A 38mm cap and neck set for fabricated coolant tanks. The mid-size 38mm neck is a sensible general choice: wide enough to fill and inspect coolant level without funnelling awkwardly, but still compact enough to fit a tank tucked into a modified engine bay. Supplying cap and neck as a matched pair means the sealing faces are made for each other, which is what keeps a pressurised cooling system holding pressure rather than seeping at temperature. Weld the neck square and upright, position it as the high point of the system for proper bleeding, and never open it while the system is hot.