From my readings it is not unusual for old prechambers to crack, particularly wandering out from the various burn exit holes. Obviously if a prechamber broke into pieces this would wreck that cylinder, head, rods, maybe crank. I'm sure there are newer methods designed by metallurgists for coatings to make parts less vulnerable to breaking. A process called "magnafluxing" was used on the cranks of alcohol & nitromethane drag engines many years ago. I believe on the crank rod & main journals to the 'left & right' of where the bearing contacted. This was probably protection against tortional breaks more so than bending breaks. But, there are coatings that are basically one molecule thick and are highly resistive to wear, thus protecting bearing contact areas, cylinder walls, precombustion chambers, though I have no idea as to cost, but I suspect it to be prohibitive. Back to the prechamber: it sees extremely high stresses and has ideal areas for cracks to begin around the holes. I have a 220D and it is a ways down on my list of things to fix, but I should pull the prechambers at some point, as there was a greatly improved design that fits the 220D, that came out not long after 1969. Obviously any carbon deposit buildup around the holes is not a good thing; for that matter, anywhere, carboned glow plug, any hot spot that affects the geometry and temporal course of fuel burn. A coating basically one molecule thick is not going to appreciably affect exit hole geometry, so I don't think that would be a problem. I don't have any great experience to speak of what the mechanics on this site have had opportunity to see over many years. So, much of this is just my own thinking on the subject, for what that may or may not be worth. I must say I view this as a thread I'm going to watch to see what others have to say.