3rd Gear Chatter, rebuild or swap gearbox internals?

Just trying to get some info before I strip my gearbox looking for why 3rd gear is making a chattering sound when engaged. All the other gears are silent and work perfectly, 3rd gear also works fine with no noticeable vibration through the gear stick but I have done a few thousand miles with it with this noise across Spain and Portugal but I'm looking to head abroad again at the end of the month so would like to try to rectify the problem before it does fail or gets worse. It sounds like a bearing rattling but I'm not sure if it is or a gear but I won't know until it's apart? I have a spare gearbox that was rebuilt a few years ago but this has a cracked bellhousing/diff casing, I think my plan is to swap the gear clusters from my spare box to my current box which would save the cost of a rebuild. How easy would this be to do, I'm hoping its just a case of unbolting the casings and swapping the gears over, is there anything I should watch out for as this will be the first time I have stripped the 996 gearbox down and I am trying to make it as painless as possible? Is there any information to show what gear is what within the gearbox and would it be easier just to repair the gearbox I have, obviously this would need a press though to get individual gears and bearings off?

I'm going to upload a video in a bit about the gearbox internals. MC

:thumbs: Great timing then.

I think swapping the good/rebuilt gear cluster in to your current casing is totally doable - I've had a spare gearbox apart recently and from what I can see none of the internal parts are machined to fit a specific casing. EDIT - this is what I mean by cluster. Fully assembled mainshaft and layshaft on the spider. (Reverse gear missing from above, but you get the jist)

This would be my preferred option as it is zero cost, but may want to rebuild this noisy gear cluster at a later date, again just to have as a spare ready to go in if required. This does all depend on the condition of the gears I have in my other box, I know the input shaft was replaced and a couple of other things too but I would need to dig the invoice out to see what exactly.

I managed to strip most of my spare gearbox down and inspect the gears today although it is quite fiddly getting a couple of the bolts off the retaining ring. I noticed the previous time is was rebuilt it had three allen head bolts that are almost impossible to get at and they had previously been undone with a punch, I decided to do the same as I didn't want to round the head off the allen key hole as they were quite tight. All the gears and syncros seem to be in perfect condition, no chips on the gears or pitting and no teeth missing. I just need to work out how to allow the gear cluster to come away from the front housing, I haven't done any research on how this comes away yet so only got as far as I could work out what can be removed easily. Also I compared my previous input shaft against the one in the gearbox and you can see the pitting the old one had on (I think) 2nd gear?

The small gear is used for reverse at the top and first lower down. The larger gear is second. I believe you need to undo the retaining plate, then warm up the housing and lever it out. It is mentioned in the links I sent. MC

Boiling hot kettle of water on/around the housing where the end bearing is retained and it just dropped out for me

Just reading through some of that info now, I have all the retaining ring bolts out but I haven't punched the pins out from the selector rods. I was hoping it would all come out as one assembly as this would make life a hell of a lot easier just swapping the whole lot over but it seems I'm not able to do that. I can then hopefully heat the housing up to remove it all and then get on with removing my currently installed gearbox.

Got the spare box all stripped of its gubbins now and all the bearings feel smooth so nothing needs changing at all luckily so now I just need to roll the car in remove the box and swap internals over, easier said than done.

Good job so far - How do you plan to shim the bearings back up ? The pinion shaft bearing and input shaft bearings have shims to set clamping force on the outer races and set the pinion shaft height ? There is a tool to do this but they have to be removed. Also you are now swapping a pinion shaft into another gearbox with a different differential.. ? They are matched when new and have matching numbers with installation heights and backlash figures. The differentials all need pre-loading and then the backlash setting. This really is a specialised job as quite a lot of specialist tools are required in order to set the gearbox up properly.

I have read about the different shims for this, and seen photos of cut-away cases being used so that the clearance can be measured with the case fitted. What I haven't found yet though is the required clearance figures. In fact there generally seems to be very little information on rebuilding these gearboxes. MC

I'm not sure of the clearance figures, it would be interesting to know how they are measured, I can get the shims (plus the ones I already have) but getting the clearance right will be difficult. The end shims are sort on conical, I'm guessing to take up any slack, is the how they should be or should they be perfectly flat? How does it set the height of the pinion, I'm not sure how that would be affected by shimming the end of the bearing, or are you taking about something else needing shimming? I also heard about the open casing idea when talking to MC recently to be able to visually measure clearances, but I don't get how that casing would then be the correct dimension before putting it back in the box you are rebuilding? I know about the diff being set up and others have had problems when fitting a new diff with it making noise as it was loose, I assume more research will be required on this? Would the best way around this be to change just the damaged parts of my original box, would that be the best option, then as it should all go back the same as it came out?

I am not sure that's what we are talking about, we have casings with the side cut away in order to check engagement depths of shift sleeves. Nothing to do with pinion heights or pre-loads.

Ah yes, that was it. Shimming the shafts to check the engagement. Apparently incorrect setting of these was thought to be a cause of the 2nd gear popout on early gearboxes. Is there a good document for required clearances for setting up the bearings? MC