100K to 150K miles on an air-cooled engine is not what I have gathered is the norm. "Top end" rebuilds appear to be a 30K to 50K mile thing with bottom end rebuilds approx. double the miles at which the top end needed doing.

Even at 100K to 150K miles that's not a selling point to me. My 2002 Boxster has almost 300K miles and while one valve (make that camshaft) cover was lifted to replace a VarioCam soloenoid/actuator the valve hardware has been just fine.

My 2003 Turbo has just over 140K miles and I don't expect to have to have a camshaft cover lifted at all. I am hoping my experience with my 996 Turbo engine paralells that of "T2" who covered just over 400K miles in his 996 Turbo with AFAIK not having to open the engine at all. And when he did to address some leaks and to address some wear he found no measureable wear.

 

I did not read the thread, so sorry if I am posting double, etc.

You make it sound as if the air-cooled motors were unreliable. While this is absolutely NOT the case, it depends what air-cooled motor you are talking about. The air cooled cars had their problems as well: 30-50k figures for a top end are not uncommon for a 2.7 with thermal reactors. But the SC 3.0 and especially the 3.2 Carreras motors are bulletproof. They are the Mezger's direct predecessors. 250k+ motors are not uncommon. If you do not want to believe me, please flip through Bruce Anderson's 911 Performance Handbook.

And even the 2.7s with their soft magnesium cases are relatively reliable without silliness like thermal reactors strapped under the heads. And the 2.7 was phased out in 1977! 39 years ago!