Enlarging Pre-Exsisting Drain Plug Hole

I am in the process of adding some ballast to my boat and I want to take the standard 1/2" npt drain plug and enlarge the hole to accomdate a 3/4" npt plug.

I found this method online. Does anyone think that it will work?

Install the two hole saws, the small inside the large one. The small one should protrude at least 1/2" beyond the larger one. Insert the smaller saw into the old hole, and carefully guide the saw through the hole. Keep the drill on a straight, steady course as the larger saw cuts the new hole around it.

Will work with holes of 2 significant different sizes, not sure how it would work in your case, only an 1/8" difference on each side of center - I would try it first a couple of times on some scrap

Why not cut a dedicated hole? You're cutting the hull anyway, current drain plug just complicates things... Add a hole the diameter of your choice and leave the drain alone.

Agree with Nacho - whole lot simpler and less likely to screw the boat...

That will work Murphy. The risk is that you get a wobble on the inner bit you're using as a sleeve and don't catch it in time. Then you've got a big skuff around the hole you're drilling. Remember that the inner bit is going to get hot even though it's not cutting.
There is less risk cutting a new hole, but then you've got two holes.

Seriously, why not drill a new hole? Not only that but now you cannot drain the hull from the back of the boat.

I say don't do it. Go with a second hole. Normally the plug hole is super close to the bottom of the boat to allow for draining (obviously) and if you bore it out that big, I'd think youd cut more than just the transom, but go into the bottom fiberglass as well.

I've done it with speakers on my boat. i bought some 6.5's that were slightly larger than the previous and it worked like a champ.

If you want to drill the hole just plug the existing hole with a piece of wood of the appropriate size. This will guide the bit. Then drill with your hole saw...snappity snap. Your method will probably bind.

I agree with everyone else. Put in a new hole with brass fitting and brass shutoff ball valve. No plastic! Also make sure you have a stainless handle on the valve. Use 3M marine sealant through the hull. No bid deal.

If its below the waterline which I assume it is, I would use bronze instead of brass....

You 100% right, bronze. You can never trust the info you get from the internet.

Murphy,

Just like Hate, we've been using this method for years to oversize holes and it works famously.

But a 1/8" radius seems like too small of a difference to get one hole saw to fit within the other. If it fits I know it works.

If it doesn't fit then plug the hole with a pine dowel that's glued in with 3M 5200. Once cured its permanent. Now you can use the standard hole saw and pilot without using the smaller hole saw as a pilot.

In either case, deeply scar the gelcoat in reverse before going forward with the hole saw.

David
Earmark Marine

For a hole less than 1" just go buy a regular drill bit. I Bought a 1" drill bit at Home Depot for approx. $15 to install through-hull fittings for ballast empty and vents. Drill in reverse through gel-coat and fiberglass.

A standard drill bit will work fine when drilling a fresh hole. But when used to oversize an existing hole, it may grab, bind and chip away.

This is where a hole saw has a distinct advantage.

David
Earmark Marine

I may give this a try. I am only going to be using this hole for (1) Tsunami 800gph pump to fill a 600lb locker sack. It is less than $2 and if i am not satisfied, I will fire up the drill.

1/2" Mips x 3/4" Fips (IPT Adapter)}



(Message edited by murphy_smith on September 15, 2009)

Will you use any type of valve on it or just go straight into the pump? Seems a bit dicey.

Get a pipe plug to fit your existing through hull, drill a pilot hole in the pipe plug, put the plug in the existing fitting then use the hole saw as it was designed.

Yes, will be usuing a valve immediatly after the hole.

Mikeski, I think I will use use David's suggestions of 3m 5200 and a pine dowell plug.

Do not use plastic below the water line unless you have a good and fully paid insurance policy.

Use a bronze fitting followed immediately by a shut off valve. You can run plastic after that because you have a secure method of shutting off the water should a leak develop.

Thanks time. I found one in Brass for $5 so that I don't have to use plastic.

btw - on my 99 Maristar - the stock thru-hull for the engine coolant line contains no shut-off valve of any kind... no kidding.

You really should use bronze in the bilge area. I know you are trying to do this on the cheap, but you really should leave your drain plug as is and just drill another hole. Most new wakeboard boats have 12-15 thru hulls as it is. A new hole is really not a big deal. I would reccomend getting one of these, and some marine epoxy/sealant



and one of these



Then pick your spot to drill from (make sure there is plenty of clearance for the pump and to be able to open or close the safety valve. Drill a pilot hole from the inside out, the use masking tape and a hole saw and dill the larger hole from the outside in.

You will be done in 30 minutes and never have to worry about it again. You will easily be able to upgrade your system as well.

Hopefully, you have planned for draining and venting as well?

Based on every one's feedback, I am going to put bronze thru-hull in the ski locker. New hole and new fitting.

I checked with Malibu and this is what they used to go with their Wakesetter Sac Systems. I was worried that there might be a void b't the bottom of the sk locker and they hull - but they assured me that there was not.

I had a 2000 malibu with the "MLS" in those days it came without a shut off valve. I added one and it worked twofold, once th pump just fell off and it was nice to close the valve and calmly deal with it. The other nice thing was that the bag would tend to fill on it's own to the point of causing damage, I would fill the bag and then reach down and close the valve.