Increasing Diamater of Skin Fitting

[JDR]

I've picked up a Rule bilge pump and have an existing skin fitting but the ID is only about 3/8". The pump takes a 1 1/4" hose so I need to fit a larger skin fitting, maybe 1" ID or so. I know how to drill a hole from scratch, pilot hole then hole saw but how do you increase the size of an existing hole with nothing to centre the hole saw?

[blackrose]

I've picked up a Rule bilge pump and have an existing skin fitting but the ID is only about 3/8". The pump takes a 1 1/4" hose so I need to fit a larger skin fitting, maybe 1" ID or so. I know how to drill a hole from scratch, pilot hole then hole saw but how do you increase the size of an existing hole with nothing to centre the hole saw?

 

If you can weld a small strip of steel across the hole then you could put your pilot hole into that. There must be an easier way....

 

When I needed a 1 & 1/2" skin fitting I just cut a new one and plugged the old 3/4" fitting with a plastic wine bottle cork and plenty of marineflex.

[David Mack]

Or hammer a piece of hardwood into the existing hole and drill the pilot through that.

[Boaty Jo]

As above or something like this....

 

http://www.amazon.com/Neiko-Titanium-Step-Drill-Bit/dp/B000FZ2UOY

[NMEA]

I have to do this regularly when upgrading transducers and the like in hulls, I use an Oops arbour with my hole saws. Available from Starett it replaces the pilot drill in the main arbour, you then can fit a hole saw which is the same size as the existing one which centres the hole saw for the larger hole to be cut around it, I wouldn't be without it now. Sounds a bit complex but this video makes it very clear http://www.youtube.com/watch?v=9t9wnEQ_888

[notaminga]

Or you could chain drill the outline of the new hole and join up the holes with a jig saw then finish off with a rotary file.

[by'eck]

I've picked up a Rule bilge pump and have an existing skin fitting but the ID is only about 3/8". The pump takes a 1 1/4" hose so I need to fit a larger skin fitting, maybe 1" ID or so. I know how to drill a hole from scratch, pilot hole then hole saw but how do you increase the size of an existing hole with nothing to centre the hole saw?

 

With a GRP boat the answer was to make a wooden plug to fit the existing hole. The larger size hole cutter could then drill into the centre of the plug with its reference bit. Don't see why this method couldn't be used on steel.

[Kae]

Or fit two hole saws onto the arbor.

 

Fit the bigger saw first (the one for the new hole) and the smaller one second inside the first (this being the one that fits inside the current hole). Take it steady and use lubricant.

 

Edit: Just seen, exactly as NMEA has suggested, didn't realise they actually did a tool for this! I can see a new toy coming on...

Edited September 20, 2013 by Kae

[Wrinkley]

Can you not pilot drill next to the original hole and when you cut new hole remove old 3/8 hole. Sort of thing?

[JDR]

I've removed the old skin fitting and there is just over 3/4" to play with so going to try a 3/4" fitting then screw on a 3/4"-1" adapter and clamp hose on that. Probably a bit on the skinny side for a pump that would have kept the Titanic afloat but it involves the least expense and effort. Thanks for the answers, if I do it properly in the future I know how to do it.

[nb Innisfree]

Using hole saws make three metal discs, one to fit snugly in the present hole and two more that will fit easily inside the new hole saw, place small disc in hole and sandwich it with the other two discs and then drill and bolt them together. Central holes already made in the discs will act as a drill guide for the new hole saw which will clear the two larger discs.

 

ETA: The larger discs could be cut from ply.

Edited September 20, 2013 by nb Innisfree

[smileypete]

Lots of good ideas here, another is to fix a piece of 3/8"+ ply across the hole and drill thru that.

 

cheers, Pete.

~smpt~