The invention is directed to an adapter used in a marine fluid handling system to prevent thread mismatch between a thru-hull fitting and in-line valve. The adapter includes a head with a male NPT thread and a bore with a female NPS thread. The adapter is threaded to a thru-hull fitting with a male NPS thread that extends through a hole in a boat hull. The adapter is also attached to an in-line valve having a female NPT thread. The valve may be mechanically fastened to the adapter to prevent thread disengagement. When the valve fails, the valve is easily removed from the adapter that remains attached to the boat hull, and a replacement valve is attached to the adapter.