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Gearbox rebuild. What to look for and what to replace?

I've never stripped a gearbox before but I want to give it a go. I'm intending to replace the synchros and also change 6th gear for a longer one from a diesel. I have access to a full workshop so doing the job is no problem. The only issue I'll have is the special tools required...

So, what exactly am I looking for once I've opened it up?  Obviously I'll be checking gears for wear but I have no idea what a healthy synchro looks like versus a worn one. Is there anything else worth checking and/or replacing whilst I have it out?

Thanks guys!

Oh btw is anyone able to tell me how much I should expect to pay for a new set of synchros? And if I change 6th gear are the synchros specific to the gear, or will the S4 ones fit/work?

You'll know a worn synchro when you see it.
They should have a rough surface - either very fine grooves (those ones don't wear out so easily) or some sort of friction compound.
You'll find that is worn thru to the metal below. You can measure the axial gap between the synchro and the collar with a feeler gauge and there will be a minimum gap. If they are too worn, they fit too snugly.

You'd want to change at least 2nd and 3rd gear synchros as they get the most stick. 1st doesn't as you very rarely change into 1st from anything but idle. Perhaps 4th as well. Rarely 5th or 6th.

New synchros can run from 35 quid to 70-odd, for the B5 boxes anyway. The B6 box has 3 piece synchros but I've never bought one.

You will want to replace all the seals, gaskets, orings that you find, although drive shaft seals are reusable.

You'll want to check the pinion and ring gear for wear - scratches across the surfaces of the teeth - and the same for the 1st gear part of the input shaft - the small part of 1st gear with only a few teeth - this gets worn with cavitation pits.

Keep everything in tubs and clean it all with rags as you dismantle it.
Clean the magnets you'll find dotted around the box too - they should be caked with a metallic grease from gear wear.
Inspect all bearings for grooving or pitting or other damage. You'll likely just find that they are 'misty' looking (not shiny), but otherwise fine.

Unlikely you'll find any other worn gears. Might find a broken tooth I suppose.

Special tools can be a bugger. If you need to replace bearings or some shafts, you need to reshim the box and those tools can be expensive.

Synchros should be the same for your new 6th gear.

This box is simpler in some ways than the 01E box in the B5 - far fewer of the most horrible snap rings you'll ever find.


You do need a couple of special tools really althought you can get away with ghetto methods.
There is a special 12 sided nut on the output shaft that you can buy a special socket for, and there is also a special tool for removing the detents but you can make one fairly easily.
A big press and a good selection of drifts & press plates too

Jock that's an amazing reply. Thank you very much!

I'd rather not buy any special tools if I can get away with ghetto methods lol.
As stated previously, I have access to a full workshop. Even a 200 ton press if need be!

So I'm.about to start this job. Does anyone know the tightening torques or where to find them?

Bentley manual has the specs.
Typically you're looking for 25Nm on the T40 torx bolts which hold the casing parts together. Stretch bolts (driveshaft bolts maybe?) will be less, eg 10Nm, followed by a quarter turn.
Dunno about this box but the B5 box has a 150Nm (!!!) 3-square bolt on the input shaft.
The big detente bolts are 60Nm.

Thanks. But what about the 2 internal nuts on the shafts?

The M30 and M38 double hex nuts?

for what they do and their size, ... I'd say a Lot... 100Nm+????, but better to ask on the forum for someone with that version bentley manual, for the torques.

or talk nicely to your local audi specialist.

Take some pics as you go along. I'd look at them Forum Index -> Audi S4 & RS 4 (B6 & B7) quattro
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