by Robert S. Hoover
If you have time could you give the lowdown on what causes that, and
whether it's a catastrophic sort of thing?"
"That shaft that comes out the back of the trans and goes into the pilot
bearing? I know two people (Joe and Marc) who have tried to pull the engine
and ended up unintentionaly pulling that shaft out just this week. Marc
tried to put his back together but now his bus doesn't move. Joe hasn't
If you have time could you give the lowdown on what causes that, and whether it's a catastrophic sort of thing?"
Ken (and the List),
The mainshaft comes out when the circlip and coaxial stud has failed, which usually results -- although not immediately -- from a rear-end collision or colliding with something while backing up.
If the damage is isolated to the mainshaft, reversing gear, keeper and coaxial stud, the repair is relatively easy although the disassembly/reassembly is a pain in the ass.
I don't know if any of the manuals address this specific repair procedure. In most cases of main-shaft disengagement there are other, more serious problems to be dealt with. Repairing only the mainshaft could be the waste of a vast amount of effort, if after the job you discover you also have a shifter-fork or gear-cluster problem.
However, with the above as a precautionary prologue, the repair goes something like this:
Drop the engine and pull the transmission. On a bus, pull the outboard gear reduction units and strip everything down until you have bare axles. Unbolt the driver's-side cover plate from the tranny, seat the PASSENGER-SIDE axle fully into the differential side-gear and GENTLY drive the differential and driver's-side cover-plate out of the tranny using a lead or copper mallet.
Keep the shim stacks together. This is vital.
With the differential out of the way you will be able to diagnose the root problem, which is usually a sheared coaxial stud -- it is a dinky 6mm stud, treaded on both ends, that screws into the center-line bore of the tranny's input shaft, and into the mainshaft.
The coaxial stud is only for alignment, not power transmission. Power from the engine to the transmission is transmitted via the reverse-gear coupling sleeve which slips over BOTH the mainshaft and input shaft. The reverse-gear coupling is held in place by a circlip on the mainshaft.
If the coaxial stud is sheared in the input shaft the repair may require dismantling the transmission (so far we've only been dealing with the differential). The typical bus owner is unlikely to have the tools or skills needed to tackle this level of tranny repair. But if the failure is limited to a problem with the mainshaft, reverse-gear coupling and circlip, the repair usually involves only the replacement of any damaged component.
Taken slowly, with constant reference to the manuals, this repair is within the capabilities of the novice but there is such a range of variables, any one of which can lead to more difficult tasks, that it would be wise to consider replacing the tranny as a unit, allowing you to repair the removed unit in a bench-top environment.
PS -- It should be obvious that any repair that requires removal of both the engine and transmission provides an opportunity to perform maintenance, repairs and replacments to related systems, such as the shifting linking, outboard gear reduction units, rear brakes, heater ducting and so forth. -- rsh