Nice post Aidan.

Quote...

""but whats to say the first torque is any more accurate than the last??""

That is a good question Bradley.

The answer I believe is that the stresses that will influence the torque wrench "click off", that are imposed on the bolt are less at the lower torque range.

Take for example the effect of static friction.

I am sure that we have all experienced the loud "crack" that a head bolt makes when it subjected to the final stage of a tightening sequence. This loud crack is generated when the static friction threshold is surpassed.

If you think about the amount of effort required to start the bolt turning, in that final head bolt tightening sequence, compared to the effort required to start the same bolt turning, when it has only been tightened to 25 Nm, you then begin to understand why we now use a low torque in the first instance, followed by several angular tightening sequences.

I am sure that in the old days, there were many examples where technicians had inadvertently thought that the head bolt was tightened correctly, when in reality the bolt was not tightened optimally due to variations in technique.

This can be demonstrated nicely in the workshop, If you have a sacrificial car & the inclination to try it.

If you set your torque wrench to lets say 150 Nm & proceed to tighten a head bolt, you will see a significant difference in the final resting position of the bolt, depending on your technique. For example, if you perform the final tightening action, in one fluid movement, without pausing, you will find that the resting position of the bolt will be further (read tighter) than if you were to pause momentarily before the final 150 Nm torque wrench "click off" is heard. The momentary pause will often cause the torque wrench to "click off" prematurely before a further movement has taken place, giving an misleading indication that the bolt is tight enough.

On top of all that, imagine the influence of thread condition & lubrication used. Not all technicians apply the same standards. A tight thread & a dry head bolt washer will influence the final torque reading more so, when compared to a bolt that is tightened thus.....

25Nm
45 degrees
45 degrees
45 degrees

I hope that makes some sense.