How do you determine bolt fastening torque?

Answer 1

It depends on what the bolt is made out of ,if it is metric or standard, the thread pitch and what you are screwing into and if you torque it dry or lubricated... but ALL BOLTS HAVE A TORQUE:

If it is a bolt going into the block for example: the manufacturer has a specific torque it needs to be at. Alot are torque to yield and have a torque and torque angle the bolt needs to be at.

Another example is on BMWs: they use alot of Aluminum bolt to attach accesories like alternator to the block, they don't have marking(thier usually painted blue and E torx). The torque spec. is crutial do to how easy the can break.

The material your screwing into can be the limiting factor, For example an aluminum block without a steel insert. You can easliy pull the threads out, especially if it overheated(again important on BMW's).

Their are general torque specs. charts avail. from the manufacturer, listed By the markings on the bolt head and the thread pitch: 1/2 -20 grade 5 bolt for example, from one manufacturer is torqued to 90ft-lbs DRY, 65 ft-lbs LUBRICATED.

the lubricated itself can cause the torque # to change, torquing engine bolts with MOLY lube requires less torque than oil do to how slippery moly is.

On high end connecting rod bolts, torqueing isn't even recommended... These bolts are under such stress that the manufacturers recommend measuring BOLT STRETCH.