I've been thinking lately on routing cables on a frame of mine.

There is a cable guide right next to the rear derailleur. The frame uses old style shifters mounted to the frame.

To connect the rear shifter with the derailleur, I would need to run under the bottom bracket.

There is a hole at the bottom of the BB, but its diameter is 7-8mm. Now, is there a guide I could actually mount to such a huge hole? It is not threaded, and I am not even sure whether it should be actually used to mount a cable guide.

What I should be looking for? There are dozens of plastic under BB guides, but they all come with a small-diameter mount. Any phrases I should search amazon / ebay against?

Is the guide you are talking about for the rear derailleur cable on the top or bottom of the chainstay? If it is on top, you don't use a guide underneath the BB.

Since every bike can be different, it'd really help to have a picture or two. Or at least know what bike and components you are planning to use.

post pictures, impossible to give good input without info.
Underside of bottom bracket and overall pic of bottom bracket
"guide: near rear derailleur (typically bare cable runs from shifter to this "guide" and then there is short length of housing between guide (cable stop more likelly) and the derailler

Absolutely. Here are the pictures. Other than these, there are no other guides on the frame. The cable to the rear brake goes internally through the top tube but that's it.


That little pimple is the guide.

Is this a guide, too? What is it used for?

Second pic is to hold your chain when you remove/replace the wheel. I had to ask the same question here on BF quite a few years ago.

You probably do need a guide on the bottom of the BB. Some bikes from vintage days just wrapped the bare cable around the steel BB shell and made notches to keep it in place or had little bend pieces of metal to run the bare cable through.

On my bike that I also learned what that chain peg was for, it also just wrapped bare cable around the BB. However after a few thousand miles, I started having some issues shifting and it took some time for me to discover that the cables were sawing into the BB. One of them almost cut completely through. So then I spent the big bucks to get this Shimano BB guide and drilled a hole for the scew where I felt it belonged and ground off the pieces of bent steel they'd brazed on it.


https://www.jensonusa.com/Shimano-SP...RoCRhYQAvD_BwE

$3.00

And the BB:


The hole has a diameter of 7-8mm.

Oh, OK, not something you would use on a daily basis, then. Thanks!

The hole in your BB is for drainage of moisture. That's a big hole though. Usually I see them half that size.

If you use a guide like I showed, then that hole in it that doesn't have a screw in it, should probably match up with your drain hole in the BB. Unless that puts your guides in a very bad position for the DR's. Particularly the front DR.

Bottom bracket cable guide issue



Adhesive or find one with a big snap in like this.

​​​​​​https://www.amazon.com/Cannondale-Do...082168&sr=8-11

That may not be a guide but rather a stop. You might need a reduction feral to properly mate up with a housing end. Look at this thread for reference to another that had an issue with that type of stop
How to get rear derailleur cable house to run straight on vintage steel frame?

Definitely for a cable guide, you can see the rectangular outline on the bottom of the BB of where the old one used to sit.

can you show a picture of the BB from above.....are there metal guides there?

concur this is a cable stop, (sometimes called divers helmet style) 99.99% will need to use a reduction ferrel

Of, relating to, or suggestive of a wild beast.

Colin. An Irish actor

Ferrule: A metal ring or cap placed around a cable, pole, or shaft for reinforcement or to prevent splitting.

forgot about ferret....would be fun to have a ferret on a bike, but not in bike shorts

A bike shop that has been around quite a while will probably have one or those reduction wild beasts <grin> that fits that old bike in one of their old parts bins. That's where I got mine when I built out my old Paramount with 11 speed stuff.

I've been thinking that the big hole on the bottom of your BB probably served both as a drain and something for the cable guide to snap into. I never had nice bikes like that BITD though. Mine where all the looks like a light weight racing bike, but in looks only.

I didn't realize they were becoming so scarce! Might have to stock up on a couple, just in case!

Maybe a feral ferret?

I don't know that they are getting scarce. I just find that the older bike shops have bins where they keep miscellaneous old and new parts that they will always have extra left over from components they've had to install or such.

They seem more likely to just give me such things than the big shops where you barely ever see the mechanic. I usually buy gloves, socks, chain oil, tubes or something when they do stuff like that for me.

https://www.ebay.com/itm/28403507325...UAAOSwdipfevuE

You could also put a clamp-on housing stop a few inches up the downtube from the BB and run the cable through the stop and then through a short piece of cable housing that is long enough to run under the BB.

It will serve as a cable guide and keep the cable from rubbing the BB shell. I am running this sort of thing for both the RD cable and rear U-brake cable on my RockHopper and they work well.

Otto

Well, we got one donation in today that had the same "Diver's helmet" fitting but missing the reduction ferrule (thank you [MENTION=526099]Rolla[/MENTION]), fortunately we have quite a few variations and was able to find one in our salvage stock to get the bike back on the road. Last time I went to that box was help someone else here on the Bike Forums.