Bolt torque estimation based on angle and material

Question

Is there a valid generic formula to roughly estimate a bolt's tightening torque similar to the "tighten by hand until seated, tighten XXX degrees more with your wrench"? I have used for many years the "tighten by hand until seated, then tighten another 180 degrees with wrench" on my gasket-type spark plug when new or "tighten by hand until seated, then tighten another 90 degrees with wrench" on retightening after inspection etc and the truth is that given the spark plug's position on my dirt bike it would be almost impossible to tighten it with a torque wrench.

So I was wondering if, given the materials included, the thread geometry (eg A2 M10x1.5 or 8.8steel Μ10x1.25 bolt to be screwed on aluminum triple clamp etc) and any washers/stoppers/gaskets included there would be a rough generic way to estimate the applied torque by tightening angle.

Answer 1

This can be calculated with any bolt calculator, although if you want it in degrees as opposed to Newton meters, you need to calculate a bit more.

First, use a bolt calculator, tell the exact materials of the bolt and nut/hole. It will tell you the preload in Newtons and the torque in Newton meters.

You don't need the torque in Newton meters here. You need the preload. Calculate with the Young's modulus and the average diameter (note: 6mm bolt will have average millimeter far lower than 6mm because it's only 6mm at the widest parts of the threads) and the average length of the bolt from its head to the top of the threads/nut, how much the bolt elongates in millimeters from the preload value in Newtons.

Then you know how many millimeters you need to tighten it past tight. Use the thread pitch to convert that to degrees. Example: if the thread pitch is 1.5 mm, and you need to cause elongation of 0.9 mm to have the desired preload, that's 0.9/1.5 * 360 = 216 degrees.

If there are some gaskets that are intended to deform, then this method won't work. In this case, the first tightening will require more degrees than if you remove the spark plug and install the same spark plug again with the same gasket. However, for ordinary bolts this method works.

Answer 2

I mean i guess there is. You cant always trust it though. my rangers caliper bracket bolts were "torqued to spec" but the one still came loose on my twice so i used threadlocker and then torqued to spec. I dont really think theres a generic torque spec. Not every bolt can be turned 180 degrees past tight. I say the best thing is threadlocker and just tighten it a little past tight.