Well, I finally completed the rebuild on my 186f diesel. The sudden loss of compression was due to failure of one of the lands between the piston rings. After a lot of research, it appears that the failure may also be related to the fact that my engine was not originally fitted with valve stem seals. This resulted in oil leaking into the combustion chamber via the valve stems, causing heavy carbonisation around the intake and exhaust valves and fuel injection nozzle. Apparently this condition, combined with continuing introduction of oil into the combustion chamber is the source of the piston failure. Well, I fitted new Viton valve stem seals, honed the bore, fitted a new piston, rings, gudgeon and conrod (all bearings and seals, bore, valves and guides were still well within service limits), decoked everything, ultrasonically cleaned the injection nozzle and reassembled the whole thing with a new crankcase gasket. Filled it with BMW spec LL4 5w30 fully synthetic oil and it starts and runs better than it did when new - not a hint of smoke and very smooth (for a one lung diesel!). Glad my generator has an electric starter though, the compression is so high now that Tarzan would have a job starting it manually via the rip cord.
