These bayonet style gas caps (also known as cam gas caps) are vented and are black steel

found on K Model bikes and Ironhead Sportsters 1952-1980

Center to center hole mount distance is 3

Internal hex for easy installation

While these tanks are pressure tested during manufacturing

Diamond Tail Light - Chrome Bar Clamping Diameter_1 inch These bayonet style gas capsThe Diamond Tail Light features two threaded studs for easy mounting on most flat surfaces. A 12 Volt 21 5 watt dual filament bulb allows this to be used as a running and brake light. Features a chromed plastic body, red acrylic lens, and a clear acrylic lens for license plate illumination. Measures 2 3 4" high, 5 1 2" wide and 2 1 4" deep.