Parts - Engine Damage
HarryMann: Cylinder heads on the VW 1.6 and 1.9 diesels are well know to develop cracks between the valves and from the valves to the pre-chamber. The former can eventually end the life of the head when they break through to the valve chambers. This is a heat and detonation problem, the heads being alloy.
Here is a 1.9TD (AAZ) cylinder head that has suffered severe carcking damage from overheating, but not thro' failed cooling, usually excess boost and fuelling. Also from 'running on oil' - resulting in detonation, a not uncommon problem, particularly with these canted 50 degree installs, though with bad breathing can occur on upright installations. This head finally gave up the ghost when the head gasket went, even though the cracks linked the exhaust and inlet ports! So although they crack, they're toughies...
Piston edge erosion
HarryMann: Either excess heat (v. high EGTs) or bad injector spray pattern