MuZ Skorpion bikes

Good question.
lots and lots of japanese cars run the cams without bearings and so do very many motorcycles including all Yamaha 4 cylinders.
Basically, the cam running directly in the aluminum works fine a long as the oil pressure, oil temperature and the oil quality is right. The cam runs on an oil film which should not allow metal to metal contact. Obviously, this does not always work. That one reason why correct oil and allowing the engine warm up is so important. Modern oils need temperature of at least 95º to work properly and are work better around 105-110, some even higher. It is not a question of how thick (or thin) but one of how the film holds up under pressure.

Just obviously, the oil used muzzer did not hold up, did not demonstrate any "Notlaufeigenschaften" (sorry if I can't come up with the English off hand).
Look closely at the three bearings:
-The right one is just like new! That is where the oil feed from the pump comes to the oil galley ( the small hole in the race) and it is the bearing with the least load. The oil is directed into the cam through the groove around the cam inside this bearing.
-The middle one, which is widest, gets its oil from within the camshaft. It shows considerable metal-to-metal abrasion on the sparkplug side but almost nothing on the other. The sparkplug side is "at the end" of the turn as it were for the cam cover. The oil pressure under the cam is enough to support the cam but the film has torn off at the other side. The cylinderhead half of the bearing will be the same, i.e. the trailing edge will be torn off and heavily abraded. This bearing has a normal load to take.
-The left bearing, the timing chain side, is the narrowest bearing with the highest load by far. It has abraded all the way around. I am sure the cylinderhead side is even worse. The left bearing gets the least oil, is farthest from the pressure line at the other side. Between it and the pressure line there are the middle bearing and the three holes for the lobs as well as the two BIG holes for the auto-deco in the cam and the hole at the end of it.
that is the main reason why I am so decided about removing the auto-deco.
The oil pump delivers enough pressure if it is in good condition. THE camshaft has the only plain bearings in the engine, the only bearings requiring any pressure at all. The conrod small end is also a plain bearing but it is directly lubricated. Nonetheless, it dan be subject to the same oil failure and in fact is much ore often defect than the cam.

A certian speed in m/sec is necessary to build up the oil film and this speed is not given at idle. This is another reason for this engine not being a thumper. And one explanation why it is true that the harder you push it at (for it) high revs, the longer it lasts.
All of the modern designed single use roller or ball-bearings for the cam like the XT660R or a KTM. This not so much because the plain bearing is bad - it is not - but because the much shorter camshaft with only two lobs and the ball-bearings are so much cheaper and the one-piece head is also cheaper to manufacture at the bottom line. The casting itself is, of course, much more expensive. The 5-valve 450 Yamaha engine, a very modern DOHC design still has the two cames running in plain aluminum bearings.