Any common circumstances when the valves dropped i.e. high revs, high temperatures ? .
Yes, High mileage and never had a guide re-line done.
Presume they go at the join.
I used to think that, mainly because for a long time I had an on going affection for VWs. once upon a time VWs experienced a number of failures that were due to incomplete welding at the join.
The above sentence needs some explanation so that some people do think I have gone mad.
Although neither is a "performance" engine, the Airhead boxer shares with the boxer VW the difficulty in maintaining exhaust valve temperature control. Both have solve the problem by using hollow exhaust valves, the hollow being partially filled with the metal Sodium which liquifies at a relatively temperature and "sloshes" around inside the valve and acts to transfer heat from the head of the valve to the walls and thence into the metal of the cylinder head.
The valve is formed in two parts, the sodium is introduced to the stem and then the stem and the head are rotated in opposite directions and then brought together with a certain amount of force. The result is that the two parts friction weld together, the completed valve is then ground to size and off it goes.
When VW (and for that matter BMW) first introduced Sodium filled exhaust valves they were a very
avant garde thing found only in performance engines or engines of relatively high output. VW invested large in developing the technology to make sodium filled valves to the point that they could be fitted to a "budget" engine.
They succeeded, but along the way they had some teething troubles when valve heads that had been completely welded fell off with obvious results for the engine.
That problem was quickly sorted, but the myth of exhaust valves failing at the join remains to this day.
What actually happens is that the guide wears which allows the valve to [and here comes the word that the last time I used it this nanny state inspire board protected all readers from] c0ck over meaning that the valve head does not strike the seat truly square.
Now the exhaust valve is running at a nice cherry red heat range and it suffers an off-square landing on the seat 25 times per second at 6,000rpm. You can replicate this by feeding a piece of wire through a pipe and then repeatedly bending the wire at a point just below here it is supported by the pipe.
Eventually the wire will break, the time of breakage being determined by the quality of the metal in the wire and the amount of "bend" being applied.
When you think about it, exhaust valves are the "tough guys" of the internal combustion engine world - they take this kind of abuse for years and years.
But eventually, as you, I and sadly 1,000s of others know, they do give up..
