Bolt torque chart

T = K x D x P

T = K x D x P. D is the nominal diameter and P is the target preload, taken here as 75 percent of the proof load. K is the nut factor: a single empirical number standing in for the friction in the threads and under the head, and it is where every disagreement between torque charts comes from. Nothing on these pages is a torque measured on a joint - the published columns are what named charts printed, and the estimate columns are this equation with the K, D and P shown beside the answer.

Preload target on every page in this section is 75 percent of proof load. Every chart cross-checked here targets 75 percent of proof load, so the disagreements between them are about K alone. A chart that targets 65 percent, as some do for reusable joints, will give figures about 13 percent lower for the same fastener and the same K. Always read the clamp fraction before comparing two charts.

What a torque figure is worth

A torque figure is an estimate of preload, not a measurement of it. Even with the right K, torque control typically lands the preload within about plus or minus 25 to 30 percent, because almost all of the applied torque goes into friction rather than stretch. Where the preload actually matters, measure it - turn of the nut, bolt elongation, a load-indicating washer or a tensioner - rather than trusting a wrench setting. And a torque table cannot know what your joint is made of: it assumes the threads and the clamped material can carry the preload the bolt can.