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Transducer Upgrade Needs Bigger Hole

Old 02-06-2012, 05:44 PM
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Default Transducer Upgrade Needs Bigger Hole

I'm moving up from an AIRMAR through-hull with a 33mm shaft from one with a 31mm shaft. My problem would be a 31mm hole in my boat. I need to make the hole bigger without destroying my hull. Trying to over-drill an undersized hole in wood is usually a disaster and I don't expect fiberglass to be much better. Ideas?
Old 02-06-2012, 05:52 PM
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One way - get a long hole saw blade that fits your existing hole and a shorter one the diameter of the desired new hole. Put them on the same mandrel. The longer one wiill act as a guide through the existing hole to center the shorter hole.

Going from 31 mm to 33mm may not be possible with this method. A router with a template may be an option. I used this method to enlarge cup holder holes from 3" to 3 1/2" or something like that.

Or, just a rotary rasp in a drill motor and some skillfull handling might do it. There's a lip and caulk on the outside and a washer on the inside, right?
Old 02-06-2012, 06:21 PM
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5 min epoxy a scrap of wood on the inside of the hull, use the wood to center the new hole saw. After you start the hole it should guide in fine. Don't put too much epoxy on since you will have to remove the wood.
Old 02-06-2012, 06:39 PM
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Just use 5 min. Epoxy! And don't scuff or prep, you only need it to steady the centering bit for a short while! Usually I use an oops arbor, but you don't have enough size difference for it.
Old 02-06-2012, 10:33 PM
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two millimeters? I'd use an abrasive cylinder on a RotoZip or similar to make it a two minute (1/2 beer) job.

In fact, the new transducer I installed last weekend called for a 60 mm (2-3/8") hole. I had a 2" hole saw, and a 2.5". Used the smaller one, and then ground the hole bigger in about 5 minutes with the abrasive bit and the blower running to keep plastic dust off my face.

Good light, good marks, and a light touch are all you need.
Old 02-07-2012, 05:15 AM
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2mm is what 3/32" (about 2 nickels stacked) - get out the round file - it won't take but five minutes to get it to the right size.
Old 02-07-2012, 09:13 AM
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A trick that is used is to get two hole saws, one for the smaller size and one for the larger. The smaller one should be taller than the larger one.

Then, nest the two together (will look like a wedding cake with the smaller diameter taller than the wider one). The taller and narrower one will fit inside the existing hole and act as a guide for the larger one.

May not work for you since they are so close in size (may not nest together), but I've seen it work in other cases.
Old 02-07-2012, 10:55 AM
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I would go the round file route. Might take a while, but better chance of not screwing up.
Old 02-08-2012, 11:33 AM
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gluing a piece of wood to get the pilot started if the best bet outside of stepping the diameters on the mandrel(Which i dont think you can do with this close of a size)
Old 02-08-2012, 12:18 PM
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Get a one inch dowel cut to about 3-inches. Drill a 1/4-inch hole down the center (use a lathe if you own one). Mount the dowel on your assembled hole saw bit. Wrap masking tape around the dowel until you have a snug fit in the existing hole. Then have at it with your drill. Easy-breezy.
Old 02-08-2012, 05:44 PM
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I just have the same problem.. Just change my Airmar B60 to a B164 that its bigger hole ... After thinking a lot how to do it I go with the abrasive bit and job done!!! works for me!

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