A weld-in bung threaded M22 x 1.5 to accept a thermatic fan switch. Custom cooling systems, alloy radiators and hose-routed conversions frequently leave you with a switch and nowhere factory to screw it into. Welding this bung into a radiator tank, header tank or fabricated coolant pipe gives you a proper sealing port instead of an improvised adapter stack. Site it where coolant genuinely flows past the sensing tip, not in a dead corner, or the fan will trigger late. Clean the parent metal thoroughly before welding, and protect the thread from weld spatter so the switch seats correctly afterwards.