Originally Posted by Stephejl
1. It isn't important to get it exactly xx degrees. Important torques are never given in degrees. The place you'll see that spec given most often is in structural steel and they usually give it in flats, i.e., 1/6 of a turn. It is an approximation and stretches the bolt differnt ammounts depending on how long it is. I don't like to use this spec and will generally calculate a torque to replace it. It's only advantage is that it doesn't depend on the condition of the threads to achieve correct tension in the bolt.
2. ALL bolts stretch, not just special bolts. That's why you torque them. Stretch, called preload, is what keeps bolts from backing out.
3. Torque goes up lineraly with stretch unless you reach the plastic region, i.e, past yield point. That will not happen on any motorcycle bolt. I've heard of aircraft parts spec'd this way but have never seen it myself. So your 3rd paragraph above it totally incorrect.
My advice is to always clean and lubricate your bolts well and use the torque wrench.
I'm not "off" at all. The BMW factory specification for tightening the big end bolts in a K100 (for example), states SPECIFICALLY that the bolts in question can only be used once, because they are designed to stretch PERMINENTLY when tightening. This specification also SPECIFICALLY states that it is necessary to tighten to a specified torque, AND THEN to tighten EXACTLY a specified number degrees further.
This is also stated very clearly in the Haynes manual for the bike, where they take care to explain why all this is necessary.
For bolts such as these (which are not uncommon for big-end bolts in many engines), it is absolutely impossible to get the required amount of stretch using a torque wrench. Once the bolt starts undergoing this required permenent deformation, torque no longer increases as you turn the bolts more. In fact, torque actually starts to decrease a bit when you pass the yield point, so there is no way to tell when you have stretched the bolt to the correct degree, except to measure the number of degrees you have turned.
Of course, torque is relatively linear until you reach the yeild point. Anyone knows that! The point here is that these bolts are DESIGNED to undergo a carefully defined amount of plastic deformation, meaning that a torque wrench is useless in this case for accurate tightening. This has nothing to do with the way "normal" fastenies are preloaded, and torqued.
So, for those few, highly stressed bolts that are INTENDED to be stretched past their yield point when tightening, your "advice" of using a torque wrench alone contradicts the manufacturer's requirements and can lead to failure.