MP4.2+6.0

The core belt function is to turn the cams one revolution for every two of the motor/crank. That happens over a certain number of running inches of belt for a given number of crank gear teeth. The longer the belt, the more inches there are before the belt completes one full cycle from a given mark/position on the belt back to its starting point. Thus longer belts = less physical wear per running inch per mile driven--by a lot. And as a secondary point, newer motors go a lot fewer revs per mile (or RPM) at a given speed than older ones. My C1 (chain driven OHV) did 3000 RPM at 60 on its little I4 tractor motor with a 3 speed; my C3 (with an early gen. Audi SOHC belt drive) even with a 5 speed stick did like 2500 RPM at 60. More like 2000 RPM if I remember my C5 4.2. So just in terms of cumulative engine turns, my C5 had ⅓ fewer motor turns per 100K miles than my C1, and presumably all the less wear in many related mechanical systems.

Returning to the core point on belt wear for a given amount of engine RPM's--plus general design--on my prior C5 4.2, that way longer belt made many fewer total pulley revolutions than my first C3 I-5 5000 belt drive that was not much more than a slight triangle as it wrapped the water pump--where one stupidly had to make the tension adjustment by loosening the pump housing and swiveling it while praying the sealing large o ring held to the flat surface of the iron block. The 4.2 belts are also a lot wider than the old I5 ones were (or an ancient Rabbit gen. 1 diesel I dealt with). Yet IIRC the "old" general belt interval, including for the earlier D2 4.2's and the very early D1's based on the C3 body was 105,000 miles. Audi conveniently moved it down to 75,000--including retroactively--right when they also (at one time) moved the last service interval on what is now known as extended AudiCare down from 75,000 miles to 65,000 miles. That was also shortly after a bunch of Honda Preludes made the news for often blowing belts and eating interference motors that forced a service action and a bunch of large dollar lawsuit payoffs. And now that the belts are largely gone on Audi motors (except for one or two now out at 105-110K miles IIRC), funny how extended AudiCare that covers anything on the regular service list went back to covering 75K... Adding to the conspiracy theory, they actually kind of slipped up when they shipped the C5 RS6 with a 30K mile belt interval. Left them on the hook for up to two belt changes with extended AudiCare. And guess what, per old C5 RS6 board posts back then, Audi took the position thy would only cover the belt itself on RS6's under paid Audi service, not any of the pulleys and other components.

My big picture net FWIW is to expect that--WITH OE/OES quality components throughout--these can go a very long way and well past the current service interval. In the end it's a percentage game, rising from an infinitesimal percentage early, to a small percentage later. My first experience with them with the C3/I5 in the mid 80's was that the water pump was pretty consistently the weak link--and the fan relay pack, and the fan, and the radiator, and the coolant switch, and... Back then you could get at the thermostat a lot easier too. Likely on the 4.2 motor era--again with OE parts--it would be the thermostat that may give first and de facto prompt the job. Each time I have pulled and replaced any OE pulley or tensioner, at most I wondered about whether it might have little squeaks, but never saw any sign of bearing issues. I really haven't had any really fast Audi water pump failures since the C1, and usually the front seal blow out starts peeing coolant anyway as it shows early signs. I wouldn't ever push it that far, but as these cars age and drop in market value, it starts to be more of a judgment call on putting $ in at OE parts prices. If it were a classic or big bucks almost impossible to find motor like an RS6 I would stay on the belt change treadmill a lot longer.

On my W12 I remain timing belt liberated. I did the serp belt at 105K when I thought I might have a minor water pump drip that was coming down to the pan area--cake walk to get at compared to a 4.2--but it was apparently just an upper hose o ring in the same area that I could only diagnose as I started to drain coolant and pull parts off. Frankly the proverbial mile long serp belt still looked very clean and pulleys and tensioner great, so left it all be--other than serp belt I had already bought from AudiUSAParts/Sunset.

mishar

Speed (not rpm) of timing belt depends on pulley diameter and engine rpm, so it is pretty similar on all modern engines. Longer belt makes fewer revolutions and spreads wear over greater surface that definitely helps, but makes more bends and drives four cams that does'n help. So I am not sure if I was wrong.

MP4.2+6.0

I tore down a C5 4.2 head on a bench to do valve stem seals. Also did a simple I5 C3 SOHC head with an old 10V to change to sodium filled turbo exhaust valves when I bumped the compression, and changed the cam on the same motor twice to try a more aggressive duration set up. Plus many C1 OHV heads with old school ball rockers and push rods.

The net is the individual spring pressure on a 5 valve 4.2 head isn't even close to the old 2V heads, yet belts go back to those 2V days starting as early as the Audi Fox/B1 80 and the Rabbit/Sciracco/Dasher in the mid 70's. The modern springs are all simple small single spring set ups instead of the old nested dual spring stuff, at least in the Audi heads I did. Of course the whole valve train has been way lightened over the years, and they moved from shim based OHC with direct mechanical cam lobe force to the more buffered lifter set up starting with the early V6's. Plus, with 40 valves, at any given point in the motor rotation you often have simultaneous but somewhat varying movement of different valves acting off the many cam lobes, with slight differences intra cylinder overlap for either exhaust or intake as the valves ramp on and off the cam lobes. Meaningfully different intra revolution mechanical forces, impulses and total valve opening than the very distinct on-off lobes of the old 8 and 10V stuff. And along the way my valve spring tool went from this clunky ginormous C clamp looking thing with a foot long lever arm, to a small jawed screw in thing looking more like modest nice gear puller with a 2-3" turning wheel. Still have those obscure tools and the necessary head jig by the way for someone experienced doing a deep motor dive.

mishar

You are right that more cylinders makes smoother stresses on everything, including timing belt, but two plane V8 crankshaft has uneven firing order per cylinder bank that includes bumpy camshafts torque too.

