On Tue, 03 Mar 2015 17:39:14 +0000, The Medway Handyman wrote:

I use M17 sockets/spanners a lot for fixings.

Do you mean M17, or do you mean 17mm? 17mm would normally be M12. M17
would be one weird ******* non-standard thread, and probably about a 25mm
head?

Preferred hex-to-thread chart
https://www.boltdepot.com/fastener-i...ric-bolt-head-
size.aspx