Re torqueing head studs/bolts

To do a proper bolt stretch, you have to have both ends of the bolts ground flat and parallel to each other with centers drilled.

Then use a ball micrometer to measure the bolts before the first stretch cycle, and then every one thereafter for accurate results...

I do not know of any way to pick a used bolt and measure how much it has stretched.

I've set up the multiple spindle machines on assembly lines to tighten them.