316 stainless steel. Hand polished and electro-polished to protect against corrosion Each Deck Fill is marked with either: DIESEL, WATER, WASTE or ESSENCE. DIESEL Deck Fills come with a stainless steel tang for easy attachment of grounding wire. Each Deck Fill has an o-ring gasket for a tight seal. Your choice of Deck Fill cap: Standard type which opens with a key provided or Sailing yacht type which opens with any standard winch handle.