesn't seem to be a problem either.

John

By Glen Henderson - 18 Years Ago
Back in the early 60's the instructor at the Auto trade school that I attended had the following to say on the subject. His reasoning for not going  over .030 on rod or mains was that the shell material had to be thicker than a std bearing. His theory was that somehow this would cause the bearing materal to fatigue and flake out. Old wifes tale or old mechanics tale maybe or there could have been problems with bearings of his day that lead him to this conclusion, he was in his 60's at the time and had been a mechanic all his life. With modern materials I don't see a problem as long as the shaft is turned properly and the rod big ends are in good condition.