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[Nick W]

don't understand why a bolt will move by hand when the head has been cut off?

A bolt thread is a wedge wrapped around a cylinder. Screwing the bolt up tight stretches it. If you then cut the head off, the tension and therefore the stretch is lost and you can easily unscrew what's left. This doesn't work if the bolt won't undo due to a knackered thread!

[Andy B]

don't understand why a bolt will move by hand when the head has been cut off?

It's the back of a bolt head that you pull against to tighten it  ..... assuming it doesn't bottom out first into a blind hole.

[robson]

Thanks I can see it now

[serek]

It's streath bolt so ones you grind head off you take tension of bolt
Simple as that
I find gas torch do job every time , new bolt only cost £6 from Gm

[LC0112G]

It's streath bolt so ones you grind head off you take tension of bolt
Simple as that
I find gas torch do job every time , new bolt only cost £6 from Gm

Two way's of looking at that :

1 : There is no such thing as a stretch bolt - it's a normal bolt that has been tightened past yield.
2 : All bolts are stretch bolts - if you tighten any bolt past yield they will stretch permanently.

If you cut the head off any bolt you will relieve the tension in the shank  which should make it easier to remove - provided it hasn't been Loktite-ed or rusted/corroded in.

Stretch bolt is a misnomer for a bolt where the manufactures installation spec deliberately takes the bolt past yield, and is usually identified by a tightening spec of X Nm + Y Degrees. It is unwise to re-use a bolt that has been tightened past yield.

[Shackeng]

https://en.wikipedia.org/wiki/Bolted_joint

[terry paget]

https://en.wikipedia.org/wiki/Bolted_joint

Very interesting. Looking back, when changing front wishbones I have always got away with not changing the mounting bolts, and come to no harm, and and similarly when replacing front brake calipers.
 I enjoyed the final sentence in Wikipedia 'Each method has a range of uncertainties associated with it, some of which are substantial.'