Hi

Re stretch bolts, my opinion is that this is a slightly misleading description and it would be better to refer to them as > spring bolt < as it reflects there purpose.
However as they operate in there 'plastic' region this can result in a permanent stretch when tightened.

These type of fixing bolts will be relatively long and thin, the idea is that when correctly tightened it extends like a spring. The result is that when the head and gasket expand / contract/ settle in service the ' spring ' effect of the bolt maintains its clamping force.

The advantages are clear, no re torquing needed after running, clamping force maintained through operating temperature range.

Re torque / angle tightening. The sole point of tightening a fixing correctly is to obtain the correct clamping force Angle tightening does not automatically mean that the fixing bolt is 'stretch type '

The problem with traditional torque method is that it gives inconstant (clamping force) results as so much depends on condition of threads and shoulder i.e smooth / rusty / dry / lubricated.

As an example we will all be aware of the difference oiling the threads on a humble puller has.

Tightening by angle will give more consistent results as condition of threads has no effect, the clamping force being predetermined by the pitch of thread and type of bolt.

Agreed a pre torque is used but the % error on total clamping force will be much less.

cheers Aidan