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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