Originally Posted by tap
I don't think the pump out guy used any kind of adapter.
Some pump-out hoses have a rubber conic section that gets pushed into the waste deck fitting. One holds it down to make a good seal.
Other pump-outs have a threaded adapter that is tightened into the deck fitting. The rest of the suction hose snaps onto the threaded adapter.
With respect to your cap, perhaps it is a metric cap? I know mine (Swedish boat) often create some confusion at places that use the threaded adapters. Since Crown's were made in Canada it is conceivable that it is a non-SAE (English) thread and diameter.