In practice the stainless steel plate and the flange around the port are not always perfectly parallel. If theIy aren’t parallel there won’t be a good seal. If water can flow in and out of the port as the boat heals the ballast will not be as effective. The foam layer compresses more easily than the harder rubber and accommodates any mis-alignment.

From bottom up:

1st: Large neoprene foam gasket with two holes, center hole (over the long vertical stem) and off center hole (over the shorter anti rotation pln), is fitted down onto the large plate. The soft neoprene foam ccommodates any slight misalignment between the stainless disk and the underside of the hull.

2nd: Large rubber gasket (with same two holes as above) creates a good seal to the underside of the hull.

The long threaded rod is insterted from below the hull with the two previously installed gaskets in place on the plate. The anti rotation pin is allinged with the existing hole in the rectaangular plate, with the larger rod passing through and out the top of the tank, inside the boat,

Last (inside the boat): The small rubber gasket with only one hole is fitted against the top of the tank. Next is the flat washer, then the wing nut.

Finally, after all gaskets, washer and wing nut are installed, insert the keeper ring to prevent the wing nut from coming off the top, which would result in the loss of the entire assembly. 

If you purchased the complete kit #E1877, a rectangular plate is. provided. It holds both the vertical stem of the valve plate assembly and the anti-rotation pin. It is installed across the diameter of the port in the hull at the bottom of the tank. You may not need it, but it is always included with the complete kit just in case yours is missing or damaged. It is not available separately.