1 5/8 dia Stainless Steel Filler Cap and option for raw steel or aluminum weld in Bung. Great for building custom oil tanks and gas tanks. cap is now vented so if you plan to use it on a gas tank you can remove a set screw to reveal a small vent hole. also you can use the vent hole as a way to tighten or loosen the cap by sticking an Allen wrench or pick in the hole.