If you manufacture your own aluminum radiator, the Mishimoto aluminum coolant fill neck is a must-have. The Mishimoto aluminum coolant fill necks are CNC machined and ready to be welded for use with your custom project. These aluminum fill necks are compatible with our Mishimoto radiator caps. The large Mishimoto CNC machined aluminum fill neck is 42 mm and includes a screw-in nipple for overflow.