"Replaced the rusty old ones on my bike and they look great now. Would advise you to be careful when threading your old cable through the guide as there is a rubber insert that is quite delicate and can easily be pearced with the odd stray cable fibre.
My advice would be to make sure that the cable you are threading is cut so it has a clean end and as you insert the cable to gently twist the guide and push it through at the same time to make it easier."