So the boat had logs and now going to headers, any tips on how to make the holes bigger? need to go from 3 to 4 inch, some have said mount plywood and use a hole saw but seems a little sketchy. I was thinking using a router and slowly open it up. any tips or help would be awesome.

plywood and hole saw b


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so should I use plywood no both sides of the hole? how do you mount the wood? I take it start from the outside and cut into the boat?

Just the inside.

I use project hardwood plywood.

Very cheap,  screw it to the inside and have your way with it.

Will the holes be close enough in proximity?

GT

Do you have to cut on every boat so you can add headers?  My boat has water logs now.


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so should I use plywood no both sides of the hole? how do you mount the wood? I take it start from the outside and cut into the boat?

I've never done drilled into a hull, so if I'm wrong maybe somebody with experience will correct me.

You should just need wood on the back. The hole saw should have a drill bit long enough to reach through the existing hole to the plywood. Maybe pre drill the hole in the wood, center it to the hole in the transom and attach it. I would think tape would work.
The plywood should be just to align the holesaw. Once you start to cut into the hull,  I don't think the plywood serves much of a purpose after that.
Don't push the drill too hard and let the hole saw do the work.

I didn't even think about the hole location. It was an olds now going to BBC. Take it that's why a lot of goes go OT headers after a swap. Is there a way around it?

The inside holes will be different. Hopefully you have the TT headers to mockup their final exit location on transome. I'd be so gentle pushing the holesaw that last 3/8". Ripped up gell coat,fiberglass,paint is a beautiful sight. LOL. Dude just bite the 7.62 round, spend the maybe $300.00 and have a good shop or a close site member help u. Juzz my 2c$ worth,round file it if you care to. Lol

Just for general information.

When cutting through gel coat,  always tape it up. I use something call clean room tape. Basically very tough vinyl tape.

GT

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I went from 3" holes to 4" holes for my new tips. I was nervous at first. But it was no big deal. Mark the circle where you will be cutting. Use a hole saw on a drill. TAKE YOUR TIME! PATIENCE! I used VERY light pressure. Little by little. No plywood for pilot drill. Just make sure hole saw is as square to the surface as can be. I started on the gel coat side. Once you get a "groove" going the saw is set and just keep going with it. Patience I say! After I did one, the other was no sweat. Plus, my tips have a nice flange to cover the edge of the hole. It came out looking factory. Good luck.

Put a 3 inch hole saw on the inside of the 4 inch hole saw to use as guide and take your time. Worked for me. Or mount the headers put exhaust hose to the transom and draw a circle also works

Most holes in the trans are already 4" even with a 3" tip. Measure it first. A reamer bit on a variable speed drill will grind it out quick if you still need to open it up more. I went olds to ford... I shaved the ends of the hose slightly for the light angle difference. I had 3" aluminum tips an went to 3.5 ss without modifying an still have room in original hole to go 4".

When your finished,you'll be pleased😅