Tech 101: Why Fastener Torque Matters "Güdentight" isn't a torque rating

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My introduction to "how tight" was gleaned originally from using Mechano as a child, where you could make basically anything using nuts and bolts to join various standard shapes of metal (with lots of holes), plus wheels, axles, gearwheels, and, if you could afford it (I couldn't) electric motors. More seriously, when I "inherited" my first motorcycle from my brother, a Tiger Cub, all maintenance was down to me. This included stripping the whole engine more than once as bits broke or wore. I learnt the hard way about doing bolts up too tight (aluminium crankcase threads strip surprisingly easily), or too loose (the usual seepage of oil from the joint faces would become nearer a flood).

I still have the original instruction manual that details everything to strip, check and rebuild the whole bike. As far as I can see, not once is the word "torque" used, just phrases such as "fully tighten".

So, as far as my normal maintenance is concerned, I believe I have a good "feel" for how tight to do up fasteners. This is particularly important where a thread may or may not be lubricated in some way, the necessary torque differs significantly, which is where, in the general case, "feel" is better than a torque figure in a manual.

Where a torque wrench may be needed is on things like cylinder head bolts, where a significant stretch is needed to cope with differential thermal expansion. I've seen instructions on these, tighten to a specific torque, then tighten another quarter turn. Even here, the initial stretch after the first torquing may well be different depending on the thread lubrication. My guess is all quoted figures are for dry threads in the factory (unless lubrication such as by a tread locking compound is specified), so too high for normal maintenance where threads have oil or grease on them.

For all normal maintenance, a good feel is adequate, but it must be a "good" feel. I am perfectly happy to trust my tightening of critical components under any conditions where I am prepared to ride my bike.

PS. For anyone who wants to see a (well thumbed and oiled) manual for a 1954 Tiger Cub, I put scans of it here. Please be aware this may be copyrighted material, so no use other than personal, please.