Spring forces are conservative. Whatever you put in to open a valve you get back while it is closing, minus friction. That generally doesn't depend on camshaft profile. But I5 10V single camshaft has 6 camshaft bearings while V8 40V four camshafts has 20 bearings and each one creates resistance.

But all this is more of scholastic discussion. What hurts timing belt most is oil, heat, time and old toothed pulleys. They are aluminum, prone to wear and should be replaced if there is any sign of uneven teeth surface.

dvs_dave

Oh the maintenance free dream of chains. Except the nightmare of the tensioners crapping out and the chains are at the back of the engine in true Audi style.

Mister Bally

Not wanting to get into a big debate; I've had a '78 VW Rabbit (gasoline), two (yes, two) 1980 VW Scirocco S models, an '81 Audi 4000 with the I-5 engine and an '87 Audi 5000 quattro turbo I-5. With my calibrated eyeballs and photographic memory; at Idle, the timing belts on all of the engines prior to my A8 moved faster at idle. Since the engine idle speeds were similar and the A8 belt seems to be triple the length of the other belts, it is safe to say that over any period of driving and idling times, the smaller engine belts are prone to more wear. As MP points out, the width of the A8 belt is greater than that of the smaller belts.

As an aside, since I had two of the I-5's with the silly water pump tensioner, it was a joke how you had to loosen and move the water pump to tension the belt. As MP eluded, the O-Ring was flat as a pancake where it contacted the engine block. It would not reseal once moved. You had to remove the timing belt and the water pump to replace the O-Ring. On the '87 turbo engine, a very special crank holding tool was required to remove the crank pulley bolt. Also a torque extender was needed along with a 3/4" drive breaker bar. Fun stuff.

dan87951

Decided to pass the oil changes were not done regularly and carfax came back with 2 accidents. I also called on a 2004 A8 with 18k miles and it has only had 2 oil changes since new. Wowzers! Why do people treat there expensive cars like this? The hunt continues...

mishar

If it is really only 18K miles two oil changes is all that was required by millage. Actually only one would be enough. 10 years would require 9 oil changes, but skipping those is not really that bad. Being so peaky usually doesn't end up well.

P.S. People treat expensive car like that because they are not expensive for them.

MP4.2+6.0

I'm not a fanatic on oil change needing to be generally annually if the miles just don't warrant it. If you look on the Euro side where the oil condition system is enabled, it can run out to 2 years or almost 20K miles. But, the longer time line will probably involve an algorithm that shows good/thorough warm up when it is driven--rather than just short in town driving cycles where it can't warm up and burn off the crankcase condensation. The longer miles side will be when it is both the former, plus lots of easy highway only miles without extreme operating conditions. BMW's and Mini's have those systems enabled here for example, and my Mini would typically report a change around 14K miles and well past a year with mixed suburban and highway. It's on a peaky high HP per liter turbo motor actually, so unlike the more robustly powered A8, frankly I never would run that type of motor past half what that system says. You have more latitude on the D3 with a 4.2 or other (gas) motor. But, a guess is that may have been a second vehicle, a "little old lady (or man) once a week to church" actual example, neither of which may have a lot of long mile, thorough warm up cycles more conducive to long oil intervals. And worst case, it could have been tampered with. Really unlikely statistically, but yes I've become aware there are systems now that can play games even with supposedly not changeable stuff like on Audis, coupled with occasional busts that make the news for that kind of stuff even with electronic odometers.

Nonetheless, probably along some lines you were thinking, only two changes even with low miles suggests it probably got minimum service attention for warranty repairs, things that relate more to time than miles (like brake fluid changes), etc. Then two accidents reported, let alone any minor off the books stuff, doesn't sound great. Hmmm, maybe the 90 year old church lady stuff?? Of course there are the no big deal scuffed bumper or broken tail light or keyed paint kind of reports, and then the big smash ups where when newer and the cars take a lot more damage before they got totaled, so it could be a much bigger accident.

These are getting older now, but if at all possible I would put value on either a one owner car, or one where the current owner has had it for years. Should be correlated with more upkeep, better records, etc., at least for a decent condition one. The many turn over cars, lease returns (lots of those pre 2008 crash BTW) that got churned a few more times, etc. are kind of like playing roulette if you don't already really know them, can inspect with a fine tooth comb, scan them real time, etc.

dan87951

One oil change in 10 years is enough? I'm speechless...

If the miles are low the max i'll go is 2 years, beyond that contamination starts to set in regardless of miles or at least thats what my blackstone reports show. I'm sorry but I totally disagree, anyone who is willing to go 5 years between oil changes is definitely a gambling man. I'll pass, doesn't exactly inspire confidence that the car was maintained or treated well.

Both BMW and Porsche had a hard lesson in their high drain interval recommendations. If anyone has taken apart a BMW engine that has followed their high drain interval recommendation knows exactly what I'm talking about. I know Porsche has lowered theirs to 10k-12k and I believe BMW did the same. I personally, won't put more than 7.5k miles or 2 years whatever comes first on my oil. I guess it all depends on how long you keep the car...