Looking back over some other past posts on this subject I think they can be Imperial OR Metric depending on manufacturer, some of mine are 3mm across the threads, some are 4mm, and the threads themselves seem to be what used to be called "Bicycle threads" - they look "rounded off" and very coarse, and I have discovered through a lot of fruitless internet searches it is now quite hard to find taps and dies to cut these threads. I am pretty certain that someone on the forum once found a supplier for these tools but don't remember who. I'm pretty sure it was a UK member. I would certainly like to make my own if anyone knows the detailed specs.