this is a Lemond Zurich from around 1999 that I'm converting to 10 speed in rear. I can't get the cable to thread past this metal part that seems to be a fine adjustment for rear derailleur. I can't pull it apart. There is a spring mechanism between the part attached to bike frame and the rotating adjustment. Any experience with this? Ideas? Thanks, Bob
They don't pull apart; the idea is that you're getting the couple mm of adjustment range you actually need, and the upside is it's an attempt at the easiest possible design to adjust on the fly while riding. If there's something physically blocking the hole for the cable, you could presumably clear it by poking a pin or needle through. Make sure the cable isn't actually getting stuck at the end of the housing though, either by the ferrule or by the end not being prepped properly.
The shifter cable come in two parts, the cable housing (outer) and the metal cable (inner) that slides inside the cable housing. The adjuster on the down tube acts as a "cable stop." It stops the cable housing (outer) from passing through, but has a small hole to allow the inner metal cable to pass through and continue to the rear derailleur, where it enacts shifts. The adjuster is threaded so that screwing it in it out changes the effective cable housing length, which in turn changes the tension of the inner metal cable allowing fine tuning of index shifting.
If you are able to pass the inner metal cable through, then everything is working as it should. In no way should the black cable housing pass through. If you cannot pass the inner metal cable though look for blockages (as suggested in other answers) or replace the downtube adjuster (they are inexpensive).
Thanks, everyone, for your answers. I ended up pulling the whole adjuster and housing, which is inseparable from the adjuster, off of the down tube. Being able to spin the adjuster and housing around the cable enabled me to pass the cable through (spinning the cable within the housing, when it is attached to the bike, is very difficult). Eventually the tip of the cable found its way across whatever ledge was holding it up.
I didn't start this way because I couldn't get the adjust off of the down tube. Having decided that this was my last resort, I whacked if off with a hammer.