Recessed Allen Key Brake Bolt Drilling

bought a pair of brakes off eBay only to find they have allen bolt recessed fittings, problem is I have an old frame they is only drilled for standard nut & bolt fitting so I need to either drill the crown/brake bridge to accept the calipers (does anyone know the drill bit size) or is there a device that can overcome the problem without the need to drill?
cheers
saxby's

8mm. Use a good HSS drill bit and a bit of fairy liquid and it'll literally take a few seconds*.

*Drilling the forks

Forks are usually quite easy, the brake bridge presents 2 problems; you need to access it from inside the rear triangle, and many old lower end frames have simple tube bridges so you also have to modify the convex washer[left]. You might try using a nut on the "front" calliper at the back, and a long Allen key tube nut[right] on the "rear" calliper at the front.

In theory, you're also supposed to make a counterbore or at least a spot face 10mm diameter to seat the flange of the tube nut. If the fork crown has a flat face, you needn't bother with this, but it would look much tidier in the case of a unicrown type fork.

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8mm. Use a good HSS drill bit and a bit of fairy liquid and it'll literally take a few seconds*.

*Drilling the forks

I think you'l find how long it takes depends on the drill bits, a standard drill bit will take quite a while, but a titanium drill bit just goes through quite easily.

Another tip is rather then start with the 8mm bit, start with smaller drill bits and go increasing the size till the 8mm. It makes it easier and gives a better finish.
Cutting fluid also helps.

You can get a stepped drill bit which increases by 1mm every so often. This is used for making existing holes larger. like this:
http://www.tooled-up.com/Product.asp?PID=109092
Or plain taper bits if you're confident.
http://www.tooled-up.com/Product.asp?PID=3798
The trick is to put some tape or tippex on the bit at the depth you need to stop drilling.

Not sure what bit you use for the flat-faced countersink though.

i did this with my mates drill no cutting fluid and a drill bit bought from the corner store. it went fine if i can do such an incompetant job i am sure nothing will go wrong

The hole is already 6mm, so enlarging to 8mm in one hit is easy enough. Making a neat job has more to do with holding the workpiece and tool in accurate alignment than the actual type of bit - a cheap but new HSS bit in a drill press will make a much easier and better job than the fanciest bit in a hand held drill.

The counterbore/spotface definitely needs a drill press, but any machinist will have the cutter . I'd expect to give my local man £10 for this job (fork only)

Quick threadjack here- mdcc_tester does your local machinist work on car engines at all?

Not as far as I know, his business is mostly making turned/milled parts for industrial machinery as far as I can tell, I've never seen anything car related in his workshop.

the allen tube nut had rounded off on my front brake and i need to switch calipers.

friend lent me a screw extractor set. it seemed to be working and i started to crack a smile as i could see the flange of the tube nut slowly appear like the head of a newborn baby. my smile was soon wiped from my face as it then came all the way out to reveal the tube nut had broken in half, leaving the brake still in place.

i need to get this brake out as it is next to useless. is drilling the only way?

thanks for any advice ls & gs

Is it possible to get some purchase on the remaining part of the tube nut e.g. pliers up the steerer tube?

nice idea chris, hadn't occurred to me to look.

if i can i assume i can just grip and turn the caliper to undo. will give it a try.

any further suggestions welcome

okay, so i tried the above but can't get it to budge. really not enough space to get any meaningful purchase on it.

so i don't want the calipers anyway so what if i was to just saw it off? would the remainder of the nut and bolt inside the fork just tap out? can't think of any reason why it wouldn't but feel i might be missing something

is it too early to bump my question above?

Don't see why that wouldn't work, as long as you can get at the brake axle to cut through it.

i reckon i should be able to. feel like there is nothing to lose