The effect that the method of tightening has on determining what size of bolt is required to fulfil a specific function is largely underestimated. If several bolts of the same size are tightened by the same method then there will be variation in the bolt's preload - they won't have all the same value. This variation is influenced by such factors as variation in friction characteristics in the thread and under the nut face, thread form and pitch variations, variations in the surface flatness etc. Hence for any particular tightening method there will be a maximum anticipated preload and a minimum given a set of conditions.

The tightening factor is a measure of the scatter in a bolt's clamp force as a result of the tightening method used to tighten the fastener. It is defined as the maximum bolt clamp force divided by the minimum value anticipated for that tightening method. For tightening with a torque wrench the tightening factor is usually taken as 1.6; i.e. the maximum preload value is 1.6 times the minimum.

A joint diagram showing the effect of preload variation and embedding is presented below.

Since the bolt is not to be broken by overtightening on assembly, it must be selected for the maximum initial preload. Hence for a given bolt size, the smaller the tightening factor, the larger the residual preload is remaining to sustain the applied forces to the joint.