How the Gearbox Selectors and Detents Work

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.

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Comments

  1. Hi Paul,

    The small screw should have a truncated conical tip, so if it looks bell shaped at all, then it has been worn evenly around the cone. Usually, they tend to get worn on two sides where they contact the edges of the pivot ball slot, but it could have been adjusted to have relatively even wear all the way around, or may have been replaced with a new one previously. It is unusual for them not to have some significant wear by now.

    The pivot ball is harder and less prone to wear. But, if you can see indentation or folding of the slot edges, then it may need replacement. Gear lever wear is more commonly a problem at the bottom ball, which should be spherical and have a slot off an o-ring or big slot for a plastic band, but they can become almost cubic with wear which makes gear selection harder and causes more stick rattle.

    Jumping out of gear won’t be related to the stick or pivot, though. Replacing the detent springs may work as they can weaken with age. Some use the reverse detent springs in all three positions as they’re stronger, which can help with a worn gear box for a while, but the jumping is normally a sign of worn bearings and shafts that require a rebuild. It is worth checking the castlated nut at the back of the main shaft (inside the transfer box) is right, though, as this causes jumping if loose, especially from 2nd.

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