rod bolt torque compared to stretch

[GARY C]

It always seemed to me that if you have to over tq a bolt to stretch it to a specific length it can't be good for the bolt.

But the actual stretch is so much more important than the torque applied.

I have to agree with you there Tom. It’s the tension the bolt is under due to it’s stretched condition that provides the necessary clamping force, not the twisting force applied to tighten the bolt. Just look at LS critical bolts tightening specs. Low ft/lb to seat joint, then angle rotation spec to set stretch. A 7/16 - 20 tpi bolt will rotate 33 degrees to stretch .0046”. Wrong lubricant can under or over rotate before the wrench click leading to under or over stretching of the bolt - under or over clamping of the joint.

I don't disagree with stretch but if you have to tq a bolt that is "designed" for 65 lbs to 95 lbs I would be dbl checking my threads and bolt head flange surface. I could see 75 on a 65 lb bolt as that could the tq wrench it's self.

[HQM383]

But the actual stretch is so much more important than the torque applied.

I have to agree with you there Tom. It’s the tension the bolt is under due to it’s stretched condition that provides the necessary clamping force, not the twisting force applied to tighten the bolt. Just look at LS critical bolts tightening specs. Low ft/lb to seat joint, then angle rotation spec to set stretch. A 7/16 - 20 tpi bolt will rotate 33 degrees to stretch .0046”. Wrong lubricant can under or over rotate before the wrench click leading to under or over stretching of the bolt - under or over clamping of the joint.

I don't disagree with stretch but if you have to tq a bolt that is "designed" for 65 lbs to 95 lbs I would be dbl checking my threads and bolt head flange surface. I could see 75 on a 65 lb bolt as that could the tq wrench it's self.

A bolt would not be designed for a specific torque wrench value. A torque wrench value is a calculation of the bolt and clamping force. T= K x D x P. By all means check the threads and bolt heads but also quantify if the extra 30ft/lb was merely friction to be overcome to get to stretch spec. Repeat with a washer to see if it ‘washes’ away some friction to achieve same bolt stretch with less wrench torque.

[gregsdart]

For a simplistic way to look at proper rod bolt tensioning, what would you rather trust , a torque number affected by a number of things, including the accuracy of the torque wrench used, or trust in ONE thing. The bolt manufacturers quality control? A bolt is nothing more than a linear spring. The factory gives a stretch amount based onputting that bolt at i think it is 75 percent of max stretch before failure, which would be the point where it won't return to its factory length. That last unused stretch is there to allow the bolt to hold under extreme loads at speed and survive to old age. For me, i will take the most accurate way IMHO- stretch