Robert wrote:
Tightening by torque is a very inaccurate method. Bolted joints work by clamping the components together and using the pre-load (or stretch) in the bolt to keep them in contact. Depending on the type of fastener, this preload can either be below or just above the elastic limit. For most of the fasteners on a car it is below the elastic limit, which means there is no permanent stretch in the bolts.
The 'stretch' is very small, but it requires a good deal of force to stretch the bolt. This force becomes the clamping force.
Typically, in a bolted connection, the torque you put in is consumed roughly, on a dry faster by 90% overcoming friction and 10% actually stretching the bolt.
The friction is both under the head of the bolt and in the threads themselves, the exact split can depend on the style of the bolt and the material it is made from
So if we oil the bolt, on the thread and under the head we might perhaps redce the 90% to, let's say 50%. (These figures are random, but probably representative)
Putting the same amount of torque in means that the effort needed to overcome friction has almost halved but the stretch on the bolt has gone from 10% to 50%, which will probably take it way past the elastic (and probably plastic) limit, and into failure.
Tightening by angle (e.g. 90º ) is much more accurate and independent of friction. For example an M12 x 1,5mm bolt tightening by (say) 120º (one third of a turn) will stretch the bolt by 0,5mm. If the length of the bolt is known then the preload can be calculated. It will always be the same preload.
If the bolt is bone dry the friction will be broadly the same, but there will be scatter and from above, you can see that a small amount of scatter can make a vast difference on preload.
Putting washers on further varies the preload a set amount of torque can deliver.
If the SAAB manual says lightly oiled, I'd go with that.
Great stuff but if the book of words says torque to 110 ft/lb then I would read that as “applying this torque will ensure an acceptable preload”. On occasions I have seen torque settings in nn ft/lb then back off and tighten to some percentage of nn plus some number of degrees. In practice, applying the manufacturer’s specified torque, either dry or lubricated as required, will be perfectly adequate. I suggest that RB211 engine mount bolts are rather more critical than anything on a car. (No, I can’t remember the torque setting or the tightening process and it may include a final torque plus a number of degrees, the last engine change I was involved in was more than 25 years ago.)