Our ST-50 knotmeter display head went T-U a couple of years ago so I purchased a complete replacement. This spring I finally changed the thru-hull to the new unit with the built-in check valve. The old transducer and new transducer are different diameters so the old one is no good as a spare. It works fine but the temperature transducer stopped working last year. Contact me at nwigen (funny symbol) comcast point net and make an offer.