
It came off with a 22mm socket, although there was a slight amount of wobble so I suspect it is imperial. It has the number 5155 on the face and the
M10 replacement I bought doesn't fit, it starts for about a thread and then binds up. Any ideas what I should be getting?
looks like one off a mini with imperial thread
http://www.cbsonline.co.uk/stop-light-switches-116-c.asp
There you go
i bought one along time ago for mu project but went for a motorbike style switch with a double babjo fitting, m10 thread. Prob 7/8" size.
I think it is a morris minor one, not 100% sure as t was years ago.
Darren
Thanks 
just checked also fits a moris minor.
Darren
They come in M10x1.0 taper thread
or 1/8-27 NPT taper thread
as mentioned they are a taper thread. you should get about 3 turns before they go tight then you need to use a socket.