Car Mechanics (UK) - - Your Letters -

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

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