For the benefit of those who suffer gear selection or jumping issues, this is a quick post with some photos of the selector mechanisms.
The gear stick sits in a pivot level with the gearbox tunnel cover. The stick extends a little further through this ball, where it sits in a row of three steel cups. Each of these cups is attached to a selector shaft. Because the nature of the movement of the bottom end of the gearstick below the pivot ball is the opposite to the top of the stick, the right hand selector shaft is for reverse, the middle for 1st and 2nd gear, and the left for 3rd and 4th. The shafts slide fore and aft, but in the opposite sense too, so the reverse shaft will slide rearwards to engage, as will 1st and 3rd, while 2nd and 4th require their respective shafts to slide forwards. These shafts have forks attached, which slide locking members into the gears to prevent the selected gear from free-wheeling on the main shaft (the reverse gear actually slides across to engage, unlike the others which maintain their position always) .
To hold the selector shafts in their neutral or in-gear positions, they have detents. These are grooves machined in the sides of the shafts, with spring loaded ball bearings that sit into these grooves when the shaft is in the required position. The detents also serve a second function: they prevent simultaneous selection of multiple gears. By using a length of steel rod with rounded ends, like an elongated ball bearing, between each shaft, a cunning extra set of grooves on the shafts and a sliding pin in the middle shaft, it’s impossible to have less than two shafts in the neutral position at any time.
With all the shafts in neutral, each shaft’s detent ball will be sitting in the middle of the shafts’ outboard grooves (top grooves on middle shaft). Each of the shafts will also have their inboard (both sides on middle shaft) grooves aligned with the detent bars.
As a gear is selected, the outer detent will sit in a new outer groove, but the inhibitor bar will have no inner groove in which to sit – it is forced into the groove of the adjacent shaft, locking it in its neutral position. If the left or right shaft moves into a gear selection, the inhibitor bar in contact with the selected shaft will press the pin in the middle shaft against the other inhibitor bar, locking the furthest shaft too.
The reverse gear itself has no detents, just that on the selector shaft. The same is true for the in-gear positions on 3rd and 4th, though their synchro unit does have three detent springs to hold the neural position when selected. The 1st/2nd gear synchor hub has a set of thee springs and ball bearings that work in a similar fashion in the sliding member of the hub as to those on the shafts. Failure of the springs on the synchro units requires gearbox stripping, but a problem with the selectors can be worked on with the gearbox still in place.