The guy at Alton was actually pretty good with me, he sent several nylon gears (at one point both bikes had the nylon gear type) and eventually just sent a new direct drive replacement, however at that time they would have been under warranty, but there was never any hassle with alton. There must be a reason why they went with a nylon gear but i don't know what it is. I don't know the dates of the design changes, maybe yours is an older unit that sat on the shelf for a while. I'm not sure how to tell which one is which without taking the back off, although there are several styles of front cover, some plastic and some ally. I think, from memory, which is suspect at best, the nylon gear type has an ally front cover with a little alton badge in the middle. If no-one else can help I could take a look at the two dead units in my shed tonight. I have often wondered if i could get a bronze gear made to replace the nylon one, but I can't help thinking that Alton would have done that if it made more sense, maybe the nylon one was meant to be sacrificial under extreme circumstances? Somebody must know?