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To add to the above comments, 'Stretch bolts' I have encountered have always been for the engine head(s), due to the composition of the metal they stretch to a set measurement at a pre-prescribed torque. ( Or turning angle )Once used they will basically not stretch to the correct settings again, due to the loss of elasticity (ductility )hence they are single use only. Generally thread lock is not used with stretch bolts unless the makers specify it's use.
On large ( Marine ) engines studs are stretched using hydraulic jacks, with up to 1600 bar pressure then the nut is tightened by hand, checked for no clearance with a feeler gauge and the hydraulic pressure released. The elasticity ( or stretching ) of the stud ensures the correct tension is applied, but due to the metal composition, these are re-usable and the term 'Stretch bolt' does not apply.
In some applications, ie Brakes, bolts can suffer from excess heat, heating and cooling a bolt will alter the metals properties. If a bolt is heated and suddenly cooled, it will effectively be hardened, thus changing the ductility, making it less elastic and more brittle, thus weakening the bolt, so safety could be affected. One of the reasons they change these bolts, plus corrosion etc.
Bolts subject to heat on say an exhaust manifold will not cause a safety issue on failure, unlike on a braking system.
If you have read this far, then award yourself a large libation or two, and maybe even some tasty comestibles, AKA a real ale and crisps. 
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23rd Apr 2021 9:19pm
