About Bolt Pretension and Moment

The usual formula is

Tightening torque (N.m) = 0.2*bolt pretension (N)*nominal bolt dia (mm)/1000.

This is on basis that thread pitch = 1/8 bolt dia, and only 10% of the applied torque results in bolt tension (the rest is lost in friction), as you can check for yourself.

codemaster, that 10% number is WIDELY variable. so much so as to make the calculation useless for almost all situations. the ONLY true reliable way to obtain pretension is elongation of the bolt. thread lubrication state as well as machining finish and tolerances make using torque as a stand in for pretension worse than useless.

Useless or not, I wouldn't mind betting that 95% of what must be millions of bolts tightened every day worldwide are done either to a torque figure or no measurement at all, just what feels right to the fitter. I've tightened plenty in my time, very few of them with a torque wrench, even less with any tension measurement, direct or indirect, and I haven't had noticeable problems.

I was trying to answer OP's question. The formula I quoted is given in suppliers' literature (with comments about lubrication, hard washers etc, which maybe I should have mentioned). In most cases it's not that critical. As you know, this subject has been discussed at length on CR4, and OP could do a search to get more background.

Hint #1, What the previous responders have advised.

Hint #2, Accurate measurement of bolt length.

Hint #3, Use a strain gauged bolt that measures elongation.

I said in #3 that this has been discussed before, but I'd forgotten just how much! If you type bolt torque into the search box you'll find enough to keep you busy for a while.

I said in #3 that this has been discussed before, but I'd forgotten just how much! If you type bolt torque into the search box you'll find enough to keep you busy for a while.