There are 2 types of nylon cam gears, they were changed on the d port motor, the later ones are better as they a metal all the way to the last about 20mm, which is nylon. The earlier ones are only metal about 30mm from the cam. I have see many early ones that have started to stress crack from the cam, tiny cranks that can be hard to see.which is said to allow the gear to wobble untill it fails. Jons is correct that all the gear that is connected to the cam, as in a wild cam with big lift and stronger valve springs can cause them to fail alot quicker. NOS ones pop up on ebay from time to time. You can buy match sets of crank and cam gears, but they can be hard to find. Cheers O