Oil Fill Caps All machined from billet aluminum and available in a variety of applications. Choose from push-on, twist-on, screw-on, or weld-in styles. Weld-in styles can be used by welding the bung to valve covers, rear end housing, etc…. Weld-in bungs are available in 6061-T6 aluminum or 1018 steel and are 1-5/8'' in diameter. Fully polished.