Just out of curiosity, has anyone ever needed to replace one? And if so, why?
I have replaced two on a 1200 Goldwing because they had gone brittle with age. I used 18mm watch glasses and Araldite. It worked quite well, but the proper gear would be better.
I suspect that we won't actually need to do this for a while on our bikes, although early 1100's that have lived in the Aussie sun all their lives might be getting close.