Torquing with just torque values such as foot-pounds or newton-meters is at best a guess at getting the torque correct. Depending on the type of fastener, materials used for both the bolt and the part it screws in to, cleanliness of the thread, lubrication used, TYPE of lubrication used, etc., the torque between two "identical" fasteners can actually vary by as much as 35% (or more) with the torque wrench having the same setting for both fasteners!
Many fasteners, such as wheel studs, water pump bolts, etc., are just there to make sure that the bolt is probably tight enough, and probably not too tight. These are the bolts that you reuse over and over without any problems.
Head bolts, as well as most of the bolts internal to the engine, are what are called stretch bolts (also known as torque-to-yield bolts). These are designed to be tightened to the point they physically start to stretch, usually by a few thousandths of an inch. This leaves them in a elastic condition, that allows very equal clamping forces to be applied to the part they're holding down.
When you tighten one of these bolts, the torque setting that is used is designed to get the bolt close to the point that it starts to stretch. The torque angle guage is then used to put the bolt PAST the point it starts to stretch and actually start to stretch it. You just can't accurately measure this with a regular torque wrench, thus the torque-to-yield/angle-to-turn method.
The reason for not re-using sttretch bolts?
The reason is quite simple. Once you have stretched the bolt once, it never returns to its original size. This weakens the bolt, and if you try to re-use it, you take a risk of snapping the bolt in two, leaving the threaded part in the block, the top in your hand, and a complicated repair required. It's just not worth the risk to try to re-use old bolts.
You can get replacement bolts that are not designed to stretch at the torque required to hold the parts together.
There are two good reasons for using non stretch bolts. You want to re-use them in the future, and/or you have an engine that is either forced induction (745i running massive boost for example) or very high compression. Since the bolt is already at it's maximum stretch without losing strength, you don't want to put undue stress on it from a high compression engine and take the risk of a blown head gasket.
There is also a good reason for NOT using these bolts, and staying with the original OEM style stretch bolts. Non stretch bolts require retorquing after a few heat/cool cycles (usually specified at around 5-6 cycles), and may have to re-torque them anywhere from every 30k to 70k miles, depending on application. If you have a normally asperated engine, there's just no reason to have to do the extra work when normally you wouldn't touch the bolts again after they've been done properly with regular OEM stretch bolts. Also, stretch bolts tend to provide more consistent, even clamping pressure than the solid bolts do.
Blatently lifted from
http://www.xmission.com/~kd7olf/torque.html. :-)
Ray