I think it's the other way around


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Posted by David Sherman on Monday, July 12, 2004 at 3:55PM :

In Reply to: Not usually... posted by Gordon on Monday, July 12, 2004 at 12:51PM :

A thread is a spiral ramp. It takes less force to push something up a gentle ramp than a steep one. Likewise, to produce a given tension in the bolt will take less torque on a fine-thread nut than on a coarse one. It seems to me that if the torque specs are designed for bolts with coarse threads going into the block, and one replaced the bolts with studs with fine-thread nuts on top, using the same torque spec on the fine-thread nuts would produce more stress in the bolt or stud in proportion to the ratio of thread pitches, which would typically increase the stress between 20% and 50% when using SAE threads vs UNC threads. Friction should be similar in both cases. Since we are always so carefull about torquing our heads exactly right, a variation of 20% or 50% above design values seems significant to me. Of course it's uniformity that matters the most, so maybe if they're all on the high side by an equal amount, it's okay. I would just like to know the real-world practical answer from people who design engines.



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