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Ideas as to how to embiggen this hole?

Xyphota

Xyphota

Ultra Member
I need to increase the hole size inside my bicycle bottom bracket shell that vents to the down tube. The hole (as shown below) is quite challenging to get to so I thought I'd put any feelers out for ideas before I start going at it with a dremel. The picture below is just an image I found of google images for reference. The vent hole in my bike right now is ~1/4", and I need to enlarge it to ~3/4"

1693331587130.png


Other than awkwardly picking at it with a dremel, I could potentially drill a hole through the opposite side of the shell to drill the hole directly with a hole-saw, and then close the outside hole back up by brazing it closed with a slug that the hole-saw removes. I'm not too worried about strength in this area as there are several examples of frame builders putting all sorts of cut-outs in their bottom bracket shells for weight saving and artistic reasons. I think the chainstays might be in the way though so I'm not 100% if this is feasible.

1693331955001.png


Any other ideas?
 
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Yeah you are probably right. Now I need to find a die grinder lol.

Maybe a forstener bit would work? I could drill a relatively small hole on the back side, stick the forstener bit in and then go to town, then close up the small holes. I believe the wall thickness is 0.083".

1693337157596.webp
 
Why do you need to enlarge it? There's often other ways to solve the problems.

A forstner bit won't work, they're not for metal.

I'd probably go with a carbide burr as well, that's the first thing that comes to mind if precision isn't exactly needed. Otherwise, it would be cut all the tubes off, machine from the outside, reweld it all back together.
 
I’ve done this exact job before to make room for an installing a Di2 junction box in the downtube.

I used a die grinder and carbide burr. If you have a good air supply, air die grinders are cheap and handy to have around. The angle was still a bit of a challenge but it worked.

I think the ultimate tool would be a 90 deg head micro die grinder with a carbide burr. With that you could actually fit the grinder inside the shell and have the burr perpendicular to the hole.
 
Xyphota said:
I need to increase the hole size inside my bicycle bottom bracket shell that vents to the down tube. The hole (as shown below) is quite challenging to get to so I thought I'd put any feelers out for ideas before I start going at it with a dremel. The picture below is just an image I found of google images for reference. The vent hole in my bike right now is ~1/4", and I need to enlarge it to ~3/4"

View attachment 37883

Other than awkwardly picking at it with a dremel, I could potentially drill a hole through the opposite side of the shell to drill the hole directly with a hole-saw, and then close the outside hole back up by brazing it closed with a slug that the hole-saw removes. I'm not too worried about strength in this area as there are several examples of frame builders putting all sorts of cut-outs in their bottom bracket shells for weight saving and artistic reasons. I think the chainstays might be in the way though so I'm not 100% if this is feasible.

View attachment 37884

Any other ideas?
Click to expand...
Don't do that. I knew a guy when I was doing triathlon, big strong as-an-ox guy, 6'4, more like a line backer. He has broken several bottom brackets when powering up a steep hill! Keep that area solid.
 
thestelster said:
Don't do that. I knew a guy when I was doing triathlon, big strong as-an-ox guy, 6'4, more like a line backer. He has broken several bottom brackets when powering up a steep hill! Keep that area solid.
Click to expand...

Fortunately I am not a strong-as-an-ox kinda guy ;)
 
thestelster said:
Don't do that. I knew a guy when I was doing triathlon, big strong as-an-ox guy, 6'4, more like a line backer. He has broken several bottom brackets when powering up a steep hill! Keep that area solid.
Click to expand...

@Xyphota - nothing like real world experience to show marginal strength.

But I do wonder if removing a bit of the cylinder inside the other cylinder would actually weaken it.

If you have to do it for some reason, I'd consider strengthening it at the same time. You never know when some dumb Yeti might get on it and break it. :oops:
 
Rauce said:
Di2 is what shimano calls their electronic shifting components. Junction boxes are the multi port wire connectors.
Click to expand...

Thank you. Any chance this could act like a clutch in a chain drive for a planter? Chain is a wee bit bigger than a bike.....

Note - I moved the rest of this planter related discussion to a new thread. It was hi-jacking too much of this thread.

Thread 'Improving a Row Crop Planter' https://canadianhobbymetalworkers.com/threads/improving-a-seed-planter.8623/
 
Hard to draw in perspective properly & not sure if you can find them locally, but these diamond coated grinding burrs would allow the shank to come in more axially to the tube & still allow a cutting edge to work around the perimeter with more control. They come in different grits & sizes. A diameter somewhat match your initial opening might be best to control. A more conventional carbide die grinder burr with teeth I suspect will snatch unexpectantly.

1693354226206.png
1693354294607.png
 
Susquatch said:
Thank you. Any chance this could act like a clutch in a chain drive for a planter? Chain is a wee bit bigger than a bike.....
Click to expand...

Like a one way clutch or engaging and disengaging?

A bicycle freewheel is a one way clutch, if you need something that engage/disengage I can’t think of a bicycle part that would work.

Do you know the chain pitch? Bicycle chains are 1/2” but narrower than 1/2” pitch industrial chains.
 
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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.
 
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