Driving down the road and (Boom) through no fault of your own bad luck attacks

a reply to: JIMC5499

Your base-thought about letting her tighten herself so she can get if off isn't a bad one.

But breaking torque is not the same as tightening torque. There are three types of loads you put on a threaded screw or bolt when you tighten it. Friction inside the thread, friction on between the head and clamping surface and stretching the bolt. In most cases you stretch the bolt during tightening.

Breakoff torque is composed of two loads and one counteracting load, namely the stretch inside the bolt works as a sort of ramp that makes loosening easier, around 10% for the typical bolts used on cars. When you discard other factors that accumulate in between tightening and loosening, like rust, then you can expect that your breakoff torque is a lot lower than your setting torque.

And the torque setting will change over time. For example camshaft bearing block bolts, I use three different gradually rising torque settings to do this, to arrive at the correct torque level throughout all bearing blocks. And it's not necessary for all heads I've worked on, but for a few. I do this because it's not really measurable if you got the right torque even with my GARANT HCT, it clearly says stop ASAP when the torque is reached because of the 10% stretch effect.

Some bolts also only stretch once and the next time they snap but these are engine block-head joint bolts though, where precision like I wrote above matters.

I always set the wheel bolts with my hands and sometimes I overshoot with a 1ft bar for typical street cars (120NM/88.5ft/pound).