1/4" nickel plated brass bulkhead with o-ring and a plug. This is our very own, custom engineered bulkhead fitting. Body is machined from a single piece and produced in low quantities. Perfect for use as a reservoir bulkhead. Big upgrade from the value plastic bulkhead. Features fine thread, allowing the nut to be tightened in smaller increments without the fear of stripping. Has a slightly wider lip to better accommodate the o-ring better. Fits into 5/8" hole. For a super tight fit in a plastic reservoir, you can wiggle it into a 9/16" hole.