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.

  2. Dean White says:

    Nick, a bit of advice please, when in second gear I can hear a slight ‘knocking’ and wondered if the detent spring is shot missing or the ball bearing worn, is it possible to remove the spring and ball bearing without taking off the top cover? just checking as if so it would be an easy check without removing the seat base.

    Can the ball bearing be drawn out through the drilling? if so how?

  3. You should be able to do that, Dean. The 1st/2nd detent is under the brass plug. The spring will be easy to remove. The ball bearing can be removed with a steel rod and a magnet, or maybe even a magnetic screw driver (probably not strong enough).

    To be honest, these boxes can make some worrisome noises without there being anything wrong; their tolerances are very large when new and most have worn parts, even those properly rebuilt, and the design harks back to the Rover P3, so is early to mid 40s technology!

  4. Mark Whitney says:

    Hi Nick, brilliant article (as usual) on gearbox faults and confirms my suspicions on my 109 as it will not go into third. All other gears are okay.
    Probably beyond my limited mechanical knowledge so now trying to work out if I look for someone who can investigate and sort out the synchro clips or if I go for a rebuild (ouch). Just need to find someone in Lincolnshire who is capable of doing the job.

  5. Hi Mark.

    I’m glad you found the article useful, and sorry you’re having trouble.

    It’s worth having a look at the detents yourself rather than spending £150 or more on someone else’s labour if you have the basic tools – the hardest part of the job is getting the floor plates out to free the tunnel cover, and that’s just because of the rusted nuts and bolts that hold them down.

    You might also have a problem with the 3rd/4th synchro unit’s leaf springs – these tent to wear in the middle and eventually shatter, and that can lock out a gear (or both, or jam the hub in a set position). Draining the oil and looking for shards of dark grey sheet steel about 1/2mm thick and very hard (spring steel). The bits can be all sorts of shapes and sizes as they get mashed by the gears. As you inspect the detents with the top cover removed, you’ll be able to see the synchro hub and remove any bits of broken spring, freeing up the hub for use. If you do find such a broken spring, you can drive with one or even two of the three missing, but the remaining springs will eventually fail too. You won’t find much difference with two springs in place, but with only one, the lever movement will be very light and little pressure will be exerted on the baulk rings and the gears may crunch more easily on changes. Once the last spring breaks, you lose synchro mesh and will have to double declutch in 3rd and 4th until you replace the springs. Some say you can fit new springs with the unit built up, but I failed after a few hours of trying and had to remove and strop the box to do it. Hopefully, you just have a bit of dirt or corrosion in the selectors!

    Nick

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