Supplier of hydraulic clutch lines and connectors

www.rallydesign.co.uk

Can supply Euroquip braided hose and fittings.

You mean the actual hardlines?

It’s no different to brake pipe stuff. Indeed some cars have larger diameter brake lines. Pre war fords and Chevys are all 1/4” for example.

Brake stuff is 3/16”. It may get metricised to 4.76mm or even 5mm, but it isn’t.
Clutch hardlines will proably be 1/4”. Or 6mm or 6.5mm depending on the seller.
You will also see 5/16” (8mm) and 3/8” (9.5mm or 10mm) These are generally used for stuff like fuel lines and transmission cooler lines.

Thread sizes for fittings can be metric or imperial, you’ll have to use your best judgement given the vehicle to determine what. Be wary of stuff like Landrovers that can be either or both depending on the age of the vehicle though.

For the amount of fittings you will need to do one car it’s proably going to be easiest just to buy odd ones in the right thread sizes and a few metres of line off eBay at the moment.