Knackered bearing on output of gearbox??

[Jesus-Ninja]

I think I have a problem :(

Took the gearbox out to do the clutch, which includes pulling out the shaft that runs through the gearbox to the transfer box.

The end of the the shaft had a great burr of metal welded onto it. The picture below shows where it was (I've since machined it back on the lathe)

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The "damage" was at the output end - ie just beyond the splined end that feeds into the nearside driveshaft. So, with the gearbox off, we had a look for the corresponding damage where the shaft sits in the gearbox.

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The pictures aren't clear, but you can see a groove in what looks like the bearing that the shaft rotates in.

What to do? One half says cobble it all back together and get on with driving it, but then the other half says "that's really not good, you should sort it out"

Anyone seen this before? What's the fix?

More importantly, what's the cause? The inner CV boot was split a year ago when I changed the clutch last. This time it's split again, and I'm wondering if the two are related.

Perhaps the car being lowered is putting some extra load on the shaft.

Another thought is that the last clutch was very aggressive, putting lots of shocks through the drivetrain. Perhaps this is responsible?

So, what to do? The bearing that the shaft has worn against looks like it can be removed, but the manual doesn't show the internals of the gearbox. Anyone got a clue?

For clarity, this is the end we're concerned about :)

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[Kenneth]

That shaft doesn't run on a bearing. The machined section (which had the burr in it) runs on oil inside the centre diff (front drive output) shaft. The centre diff runs on bearings though.

For that to happen, you would need to have had foreign matter enter through the half shaft seal (or from inside the gearbox)

I would take the gearbox apart and check it all out, you might as well while you are there. It'll annoy you no end if you have to pull it out again just down the track.

[Jesus-Ninja]

Cheers, Kenneth.

Do you reckon the worn bit in the gear box will need to be changed?

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[Nick Mann]

The hollow shaft in the gear box is one of the output shafts from the centre diff. I agree with Ken that the solid lay shaft that runs between the transfer box and the green cup on the inner cv joint does not have it's own bearing. It is supported at each end. The end with the damage is the cv joint and the diff body. As there is oil on the box could it be possible that the green cup wasn't seated in the box properly? Leading to movement and wear? I'd still be inclined to pull the box apart and check. Just removing the bell housing is enough to have a look at the centre diff and output shafts.

[Jesus-Ninja]

Cheers Nick,

Will, have a look inside. If it looks OK, do we reckon just put it all back together?

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[Jesus-Ninja]

Incidentally, not an oil leak. The CV boot was split, and lobbed grease all over the place. What looks like oil spray is where I mopped up.

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[Jesus-Ninja]

OK, more gratuitous pics having pulled the box apart.

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Everything else, visually, seems healthy enough.

At worst I can see it being a new shaft, and a new sleeve for the shaft to run in. Shaft is obviously easy to replace, although potentially expensive. The sleeve I'm not so sure about. Impossible to machine it in the box, although it looks like it might come out, but not sure how and wary of knackering something trying.

The other option of course is just a s/h gearbox....

[Nick Mann]

I'm trying in my head to remember how all this moves. My worry at the moment would be that the damaged surfaces are going to rub together again - that they no longer have a perfect surface could lead to the same problem you have already had? I think Ken is potentially right about the foreign matter causing the issue, but I'm not sure how tolerant it will be of less than perfect surfaces. I'd guess that they spin at the same speed, so the forces will be minimal and it will cope, but I'm just not happy to sound positive about it! If it was mine I'd get those surfaces as good as I could and give it a go.

[Jesus-Ninja]

I think that's going to be my plan. I've turned the shaft such that there's nothing proud, and I'll get a die grinder into the sleeve to remove any lips. Then I'm just reliant on the rest of the machine length and sleeve to support it.

I'd say there's 25mm of overlap between the shaft and the sleeve, of which 10mm is now less than perfect. I'll be relying on 15mm being enough, and being sure that the 10mm doesn't create a problem.

I chatted to Ben at Eurospec and emailed him some pics, but im thinking that cobbling it all back together is worth a punt, the risk being that it doesn't last long and lunches itself, then I'm back to square one. At least removing the gearbox is a relative walk in the park now!

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[Jesus-Ninja]

the solid lay shaft that runs between the transfer box and the green cup on the inner cv joint does not have it's own bearing. It is supported at each end.

But surely the point where the polished end runs on oil is still a bearing? ie one bears on the other. Not a roller bearing, sure, but if it wasn't bearing at all, then none of this would be an issue.

As far as I can tell, the lay shaft is supported by what whatever it's splines engage with at the transfer box end, but at the NSF inner CV joint end, it's the polished / machined part of the shaft which holds the end steady. The CV joint offers no support, as it relies on the shaft for running true.


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[Nick Mann]

Yes, you must be right. I had assumed that the inner cv cup held the lay shaft but looking more closely, it must be the other way round.

[Jesus-Ninja]

I suppose the blessing is that it's not subjected to any significant lateral loads like, for example, a big end. It's more about running true, and the seal will assist in this as well.

I'll nosey at the gearbox manual, but I think "suck it and see" is the order of the day.

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[Kenneth]

I am pretty sure that lay-shaft flat area runs like a hydrodynamic bearing, since it is the only thing holding the inner CV in place other than the seal.
/edit: can't be, shaft should spend most of its life running at very near the same speed as the diff varying with turning and wheel spin

I would not put it back together without doing at least something about it. If you don't want to replace the diff, could you machine out and press in a bushing?

It is possible that you could put it together and it will work fine. It is also possible that it will turn to custard and you will kick yourself (been there) for not doing something about it while you had it all there in front of you.

I currently have my gearbox back out because I wasn't thorough when putting it together last and did not check the sycros... :(


While the shaft is not subject to lateral loads, the centre diff is due to the helical cut of the gears. As such, the roller taper bearings have a reasonable preload on them.

[Jesus-Ninja]

Trouble is, the car is a daily, and I need it for work. Hence the feeling that I may just chuck it all back together and see how we get on. If it starts being flaky, I'll source a replacement box or internals and just take the time to swap it out down the line. I can get the box out in about 3 hours now, so 6 or 7 hours total to replace. That's a risk that is acceptable compared to the impact of having no car now when I do 500 miles a week.

It's a good thought about machining out the bore and reseating a bush. Having looked at the exploded picture below, it's item number 2, the Centre Differential Flange, that is the offending item. If I can get that out, then I could look at repairing it. Might be a good shout.

However I'm confused. The manual labels the diff as the centre diff, but I thought the centre diff was in the transfer box (ie which proportions torque between front and rear), and that the diff in the gearbox was the front diff (which proportions torque between the front wheels). So which is it?

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[Gly]

other way round,

center diff is in the gearbox,

and the front diff is in the transfercase,

[Jesus-Ninja]

other way round,

center diff is in the gearbox,

and the front diff is in the transfercase,

Cheers, Carsten. Right - so, as per the manual, the thing I'm looking at is indeed the centre diff. I suppose all I really need is replacement centre diff flange. Which evos share the gearbox internals with the VR4?

[Kenneth]

Any evo from 4-9 (5 speed) should be OK.

[wintertidenz]

You could always look at upgrading the centre diff while you have the box apart, as I suspect that would come as a complete drop-in assembly.

[Jesus-Ninja]

I thought about that, but I have 3 project cars at the moment, this one being my daily, and I have to apportion funds between all of them. And uprated diff is not going to leave me change out of 500 quid, for which I could buy another box.

Having slept on it, I think there is some good mileage in bushing it. I'll turn out the dodgy bit, press in some steel billet, and then machine it to match. Will be no worse, and potentially better in terms of life span.

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