I went to a single chainring up front on my Trail SL2 and on my first shift the cable guide on the bottom bracket snapped off. Turns out Cannondale doesn't tap the small hole and uses just a press-in cable guide. This is fine with 2 cables on the guide supporting it equally, but trouble happens immediately with just the rear cable.
I thought about tapping the 1/4" hole for 5/16-18 and using a standard bracket, but then I'd have to take my whole bottom bracket apart and drill out a purchased part... a task that always seems to end in tragedy.
I am blessed to work as an engineer with a very capable set of machinery to work with. We're slow right now and I took the afternoon off to figure out my problem. I started with a cylinder of UHMW (one of the slickest, toughest plastics made by man) on the lathe. I then added a few holes and made a cover. For mounting it to the bottom bracket, a quick visit to McMaster-Carr yielded a perfect thread insert for this application.
The final product may not look as clean as something purchased, but it's tough as nails, gives me three options for cable routing, and allows me to switch things around without taking the cable out of the mechanism.
Insert installed into the 1/4" hole (pound it in with an extra 10-32 screw):
The guide installed with the first screw:
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