Steve Rothwell responds:

Any inconsistency in the torque settings supplied by CM can only be attributed to the main dealer information, as all settings used are as specified by the dealer. It is general practice to assume that a bolt or fixing being fitted is in good condition and has a clean and mostly dry thread. While variations can occur – if, for instance, the bolt was too tight to remove and had been lubricated with a penetrating oil – this should still not affect the tightening force dramatically as the removal process normally wipes the threads of the applied lubricant.

Most vehicle torque settings are worked out by the manufacturer based on the material used and the force required. One exception that is applicable to the heavy goods industry is the tightening of wheel nuts on twin-wheel vehicles. There have been major problems with the tightening of wheel bolts which are trying to clamp two separate surfaces onto the hub, but this is a separate matter and not really applicable to the operations we cover in CM. Very often, components that require new fixings when being installed will have the fixings supplied with the new part – this is applicable to factor parts as well as main dealer units.

Stretch bolts are designed to perform at the correct clamping force. This is only possible by stretching the bolt to the designed force according to the metallurgical property of the bolt. As cylinderhead bolts cannot be measured once fitted, the stretch cannot be fully determined and so the bolts are effectively disposable after use. As they are recyclable metal, this is not a major problem.