Stainless Steel Bulkhead Fittings are typically used for 2 reasons. Either you are storing a liquid that is not compatible with standard Polypropylene Bulkhead Fittings. Or you are storing liquid in an industrial environment and prefer having a more durable fitting in case of accidental impact. These Stainless Steel Bulkhead Fittings are milled from a solid block of stainless steel and are double threaded. The inside threads go all the way through so something can be threaded in from the inside of the tank. These reverse threaded fittings also need an application of anti-seize before tightening down.