How important is it to Torque Bolts?

[Steve_God]

I've been doing the odd bits and bobs of servicing my bike since I've had it (over 3 years) including a chain and sprocket change, and never felt the need for a Torque Wrench.
I too am tight with my money lol

Personally... if I started looking at taking apart parts of the engine, or other related internal type stuff, I'd buy one. But for now, I don't see the need

[timwilky]

If I have to use my big torque wrench anywhere near a bike there is something wrong. It is a 4ft 3/4" drive britool jobby I used to use for truck wheels.

I do have 1/2" and 1/4" drive ones as well for small stuff.

why use a torque wrench. Well the idea is to maintain a fastener within the elastic region of a stress/strain relationship. Once you have overtightened 2 possible effects are :-

you strip the threads or you have passed the fastener yield point (Not fracture point) and and stretching of the fastener which gives it its clamping effect is permanent and the more strain you apply there after will not result a proportional clamping force.

It is impossible without specialist fasteners to know the stress you are exerting on a fixing. Therefore fastener manufacturers have invested time and money in investigating torque needed under specific conditions (Ie dry, with thread lock, etc) that will produce a clamping force yet leave the fastener "elastic" and therefore exerting its clamping force.

All this palavar and techno speak---learners code--tighten it till it strips and then go half a turn back NO NO NO

[yorkie_chris]

Most of torques in the haynes etc. are dry torques, so when you use copperslip, alloy antisieze etc etc you actually exert more tension force than the manual reckons.

So even with a calibrated torque wrench there is still some uncertainty.

i use a tw' for every bolt on every bike..im fastidious wi things like that..great tool

[yorkie_chris]

Inside engine nearly every bolt, outside I don't bother.

[embee]

Just an additional note to what the others have said.

Many modern engines have major/critical fasteners, i.e. cyl.hd., main bearings, con-rods, designed as "torque to yield". This means that the fastener is designed to be tightened until it is into yield, at which the clamping load is well controlled by the design of the fastener itself. This tightening is done with a torque/angle sensing system so it recognises when the torque does not increase proportionally with the angle turned and the material is yielding.

These fasteners are not intended to be repeatedly re-used, often a limit of one or 2 re-uses are specified (bearing in mind they are often already used once during manufacturing/boring etc.), then they must be replaced.

Since in the average workkshop the yield sensing wrenches are not that common, an alternative method of "torque plus angle" is specified for servicing, this applies a known torque which takes the fastener near to yield, then an angle of typically 90deg which is known to take it into yield but not so far that the material will fail.

AFAIK no commonly serviced fasteners, like brake calipers etc, are ever designed like this.

A word of caution is to be extra careful when dealing with fine pitch threads because the "feel" is much reduced compared to standard coarse pitch ones. Caliper bolts are often a fine pitch. When dealing with these I like to set the torque wrench at maybe only half the spec and see what it feels like, then work up towards the spec value in two or 3 steps, bearing in mind what the others have said about lubrication reducing the necessary/desirable torque compared to the spec, so if it says 28Nm in the book I might only use 25Nm (i.e. 10% reduction).

Often people over-estimate the necessary torque, a common one is with car wheel nuts. 100Nm is not all that much, a 1/2" torque wrench will be about 1/2 metre long, so needs 200N force at the end, i.e. 20kgf or only about 1/4 a typical person's weight.

I certainly don't use a torque wrench on everything, but like some of the others here I've been doing it for an awful long time.

[G]

I am quite anal when torqueing up bolt. To many bad experiences in my early days of over tightening the like of pinch bolts....snap

[Taipan]

Originally Posted by yorkie_chris

Inside engine nearly every bolt, outside I don't bother.

Same here. I dont like retorqueing bolts that have previously been done up with a TW. If I do I'd back off the setting a bit. In the main I have a "feel" for how tight to do things and its served me well so far....