On Monday, 12 December 2016 15:40:45 UTC, Larry wrote:
I have removed a twin socket from its back box.
Is there a trick for "tightening" the thread such that the bolt
will hold in it

Squeeze the lug gently with pliers. You can get M3.5 rethreader tools from electrical places.

https://www.tlc-direct.co.uk/Manufac...ers/index.html

M3.5 is for socket faceplates, M4 is for conduit fixings.

Owain