Bad News

When we pulled the gearbox last night, we found that the clutch disc is actually just fine. The input shaft had broken.

I have no experience with metallurgy or stress analysis, so I have no idea if this is a sudden catastrophic failure or a gradual breakage. The books I've read have described gradual fatigue fractures as having a visible progression to them; you can see where the initial crack started and how it progressed across the part, until there wasn't enough intact material to take the stress, leaving the last bit to break suddenly with a jagged edge. On the other hand, the sudden catastrophic failures they illustrate show a clean break, with no clear beginning, middle, or end.

The shaft looks almost like a piece of clay that was torn from another piece. There are jagged valleys and sharp ridges radiating from the center, suggesting that the shaft was pulled apart rather than twisted apart. Perhaps the twisting apart is what raised the ridges; I don't know. The transmission is coming apart tonight. With a little luck, the new input shaft will be in and the gearbox will at least be ready to reinstall by the end of the evening.