I recently bought a new bike that arrived with a broken shifter and - rather than send the bike back - I suggested the seller give me a partial refund and I'd do the repair myself. The refund more than covered me.

The bike has internal cables, which I've never replaced before, so I looked at a couple of You Tubes on how to do it, but didn't find them particularly helpful. So if it helps anyone here, here's how I did it:

1. Release cable from the component (having taken off the cable end of course)
2. Remove outer cabling on the 'exit' side of the frame
3. Tie 2-4lb fishing line to the cable, about 2" from the end - a couple of tight knots will do, nothing fancy and because the cable is twisted it won't slip easily
... the fishing line needs to be a couple of feet longer than the cable route
4. Attach electrical tape (any tape will do) to the free end of the fishing line - both so you can see it, and so it doesn't accidentally follow through the frame
5. Pull cable gently through frame until fishing line appears
6. Cut fishing line from cable

7. Thread the new cable through the (pre-frame) outer cable housing and tie the fishing line to the cable end - then poke the cable into the entry point in the frame

A guy on You Tube advocated using cotton thread and pulling the cable through... well, it struck me that that's the bit that can go wrong...

8. When threading the new cable, don't PULL it through the frame with the fishing line (you might pull the line off the cable)... instead, PUSH the cable gently through and merely use the line as a guide by taking up the slack - you might need to wiggle back-and-forth a little, if the cable end hits an obstruction
9. Look to the exit point in the frame for the cable end - hook it out, if it doesn't naturally pop out.

Job done.

I'm not saying I've invented all this, but I didn't find anything that I thought was simpler - and of course it only applies to replacement cables, not to building up from scratch - so if you've got your own way of doing either, please add it to the thread.
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If your bike has a continuous cable housing re-cableing is ultra easy. Simply replace the cable and the housing one-at-a-time and use the housing to guide the cable and the cable to guide the housing.

If you have a bare wire running through your frame tube, figure out how to position your bike so that the cable exit is pointing straight down. Feed the cable through the frame while you watch the exit. Gravity will guide the cable to where you can snag it with a bent piece of wire or something.

If it was only that easy .....

Some frames are difficult. My old Orbea has molded in cable stops, with a tiny hole in the center, just a little larger than the wire cable itself. This would never work with gravity, or a vacuum cleaner & thread.

The best way is to push some temporary thin tubing onto the existing wire from the back end. It makes a guide through the frame. Push the new cable through the tubing, then pull the tubing out the back.

See my summary post here.
And more details here.
The tubing:

Actually, step 1 is get on urbandictionary.com and compile a list of new swear words to have handy while doing internal cable routing.

Relying on gravity to align a cable with it's exit port is a fool's errand. All cables have some lengthwise bend/shape to them which gravity won't straighten. But to use this bend, or add a short dog leg at the cable's end, is a time honored method to try to catch the exit port while both pushing of the cable while twisting it. The twisting will make the cable end move about and hopefully let it catch/find the exit. Andy.

There are plenty of ways to get this done smoothly. The common element is to attach a "fish" or retrieval line to one end of the cable BEFORE removing it from the frame.

In some cases I employ a short cut. After getting some slack to work with, I cut the cable at the lever end so I have plain wire there. I use shrink tubing to splice the new wire to the old and push/pull it into place. It's basically the same, but less back and forth.

BTW - the other key element of the job is to remember to thread all the ferrules onto the cable, and thread the housing through any fitting it might route through. Otherwise, you'll get extra practice for this kind of work.

Internal Cable Routing Kit | Park Tool

^^^This^^^

It's kinda spendy, but if you have to route internal cables on occasion, it's worth every penny.

100% totally unnecessary if only replacing existing cables.

May be helpful, though not necessary to newly install internal cables if the frame came without pulls. But anyone with the least bit of mechanical creativity can do without spending the dough.

The alternatives are some decent string, heavy thread or fishing line, some electrical tape, maybe some shrink tubing, and a bit of the stuff between the ears.

[QUOTE=Andrew R Stewart;19076765]Relying on gravity to align a cable with it's exit port is a fool's errand. /QUOTE]

Why do you feel that it's necessary to disparage somebody else's experience?

I have personally done it successfully many times. The most recent involved a Klein bike that had some pretty small exit holes under the down tube. It didn't take me much time at all. Most times it takes longer to figure out how to position the bike than it does to run the cable.

[QUOTE=Retro Grouch;19077171]Sorry if I sounded like that. What I meant to say better is that the cable will not hang completely straight and so mere gravity ALONE won't insure a cable exit. Some internal routings have large exit ports, often with removable covers which make this far easier. But some don't. It is these poorly designed ports that are especially hard to see inside of while also trying to align the cable end with the port. Hence my suggestion of adding a small bit of a bend/dog leg to the last inch of the cable to allow it to be twisted about inside the frame, contact the sides of the frame while feeling for that exit. I hope this better explains my point. Andy.

[QUOTE=Andrew R Stewart;19077205]Thanks, Andrew, that makes me feel better.

I'll probably try that dog leg thing the next time that I have an internal cable to run too.

[QUOTE=Retro Grouch;19077266]Yes, my face had egg on it. I get caught up in replying sometimes that I don't always see the obvious slight I have written. Andy.

My bike came with "inner housing" installed in the frame. Running new cable to the rear brake was pretty simple as it was a straight shot, but the shift cables not nearly so easy. In particular, the FD cable has to make a sharp bend around the BB and it took forever to keep jabbing the new cable until finally it made it around the bend. Because of the inner housing, there's no way to attach any sort of string or fish line that I could figure out (the inner housing doesn't allow much space for any kind of attachment. I tried super-gluing but it didn't hold. And to make the bend, the FD shift cable can't be too stiff at the leading end).

scott s.
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I had to do this recently on an '87 Pinarello Montello. I used a length of Jagwire housing liner (extremely useful stuff, though I've never used it as housing liner) and bent one end up in the manner you suggest. The entire length had an arch to it from having been coiled on the roll, but I had to bend the end to get it to "find" the opening. It still took 5-10 minutes of twisting and stabbing but it eventually popped out. After that, it was a simple matter to thread the brake cable through the liner and pull the liner out (because it wouldn't fit into the ferrules).

Heh, that's happened to me.

That sounds similar to my Orbea, that had the thin tubing on the wire inside the frame. I assume it was to avoid friction with some foam pieces inside the frame. It's the same tubing I referenced in my post above. I slid a long section of tubing onto the end of the wire, and pushed the new tubing into the frame as I pulled on the wire at the front exit.