Let me be sure I have this right .....
You want to stay with obsolete, non functioning equipment that isn't worth calling around for a repair source for and no one seems to want to touch for that reason because you don't want to make an existing hole about 1/8" to 1/4 inch larger? I'm sorry but that just seems silly to me. If the boat's currently out of the water there's absolutely no reason not to do this and lots of reasons to go ahead, not the least of which are that you'll have a far superior piece of equipment. If it's in the water come up with a few more below the water line projects to justify hauling. The hole can be opened up either of two ways. The first would be to simply go around the inside of the existing hole with a curved, coarse file. The other would be to use a hole saw on a drill. The trick here is to use a pilot template. I use a piece of 1/4" plywood. Cut a hole in it with the hole saw you need, hold it in place over the existing hole, stick the hole saw into it and gently cut away. Bingo! Perfect hole. In this case I think I'd recommend you stick with the hand file working slowly and carefully, constantly fitting the transducer to be sure of a close fit. Now this next bit is a secret to a perfect installation. Using the curved file make a slight chamfer at the outside circumference of the hole. This provides a nice little well for the sealant to form a ring and press itself in place. Once you have it opened up and chamfered, slather 3M 4500 fast cure on the lower threads and mushroom and slap it in, but only snug it in. You don't watn sealant inside or on the threaded locking ring. When the 4500 has just set, (a few hours as I recall) you can finish tightening. This forms a perfect gasket. Moderately tight is all it needs!! Hand tighten the ring nut only. This ain't brain surgery. You can do it! P. S. I just did this to put in a new set of Standard Horizons which have also been sold as West Marine. Actually, I think at this point West only sells them as Standard these days. We'll see how good they are.
