I installed a new thermostat because the engine won't get warm with the original "tropen thermostat". During idle the engine to get him warm (what hardly works in idle, because of the fixed fan), I opened the oil filler cap to check the oil supply of the valve drive and found a bolt head "dancing" under the rockerarm shaft

Somebody already worked there: all bolts had the partnumber engraved, except the one, which broke.

So either somebody just replaced one bolt, or used a old one, or or or.
Just happy that I found it, although it looked like, that the bolt cracked long time ago.
So I had to remove the head and removed the broken shaft, cleaned everything and reinstalled the whole thing with new parts. Unfortuately the Zylinderwalls doesn't look so well anymore. They sold me the car with 120tkm and now it has 130tkm and I don't think, the engine has "only" that milage.
No polished parts in the wall so far, but the wear of 30years+ can clearly be seen, the hone-marks (what's the correct term for it?) are gone. The engine still runs good, has power, starts easily (for a DI-Diesel without glow plugs) and the oil consumption is within limits.
I also removed the oil pan and checked for radial play in the connecting rods. The engine has some kind of a mechanical "sound" which corresponds to the the engine speed. No play in the rods found.
During idle I loosened the injection-lines from the pump to "turn off" a single zylinder. The sound became a little bit less but was still there. Same effect for all zylinders. I hope/guess it's just the hard sound of a old DI-Diesel. They sound rough at times.
Here around 1:20