Enlarging Exhaust Tip Holes

[hickz3]

What's the best way to enlarge my 2 7/8" exhaust tip holes to 3 1/2". I was thinking of using a hole saw but thought it may jump (nothing to center it on).

[HotDogz]

screw a piece of plywood to the inside of the transom, mark the center of the hole from the outside and use your holesaw and start the bit in the plywood backing. That should keep ya centered.

[SmokinLowriderSS]

I taped my glass (to avoid chipping the gel), and jigsawed short of the marked circle, then sanded to size with a large drum sander.
Either way, tape the gell arround the holes and cut thru the tape (use several layers) to prevent chipping.

[sleekcrafter]

Another alternative is fill the holes with duraglass, this allows you to center the holesaw. Sleek

[Wicked Performance Boats]

Just use a hole saw to cut a slug to fit the hole.[ make it 1/8th inch larger and grind it to fit.] it will already have a centering hole in it and tape the gelcoat so it doesn't chip. And drill away. Or screw the smaller hole saw inside the larger holesaw and try that. Budlight

[Old Guy]

screw a piece of plywood to the inside of the transom, mark the center of the hole from the outside and use your holesaw and start the bit in the plywood backing. That should keep ya centered.
I have used this method several times. Works fine. Also, you can probably screw a piece of plywood (3/8" works good) to the outside using the three screw holes that held the outside rings to the transom (all my boats have had them). You can use the rings to locate the holes in the plywood. The screws should be outside the new hole a little bit. You start by using a holesaw that fits the old hole. From the inside, you go through the old holes to establish the exact center for the pilot drill. Then, using the new pilot hole in the plywood, you holesaw the correct size hole from the outside, going first through the plywood, then the transom. It shouldn't chip on the outside at all. I have used this method several times also. Makes a perfect hole exactly where you want it.
Roger