Gearbox problem found

[Craigb]

Stripped the gearbox yesterday on the series 3. Took me a while to find the problem, but it is clear in the picture below!



Inner synchro ring between first and second.

Now to get a new one and reassemble.

[Family_Dog]

I don't see a picture!


-F_D

[Craigb]

Better?

[Craigb]

Similar post on another forum - good for anyone in a similar position:

I have my gearbox in bits and am in need of a bit of advice. Problem is with second gear.

First question is whether selector fork is supposed to be a snug fit in the synchro hub? There is visible wear on mine but judging from the unworn bit it looks like even a new one would have a fair bit of play. (about a millimetre) Doesn't look like groove in synchro hub is worn either. I reckon that amount of play would allow the gear to move enough to jump out.

Second question is if/how a worn synchro ring could cause it to pop out of gear. Folk seem to be suggesting on other posts that this can be a cause but looking at the workings of it I don't see how wear on a synchro ring would do anything other than make it crunch when going into gear. Rings are cheap and I may just replace them anyway but I would like to know how it could make it jump out.

Third thing. I suspect that the teeth on the half of the synchro that is attached to the gear are a bit worn and that my problem may lie partly with that. If I were to change the entire gear for a new one am I going to have a noisy gearbox when the new gear is running against the original one on the layshaft. If so, I may be best to leave it alone and hope that tightening up the selector sorts it. Teeth don't look that bad.


And lastly, would I be right in my conclusion that the detent mechanism inside the 1st/2nd gear synchro hub (Series 3 suffix A box) only holds the gear in its neutral position and doesn't actually do anything to hold it in gear? If so, it looks like 1st and 2nd relies soley on the detent on the selector rod to hold it on gear, unlike the 3rd/4th synchro which seems to have an internal "in gear" detent as well as selector rod detent.

Have I got this right? Any words of wisdom? I'm thinking of buying a new selector fork, possibly a couple of synchro rings and leaving the rest alone. Any thought from those that know about gearboxes much appreciated.

spent some time yesterday figuring out the workings of my gearbox and I think I can now answer my own questions. For anyone who's interested or finds it useful here's what I learnt from my rummagings.

The fault lies with wear in the selector fork and in the dog teeth on the side of the gears. I think in my case the synchro rings are ok and that the damage to the gears has occurred because the fork has become worn. To fix it I am now intending to change the fork, 1st and 2nd gears and the synchro rings. If its noisy, its noisy. Probably won't hear it above all the other noises.

Some other points:

(i) the selector is meant to be a snug fit and despite appearances, a measurement of the unworn bit with calipers suggest that a replacement fork will sort this problem. This is partly the cause of it jumping out of gear in my case but is not the only thing.

(ii) worn synchro rings won't make it jump out on its own but it will make it crunch into gear. Each time it crunches it wears a bit off the dog teeth that actually engage the gear. When the dog teeth get worn to a certain point it will jump out.

(iii) Dog teeth start out only about 3mm wide and so a slight loss of width combined with a bit of play in selector means that the sliding collar that does the actual engagement doesn't have to move much for it to slip out of gear. As the ends of the dog teeth and the teeth on collar both have an angled "lead-in", if these are bearing against one another it tends to force itself out of engagement.

(iii) 1st/2nd seems to rely on selector rod detent only to hold it in gear and so is more sensitive to dog teeth and selector fork wear than 3rd/4th which has a different type of synchro hub with its own internal detent mechanism. Gear locking mechanism is also different on 3rd and 4th. (no dog teeth)

(iv) 2nd gear gets damaged worse because it gets a bashing on every change. Usually you'd be changing into first with box stationary (most of the time) but with changes into second, its all moving.

I think there are some adjustments for selector travel on series 11 box and on the transfer box but they aint no good to me. May help other folk thought.