road and
Page 37
If you've noticed an error in this article please click here to report it so we can fix it.
workshop by Handyman
Details to save down-time (7)
Correct use of the torque wrench is not always as simple as it appears
THE OLD TIME motor fitter, it is said, could tackle almost any repair job and see it to a satisfactory conclusion; the modern mechanic is frequently criticized for either lack of skill or interest — and certainly a number of silly failures are experienced by commercial vehicle operators.
However, a study of mechanical failures shortly after repair, or the fitting of new parts reveals a picture in which blame can be apportioned between the factors of time to do the job and inattention to essential detail.
Facts from many shop floors tend to confirm that the time element is primarily responsible: (a) in that time is not available to take many of the precautions and checks called for; and (b) neither is time being allocated for teaching these points. So I propose to look at service and repair tasks both on the bench and around the vehicle where lack of attention to detail is known to promote unnecessary expense. First, I must say, however, that today's motor vehicle repairers face a far more complex machine than did their predecessors, and to compare the technology called for nowadays and that needed by the men in the Thirties would be somewhat unfair.
Beating the clock
Nevertheless, in the race to beat the clock, earn an output bonus, etc, we are missing out on many useful tricks of the trade. To take a rather simple example to start with, the correct use of the torque wrench is not always as simple as it appears. Cylinder head nut tension is now a critical matter, particularly with highcompression oil engines, and repeat gasket failure may not necessarily mean poor joints, warped heads or upset block face. Instead it may be no more than the incorrect use of the torsion or torque wrench, or the condition of the wrench. The final task of tightening up a cylinder head after assembly and the initial warming up run, is
the point where the time element can undo the good work already done.
Before coming to the use of the wrench itself, one point must be made. Where head nuts are hidden, or access to them obstructed by valve rocker shafts, the obstruction must be removed despite the extra time involved.
Now, regarding the use of the torque wrench, when the mechanic applies the tool for the final nip-up and the engine has been warmed up, it is no use dropping the socket over a nut and giving a quick heave, as in 99 cases out of the 100 the nut will not move at all, although the torsion wrench will in fact break away, giving a false impression that the nut is at the correct tension. The warm nut will stock solidly, but if the wrench action is reversed and the nut given a slackening half turn to break it free, it will then be found that the wrench at its correct tension will tighten the nut up at least a quarter of a turn further than the point at which it broke free. This is in fact, the really critical part of the final nip-up.
The extra gain at the correct tension is what is needed to seal down the head joint effectively against the effects of operating temperatures and full power stresses. So where the torsion wrench is used, remember this practice of easing back first and see that this is applied to every nut.
Now for a second point about the torque wrench. This tool may not appear to alter its performance even to the regular user, but in fact it can begin to break at a reducing tension and after a period can be down below the tension called for. In the large firms where assembly lines make heavy demands on wrenches, a tool man and a test machine are fully employed in testing and correcting torque wrenches. Although the garage wrench is used far less intensively than the works tool, it is still sound practice to return your tools to the maker who will test and correct — usually for little more cost than the postage.