Bolt torque discussion
Well theoretically all recommended torque spec are based on the optimum stretch for the material, but as I understood it the longer a fastener, is it would need slightly more torque to be optimum.
Going back to post #5, for a head bolt, the objective is the clamping force.
- The torque or stretch is just an indirect indicator that you got there.
- The bolt size and material is to make sure you can get what you want with adequate margin and thread strength in the fastener and the matching threads.
I'll respectfully submit that there is no 'optimum stretch' per se, just what you need to get the stress (and tension) in the bolt to where you want it to get the desired clamping force on the gasket.
The reason I asked about the stretch was for the topic of the different length head bolts. If you looked for stretch to be a constant in proportion to fastener length (example: twice as much stretch for a 2x fastener length), then the longer the bolt, the more the stretch is needed to get to the same bolt stress level... and thus the same clamping force. If you need the extra torque to get to the same proportional stretch, then that is what you do. You theoretically should not have to, but the torque to stress relationship has a lot of variables that are impossible to always 100% quantify.
So different torques could make some sense for the different head bolt lengths
IF they are trying to get the same proportional stretch and thus to the same clamping force. Even if the higher torque is used to get to the same proportional stretch (and thus the same bolt stress and clamping force), then the total force on the threads in the block is the same, regardless of the torque levels.
Hope this makes some sense..... and yes, I imagine it is boring for some!