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I am installing a modern groupset on an old frame. The frame has a rear derailleur cable guide like this:
The problem is that the gear cable housing ferrules are too large for the braze-on, causing the cable to sit at an awkward angle.
My gear cables are 4.1mm (standard Campagnolo 11-speed, I think). The braze-on has an inner diameter around 3.9mm.
How should I solve this?
You need a stepped shift ferrule. You have no reasonable options if you're using compressionless shift housing. On the left is a picture of a 5mm to 4mm one, which Jagwire is the main remaining producer of. On the right is the Shimano 4mm one, which in my experience is the commonly encountered one for 4mm housing. There are Campy ones too, but I think they may only come in the Record cable sets - I don't cross paths with them very often and I kind of forget what they're like.
Shops should have them. They're still readily available new in bulk and many shops will have all they need leftover from installing the various cable kits and STIs that come with them. I believe Dura Ace shift cable kits have always had them and at least some generations of boxed D/A STIs too. Googling "5-4mm stepped ferrule" and "4mm stepped ferrule" yields some results too.
Note that there's another kind of stepped ferrule, for using 5mm compressionless brake housing on brakes and brifters that can't take a normal ferrule, which is not the kind you're looking for. The most common example of this is the Jagwire POP ferrule.
Many options - but some depend on how original you want to be.
As per Christian's comment, drilling out the cable stop will allow the new ferrule to seat better. Downside is that it will be hardened steel and difficult to drill.
Could use smaller ferrules, or even just skip the ferrule completely and let the housing sit in the cable stop without a ferrule. This is probably how it was used originally.
You might try unbrazing the stop from the frame and working on it separately. Likely you have more than one to adjust which is a lot of work. Or unbraze the cable stop and fit some clamp-on ones instead - these may give you some ideas.
http://problemsolversbike.com/products/brakes/backstop_-_1282 are clamp on cable stops but they're not small enough for a seat/chain stay.
http://problemsolversbike.com/products/brakes/clamp_on_cable_guides_-_27723 are clamp-on cable guides.
