post a picture of where you want to drill it and what size hole.

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I need to drill a hole on the outside surface of the bullhorn extension. It needs to be about 5 mm in diameter. It will need to be approximately opposite of the base bar leading edge. I need to be able to guide/push the cable housing to turn from the bullhorn extension 90 degrees into the base bar.

The new Shimano BL 78TT brake levers have the brake cable exiting the center of the handlebar plug.

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Habitual line stepper.

Drilling a clean hole with a Dremel will be very challenging. Do you have access to a drill press?

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I have no drill press. I have a hammer, nails, Dremel, and a Glock .45.

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Habitual line stepper.

 
check with you neighbors and find a drill. Seriously, Dremel tools are very hard to control. One slip and.......!!

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It's really hard to drill a hole in a round tube without a drill press.

I drilled a beautiful hole with the Dremel. Now the problem is there is not access to the inside of the base bar from the inside of the bull horn extension! Instead of trying to drill a hole in an area I cannot see, I might just swap aero bars from a different bike. Thanks for the suggestions.

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Habitual line stepper.

My uncle drilled a hole in Mavic cowhorns with a drill press. Used some sort of bracket to hold it in place (he owned a gun shop)...then drilled it again at a different angle to "lengthen" the hole more oval, afterwards he used a narrow file to take any aluminum burs off which may cut the housing. Finally, he used a buffer to smooth it like butter smooth. Worked perfectly, although it took a gunsmith to do make it so perfect--you won't get that kind of drilling and perfection attention from a handlebar manufacturer.

park tool has a nifty new tool with magnets they showed at Interbike, but maybe you could rig something yourself/

http://www.bikerumor.com/...x-full-of-new-tools/

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"...I try not to ever ride as slow as 20mph. ;) ... And even more than that, I don't race with a speedometer. My computer is set up to show Power // Cadence // Time. I don't actually ever know how fast I'm going. I only know that if I'm in 53/11, and it takes more than 100rpm to hit my target watts, it's time to coast." - Jordan Rapp on '09 IMC

Why drill at all? Push the cable until it hits, then use a hook or loop or similar to slowly ease the cable round the bend. Probably start with the brake wire, then feed the housing in to follow. No?

I normally use a 1 or 2 mm L-shaped Allen wrench to hook the cable and pull it through. In this case, the opening is not big enough at this angle to hook it. I drilled a hole and found there is no opening between the base bar and the bullhorn--the bullhorn is a cylinder bonded to the base bar. I drilled a hole in the leading edge of the base bar adjacent to the exit hole on the bullhorn. I also made the bullhorn exit hole larger. No problems after that. All is smooth and covered with bar tape.

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Habitual line stepper.