Mounting instruments in cockpit bulkhead

[geary126]

Aloha.

Replacing 4.5 inch dead instruments with something requiring 5 and 1/16th diameter hole. Anyone have any suggestions on making this happen?

The notion of using a gigantic hole cutter on a power drill scares the hell of of me. Plus, I have nothing for the bit (as in, the center guide bit) to sink into, thereby ensuring that my 5" cutter doesn't go awry.

Debating the jig saw route as another option. That, followed by sanding bit on drill, perhaps?

Help!

Thank you.

[paulk]

Jigsaw route might be the simplest, since the edges are going to be covered by a trim ring anyway. Just scribe an additional 9/16" out from the existing edge and go for it. Don't forget to seal the edges of the hole with epoxy if it's cored, btw. To use a compass bit, you'd have to attach a temporary board over the existing hole on one side, and drill from the other, once you found the center... tricky and a hassle. KISS.

[Faster]

The jigsaw will work if you have the room to swing the body of the saw as you cut. Sometimes the hole is too close to a companionway molding or a coaming for the saw to clear.

Using a Holesaw - clamp a temporary piece of plywood behind the hole , mark the center and use the holesaw in the normal fashion - the ply will guide the pilot bit.

Alternatively cut a hole the right size in a piece of ply, clamp it in place on top, and use a router to cut the hole with the ply as a guide. (this way you can shift the center slightly if that's desirable)

Good luck

[S/VNirvana]

Make sure you mask the area that the jig saw is going to move over. I have done similiar job ages ago and you can't tell that I enlarged the whole with a jig saw. If you are on the small side when you are finished or it's uneven you can use a dremel tool to smooth out the edge or to make it a more symetrical circle.

[Culinary411]

When I changed my compass on the bulkhead I used the "roto-zip" tool. I put the new compass over the existing hole and marked it with a dry-erase marker, then, took off 1/16 at a time until I got it to fit tight...a little marine caulk and that was it.
I prefer the rotary tool because it allowed me to make a perfect hole with almost no effort. Had I used a saw or a semi-round file to open the existing hole I doubt that I would have made it as nice.

[Faster]

Just one caution I've found with the rotozip - it has a lot of torque for a small tool and can be difficult to make it track straight without a router-style template to follow.

[sailingdog]

If the cabintop is cored, and most are nowadays... don't forget to remove the core for about 1/2" and then fill the gap between the two layers of laminate with thickened epoxy... this is to help prevent water from getting into the core and causing delamination/rot. Also, if the fasteners are outside of the hole you'll be drilling... drill them oversized and fill with thickened epoxy, and then re-drill for the fasteners... for the same reasons.

[Culinary411]

Sailingdog....good point. I did the same thing only filled the 1/2" gap with marine caulk. Not sure if that's proper but I know it will keep moisture out.

[pigslo]

Use a painters mask as the fiberglass dust is rough.
pigslo

[sailingdog]

Originally Posted by Culinary411

Sailingdog....good point. I did the same thing only filled the 1/2" gap with marine caulk. Not sure if that's proper but I know it will keep moisture out.

Probably not a good idea... caulk will eventually dry out and shrink...leading to the problem you're trying to avoid in the first place. Also, the filled epoxy adds strength to the area that has been weakened by the hole—essentially becoming a small structural reinforcement, which the caulk can not do. I would highly recommend cleaning out the caulk and re-installing with a good epoxy and high-density filler.