4" (small) Allen Hatch . Includes hatch cover and base but no "O" ring. Also supplied with an Allen Hatch Cover Bag
The gasket (base) is fixed to the hull of the boat and the O Ring (A.338) is placed around the gasket, under the small lip. The hatch cover lid is then screwed tight onto the gasket and forms a seal with the O ring around the external diameter of the hatch.