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

  6. Mark Whitney says:

    Hi Nick
    I made sure the elongated ball bearings weren’t fouling and then swapped out the detent springs and ball bearings, but, like a complete fool, I moved one of the selector shafts (reverse I think, as well as slightly lifting the middle shaft to poke the shaft seal back into where it should be. However, my complete lack of knowledge resulted in me losing all gear selection now. All I get is a whirring sound, where nothing is quite going in. I have no doubt moved a selector fork off of the synchro. So, unless there is a tried and tested way of realigning the selector rods, it looks like it’s going to need dropping out in situ to get it looked at by someone who knows what they are doing. I could kick myself and should have know better.

  7. Don’t fret, Mark, it’s fixable in situ – you’ll just have to remove the detents and top cover again.

    Reverse is the tricky fork to engage on its gear, so check that one is in first. Then fit the 3rd/4th shaft, but don’t worry too much about the fork engaging the synchro hub ring until you get the 1st/2nd fork into its synchro’s groove. It’s a bit fiddly, and it can be made easier by levering the synchro hubs into positions closer to the aperture and then pushing them back by their selectors before fitting the inhibit rods. You can move the front ends of the shafts up a fair bit without losing the fork alignment on reverse or 1+2 while you get the 3+4 fork onto the rim.

    A small torch and a dentist’s mirror (the small type on a stick) will make the job easier as you’ll be able to see the aft forks better.

    Once the forks are correctly engaged in their gears/hubs, you can fit the two inhibit rods between the shafts and then loosely fit the top cover. That will prevent the shafts moving enough for the forks to disengage while you fit the selector seals. Then tighten everything down, fit the detents and you should be done.

  8. Mark Whitney says:

    Hi Nick,
    I seem to be moving in the right direction, literally.
    Went back over the detent springs and ball bearings that I replaced previously and I suspect that the ball bearing for 1 & 2 may be slightly oversized, as it remained in the cover when I took it off, rather than sitting in the groove on the selector rod. So much for aftermarket products. Should have known because the reverse one was a snug fit too.
    I also had a play with the high and low selector, as it occurred to me that I had also been fiddling with that prior to losing all gears. Note to self – check the simple things first, ie could it be in neutral? I then put it all back together to find that I could get motion in 4th and reverse.
    Was just about to give up but then thought of removing the gearbox oil filler cap (it’s the military one, that gives easier access to the selector rods). I then persuaded first gear to move forward with the help of a screwdriver and, eureka, I now have 1&2 working perfectly. Still no third though. So two schools of thought are (1) get it to someone who knows what they are doing and maybe get it refurbished or (2) introduce 3&4 to my screwdriver to see if it will select 3rd. Either way, I am pleased with my efforts, even though 3rd is still not operational. Will keep you updated.

  9. That’s good to hear, Mark, but it does raise a potential cause.

    When the casing is manufactured, once their mating faces have been machined the main case and top cover are fitted together so that the bores for the selector shafts and detents can be drilled accurately. Because of how this is done, it means the case and cover are matched for life. Replacing the top cover with another can lead to bores that are too tall or too flat, the latter causing binding of the shafts or detents. I wonder if that could be your problem.

    If it is the issue, then carefully redrilling the redrilling bores to correct diameter would help. It might only be needed on the detent bores, not the three shaft bores.

    However, I’d try new genuine ball bearings for the detents first. Dunsfold Land Rover will likely have them for a sensible price. They don’t tend to wear, so second hand from a used box should also be fine.

  10. Dean Hetherington says:

    Found this post based on searching for my LT77 box in a 90 suddenly no longer wanting to go 2nd to 3rd, will go 4th to 3rd so at moment going 2nd to 4th then 4th to 3rd to progress up box, does the detents and selectors sound like a good candidate to start looking at (which I’m prepared to do myself) or is it in your experience likely to be a bigger problem that needs an experienced mechanic to potentially do clutch or gearbox

  11. Hi Dean,

    The LT77 selector mechanism is quite different from this. It uses a single shaft with all the forks hanging from it. The trick is that the shaft slides through the forks that aren’t wanted and locks to the one that is. So, moving the shaft fore and aft will only move one fork at a time, as long as the shaft and forks are in good order.

    The LT77 has bias springs on the top cover, above the pivot ball. They laterally centralise the lever in the 3-4 gate. If they are maladjusted, weak or broken, which all can happen with age, then the gate will not line up and gear selection will be harder.

    There are other internal faults on the selector shaft that could cause a failure to select a gear, but I’d expect those to happen in all changes to that gear, not just changing up from the gear below. But I have never worked an LT77, just a little light poking about once on the (similar) selectors on my R380, which is nearly the same.

    I’d start with the bias springs – it’s the cheapest and easiest thing to try. Remove the tunnel cover for access and the steel box cover around the pivot. Select 3rd gear, and then slacken the spring mounts to equalise their pressure on the base of the stick and then retighten. While you’re at it, look for broken tabs on the springs’ baseplate – it’s not unheard of the tabs to snap off, giving the spring nothing to press against.

    Failing that, it’s likely to be internal, and they’re harder alto strip and rebuild than the Series units. However, take a look at Britannica Restorations on YouTube – he videoed rebuilding LT77s and R380s and it’s a lot easier than most people make out – you just need access to some lengths of metal tubing, bearing pullers and a few other less common tools (Halfords sell them).

    Good luck.

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