Finished making the header tank today
Added a schrader valve to the top so that we can use a tyre pressure gauge to check what pressure the coolant system is at whenever we want. Also more importantly it allows us to pump more pressure in if we've ever had a leak or had to take the cap off for some reason while the car was still hot and therefore lost all of the pressure (higher pressure means higher boiling point so we don't want to run at just atmospheric pressure).
Used some polycarbonate tube for a sight glass which should be ok for the temps this will see. I've seen some people just using translucent silicone hose for this but most of those are only rated for about 70 degrees C so didn't seem like a good idea to me when I've seen 120 degrees C on the coolant temp gauge in the past lol
