Ideas as to how to embiggen this hole?

[Robert Fontaine]

Dremel with one of those extension shafts plus a carbide burr grinder and a willingness to completely destroy your bottom bracket and you are good to go. A die grinder will make it way too easy to cut right through those thin walls.

 

[Xyphota]

The potential issue I see with that is how big is the current hole? If it’s bigger than the pilot on the holesaw the holesaw is just going to walk all over the place.

I'm banking on being able to set it up in the mill so I'm not too worried about the pilot

 

[FatherWes]

Could you cut the down tube a couple inches above the bottom bracket shell, drill down inside to your 3/4”, and repair the down tube with a weld and external pipe collar/gusset? To be fancy, the gusset could be fitted into the bottom bracket, for increased rigidity.

 

[OliverTO]

Sometimes you can insert the junction box through the hole in the head tube and just fish the wire through or put the junction box in the seat tube and do the same. You can also purchase a junction box that fits into the handlebar end.

 

[historicalarms]

should I be able to drill a hole just big enough for the shank, position the holesaw inside the shell,

I was going to suggest drilling a hole as you mentioned opposite the hole you wish to enlarge but was just thinking about being able to line up the dremmel tooling others have suggested so your angle of attack to the hole needing enlarged was a straight through operation.
Your hole saw idea has merit but insteasd of relying on your mill set-up to be rigid enough for a good job, I would build a pilot insert for the old hole first to help bit line-up. I am worried that the curvature of the inside dia of the peddle tube will cause an awful lot of biting & jumping without a pilot.

 

[Robert Fontaine]

Bicycles are thin-wall chrome-moly when steel. The bottom bracket itself is a little beefier but the idea of running a pilot hole unnecessarily seems risky. I am too fat for my Columbus sl frame these days and it is like trying to ride an elastic band. They make racing bikes with minimal materials. Don't get me wrong, in my youth I drilled holes into my aluminum handlebars to run my brake cables internally so I understand the desire but having your frame fail under load can be a day-ender. If you are going to do this remove the least possible material.

 

[Xyphota]

Sometimes you can insert the junction box through the hole in the head tube and just fish the wire through or put the junction box in the seat tube and do the same. You can also purchase a junction box that fits into the handlebar end.

I am using the handlebar junction box already, but that junction box only has 2 ports, so essentially it is just connects the shifter to the rest of the system and acts as a convenient charging port. I still need a 3+ port junction b box elsewhere because the battery and derailleur need their own connections.

The vent hole in the head tube is the same size as the vent hole in the bottom bracket shell unfortunately. My original idea was to put both junction boxes and the DN110 battery in the handlebar and just run a wire from the handlebar all the way back to the derailleur. The longest Di2 cable available is 1600mm and that cable is too short to get from the derailleur to where I'd need it in the handlebar.

My next idea was to put the junction B box in the steerer tube and run the di2 wire though the head tube vent hole, but there is not enough clearance between the steerer and the inside of the head tube, so this is also not possible.

I could put the junction b box in the seat tube, but then there would now be two wires running through the bottom bracket shell (one to connect to the shifter, the other to connect to the derailleur). The vent holes are too small to do this so then I'd need to enlarge two holes instead of one (although admittedly they would only need to be slightly larger).

I am worried that the curvature of the inside dia of the peddle tube will cause an awful lot of biting & jumping without a pilot.

This is probably a healthy fear, but all of the tubes mitered for this bike were mitered without pilots (although the holesaw shanks were substantially bigger and more rigid then what I am going to attempt to do). I think it will probably be fine with a slow infeed.

If you are going to do this remove the least possible material.

Yes for sure. If I had drilled a 3/4" hole at the beginning of my build nobody would have batted an eye lol. I'm just a dummy for not have planning this far ahead. Here is the rest of the build thread in case you missed it. If something is going to fail on this bike its going to be the big hole I drilled into the down tube near the head tube lol.

 

[trevj]

Talk nice to your dentist, see if he has any old handpieces that he is about to chuck away. Same for burrs to fit. or check ebay/amazon.

 

[Xyphota]

I'm pretty happy with this! The mini hole-saw worked perfectly, although I did have to zip the pilot bit off with a dremel as it didn't quite line up. I'm going to leave the back side hole as is until I remove the brakeline in a few weeks and I'll braze it closed then.

Fits like a glove

 

[Bandit]

Hmm, thinken inside the circle! I have made custom bushings to go on the pilot drill a few times for those over size holes that need to be bigger. A few wraps of black tape to hold them on the pilot, quik and dirty.