My mates 1993 300d 128,000miles decided not to start
so was taken to a local merc specialist (comes highly recomended) Heater plugs failed (seemingly the multi-valve engine eats them!) was the diagnosis..
None of them would come out even after the application of the blue spanner! next they took the head off and took it to an engineering speciallist (hmmmm) one came out but 5 now sheared off in head!!!!! they said they were stuck in with carbon?
My mate has been given the head back + quote for new one!

Please someone out there must have some ideas? I have taken many heater plugs out and never had that problem even in high mileage cars.
Any ideas why it happened and more importantly any thoughs for a cure?