Originally Posted by
Trakhak
I admit that it can be a bit hard to get your head around what's happening when you use the two wrenches.
But think it through. Regardless of how much friction in total is in the system represented by the two sets of threads, the torque wrench has to overcome the resistance in each bolt separately. One bolt is almost certainly going to present more friction than the other, so the bolt with lesser friction hits the torque spec first, followed by the one with more friction. Once the bolt with lesser resistance is at spec, the other tightens precisely as if you're hold the crank arm instead of the wrench.
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Sorry, Trakhak. Your method will work fine for this application but is not as theoretically accurate as the one wrench method. Torque is a proxy for measuring the stretch in a bolt. Friction is the enemy of good torquing because you have to push harder to overcome the friction and that extra push can't really be quantified. In certain critical applications, part of the procedure is to oil the threads to reduce the friction.
You admit that your method continues to push on a fastener that has already been properly torqued. This is bad. There will be an equal and opposite reaction as it meets more friction. The wrench will click when the second fastener is properly torqued. The first fastener will be slightly over torqued due to that equal and opposite reaction. Again, the effect is negligible.