Wayne, transducer should be radiused to test a concave surface. Mine is. It is not very good on flat surfaces, like straight port walls, or decks. But I dont test there. They make flat, much smaller transducers for that stuff. It's only 1/2" in diameter, but it's radiased for a 4" ID. It's designed for a 4" bore, and the actual " metal things" (highly technical jargon) that send the signals are embedded in ceramic. As long as they touch the surface, it will read acurrate. It doesnt matter (so they tell me) if they sit perfectly fluch to the surface. But they need to be in contact directly with it. The gell fills in the surface features in the machined or cast surface. It's not meant to span distnace. Just provide a neutral "evening substrate" that wont change the reading, or effect the sund waves before they hit the metal. Remember machined metal looks and feels flat, but isnt. So this 4" radius test fine on under 4" (I' ve done 318s (3.91) with good results, and it will work on larger bores (I've done .030 over 400s at 4.385).