Interference-Fit 40-Valve 4.2 liter V8 Engine

Much as I love Audis, I also own several BMW E30 cars, and I sell parts for them too. My most popular item? Cylinder heads for the BMW M20 engine.

This engine design uses a timing belt and it’s an interference-fit engine, meaning: a particular valve will try to occupy the same space that the piston does, unless the camshaft is doing its job. If the camshaft doesn’t do so, then the valves impact the piston. I’ve seen the resultant damage range from not-great to downright horrible, such as valves being broken off and mashed up in the cylinder, finally making a hole and then bouncing around the crankcase, doing more damage.

All it takes for this all to happen is for the timing belt to fail. This has never yet happened to me in one of my own M20-engined BMWs because as soon as I own the car, I tend to change the timing belt.

However, a failed timing belt HAS happened to me on my 1983 Volvo 240 Turbo. Engine damage: zero. Reason: the engine is a non-interference design. Even with the camshaft not doing its job at all, the valves never impact the pistons. After replacing the timing belt, the Volvo was good to go again.

The failure of a timing belt has, to my knowledge, consistently been the result of gross neglect on the part of an owner who either knew better or should have known better.

So now when it comes to the Audi A6 rocket-ship, is the 40-valve V8 4.2 engine an interference-fit design or not? In typical automobile-forum tradition, a great many people voice their clueless opinions on the Web (including by rudely lecturing on the need for regular timing belt replacement) when someone asks the question: is this an interference-fit engine?

I prefer to get my facts first-hand. So, I walked to a dead Audi 40-valve V8 4.2 engine that I’d bought inexpensively, for analysis, a few weeks ago, and I inspected it. The cylinder heads were already off.

One valve was literally bent out of shape. Valve-shaped bright marks were obvious to see on the tops of many pistons, and the impact zone was similarly visible by there being bright metal on the sides of some of the valves. Did the valves impact the piston? Yes. Is this an interference-fit engine? Yes.

This issue came into sharp relevance yesterday, because someone in the SF Bay area was advertising his 2001 Audi A6 with the 40-valve V8 4.2 engine for $375. The engine had failed. Personally, I thought the car would be a good deal, engine or no engine. I contacted the seller but received no reply and shortly afterwards, the ad was deleted. Someone else had presumably bought the car.

Sadly I am not convinced the buyer will be happy. In the ad, the seller seemed to exude an air of being irritated and said he didn’t want to answer many questions. He did, however, mention that the car needed a new timing belt, which he’d already bought, and he indicated or implied that the new owner needed simply to install the new timing belt and all would be well. However, that’s an unlikely scenario. More likely the engine would start right up and run after the timing belt replacement, but at some point soon the damaged valves would take their toll. The hot air rushing past the unwelcome gap in the valve seats would cause burned valves pretty quickly. A clueless owner might not notice the car eventually missing on one or two of the most-affected cylinders especially since the loss of power would be gradual and there are so many cylinders.

So, from here on, I resolve to always change the timing belt on any Audi I own that has the 40-valve V8 4.2 engine. And before I buy one, I plan to diligently do a compression test, first.

Oddly, all the damaged A6 40-valve V8 4.2 Audis that I’ve seen advertised, or in junkyards with engine damage were 2001 cars, and all the damaged such cars with transmission failure were 2000 cars. Maybe a coincidence, maybe not.

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