Billet Aluminum threaded cap with o-ring seal. Can be used for oil tanks, vent cans, fuel tanks, coolant tanks, over flow tanks, or pressure tanks- this is the part you need. CNC machined from 6061-T6 aircraft aluminum. Cap has knurled edges and comes with o-ring seal for a pressure tight seal. Weld in bungs are machined from 6061-T6 aluminum, or 1018 mild steel. Cap outer diameter is approximately 1.75 inches.