Thanks for your detailed response. Others are better informed yet I think you borrow a rv timing device for your engine. Or find someone with one. I have no experience with your engine type.
I would check the archives under rv tool and the tool rental program for specifics. The tool has indicator lights on it to set your timing. Seems fairly easy and accurate to use.
That also sounded like a good investigation you performed on your engine. I wonder too if those valve seats were runing perhaps hotter than the others? Or just not lubricated as well. Or perhaps a combination of things.
Sometimes there will be recessed seats because the valves are not adjusted regularily and the adjusters were weak on certian cylinders and moved easier than the others. So repeat failure or burning of the same valve and seat required many heavy grindings of the individual seat. As the substandard adjuster was not dealt with.
This was not your case as you have hydralic lifters but that senario did apply to some of their earlier gas engines. You may have also stumbled on a possible additional contributing cause of the dreaded head failures on that type engine. Much hotter individual cylinder temperatures for whatever reason. Causing unequal stresses in the head castings.
This is an area where the milli volt method can shine. The next time you change glow plugs you could hot calibrate them off the car. Install them and read their milli volt output. That would definatly indicate if you had a cyinder or two running hotter than the others.
On your engine the position and difficulty of swapping glowplugs makes this check far harder than on the older 616 and 617 engines. In my opinion and remember it is only an opinion.
If any one cylinder had a two millivolt or higher reading than the others I would first swap injectors. If the higher reading transits with the injector you have your problem. The other approach that may be cheap and effective is just to have that one cylinders injector checked out for leakage, pop pressure, etc.
Again in my opinion it was a good ideal of yours to look around for any obvious cause. This way the knowledge base continually increases on those heads.