A8 FL 3.7Q, 4.2Q and S8 rear brake sizes
 

Can anyone clarify the standard fit vented disc diameter and disc thickness when new sizes for the above D2 A8s (FL 99-02)

The factors seem to have both 280mm diam, 22mm max thickness discs and a 269mm diam 20mm maximum thickness disc listed for these models. As a result there are two Lucas/Girling rear calipers listed for these models, both with a 43 mm diam piston, but many suppliers only quoting a 20mm max disc thickness. Did all post FL A8s of these engine capacities and 4WD move to 280 mm diam and 22mm thickness, or are some running on the smaller thinner discs.

My research suggests the change came at chassis numbers above 4D-x-004999.

Thanks

Best measure the diameter then you are sure to get the correct ones

Disc sizes
 

I'll do that. The thickness should be more accurately assessed than the diameter with the disc in situ, but I was wondering if this is an error on varous factors sites or a variance in fitted items on the cars.

I'll post my own disc size - 3.7 Q FL 1999.

The 3 brake types you will come across are Brembo, Lucas/Girling and Teves. mine is a 3.7 QS 1999 but you do need to check your own as it seems that Audi were using a "lucky dip" system for selecting brakes around the FL changeover period lol

I recently bought new front discs and pads for mine - picked up some Pagid discs and pads for £108 including Vat and carriage - euro car parts keep having 30% off weekends so try there and youll save a bit - use promo code DP30 to get the discount on brake parts currently +++

Brake disc size
 

Just managed to squeeze a digital measuring caliper around my disc with the wheel on. Can do it at a small window just above the brake caliper where the back plate is not obstructing. Its 21.5 cm - evidently 22cm when new. Diam must be 280mm - hard to judge with wheel on. Problem is all the factors selling right rear calipers quote 1999-2002 A8 3.7 and 4.4Q, but then state 43mm piston (OK) but 20mm max disc size (not ok). All except Euro car parts whose registration based system which I suspect finds the chassis number has the 22 mm disc size braks calipers but at twice the price of the factors with the 20mm size (£150 after return of unit). As you might expect not in the 30% discount items on Euro car parts. Did get the discs and pads from them . Even more confusing On 'E bay' even 'Euro car parts outlet' comes up selling the 269mm discs but 22mm thick. Looks like just judging the thickness isn't enough. Oh no - Im going to have to take the wheel off and try and measure the diameter in situ to within the 11mm difference before I can order the parts. Trying to source a repair kit is even more interesting but looking at the diagrams to strip and rebuild it don't really feel I should try this. Have done the fronts before but the screw for the handbrakes mechanism looks like it makes it much more difficult. Probably going to be new LH and RH units from ECP. Cheaper than a gearbox though. Keep checking the mass air flowmeter is giving correct load and flow rate readings using VaG com, (even lite will do this dynamically) if you want to avoid a new gearbox on 1999-2001 3.7s. I change my MAF every two years after my wife's 3.7 needed a gearbox after 15,000 miles because of this problem.

Funny you should say that - mine had a new gearbox from TPS about a year ago after the MAF failed and 4th gear kept slipping on the gearbox, was all fine until then but had never had a gearbox flush as far as I know.
Its worth checking with a VAGCom scan every month or so just to see what fault codes show up anyway. I was lucky I was using Shell Optimax fuel as the faulty MAF had also led to leaner running on the engine but the higher quality fuel had offset the effects somewhat.

MAF reading and gearbox failure
 

Its the MAF not sending the correct load reading that ruins these boxes not a lack of oil changes.

I suspect the load signal can be out without triggering a fault code. You can monitor it on block 2 of the engine section of VAG com.

Block 2 gives a dynamic read out in real time of RPH / Load / inj on time/ mass flow g/sec in this order. This block runs even with vag com lite.

I keep a record of values from 720 to 4000 rpm and watch for any changes between the times I run the test (about every 2 months). Its best to record the initial values after a new MAF and air filter change and monitor from there. The problem is remembering to do it frequently enough although £4000 to repair the potential damage is quite an incentive.

Thanks for the info, I agree the MAF can ruin them but so can lack of oil changes, its well documented on here and elsewhere, so thats at least 2 different things youve got to worry about on the gearbox alone! :(

I know we're digressing from the initial topic, but which gearboxes is this?
And what is the mechanism that messes with them?
Does it apply to PF cars as well (e.g. my S8 which is showing the odd "symptom")?

PF and FL D2 have dodgy gearboxes, though they tend to show different symptoms.

PF tend to hunt between 4th & 5th at the crosover point (like a slippy clutch), and then when you lift off about 70 it'll disengage, then whack into gear again. But it's a gradual thing.

FL failures tend to be more catastrophic on the whole, usually the F-seal which kills reverse.