It's wear on the gear teeth that usually forces the need to change to the later components. Cheap oil and owners neglecting oil changes. Airheads in the uk used to be ridiculously cheap and were used as winter hacks and run on a shoe string. There's a legacy of that abuse in the bikes that survived.
In material terms it isn't a big deal to replace the input shaft and all of its components or any of the other gearbox internal components. From a financial points of view it's difficult to justify the expense to the owner of a gearbox. In the uk an ex change gearbox is around £350 + 20% vat so this becomes the benchmark that people work to. If you tell them their gearbox needs £200 of bearings, seals and gasket and £180 for a new input shaft and that there's wear on other gear components they will generally opt for a re conditioned gearbox from somewhere else. The down side of this is that the guys turning round the ex change boxes aren't going to replace the worn internals. They generally just replace the bearings, not always the expensive input bearing, clean up the case and turn it round as cheaply as possible.
We are finding in the uk that used, late gearboxes are disappearing at an alarming rate as people snap them up as spares on the off chance that they may need one in the future. I bought a job lot of spares from somebody moving away from airheads and now have the ridiculous situation of having ten spare gearboxes. The plan is, when I have some spare time, is to rebuild them and sell them on.
Rob Farmer screwed with this post 12-08-2013 at 12:16 AM