TORQUE UP A STORM

Although torque wrenches are convenient to use, torque is a poor analogue of induced stress due to the need to make assumptions regarding thread and shoulder face conditions – for example, perfect threads and shoulder faces either dry or with a specific lubricant. Often clamping force must be limited due to the nature of the parts being assembled and sometimes the female thread is not a nut but a tapped hole in a component of unknown, and presumably low strength, material properties (eg, light alloy housings).

The recent trend in CM for tightening fasteners to be specified by a ‘nip up’ pretorque followed by a specified angle of turn is to be welcomed. However, where tightening torques are specified, CM articles tend to be inconsistent. For engine work, it is reasonable to assume that threads will be in good condition and lubricated with engine oil. For suspension work, fasteners could have varying degrees of rust, may be easily reused or may be impossible to remove without damage, necessitating replacement.

I suspect that motor trade practice is to renew all fasteners disturbed during suspension work, and manufacturerrecommended torques based on new fasteners with dry threads, whereas I imagine that most DIY mechanics reuse fasteners where possible and also apply anti-seize to exposed fasteners.

Something which puzzles me is that cylinderhead bolts on modern cars are often described as being ‘stretch bolts’ and not to be reused. In ordinary usage, stressing a bolt beyond the elastic limit of its material would render it scrap. Are these bolts actually intended to suffer permanent elongation during assembly? The only time I have come across them was many years ago when I needed to change the cylinderhead gasket on my Vauxhall Cavalier 1.6 petrol. The bolts removed looked perfect, but I did not take the chance of reusing them, instead following the Haynes manual instruction to renew them.

Martin Keating