Its much like cleaning soot from a chimney. The built up carbon can break off and get under a valve or the new glow plug will have a hard time threading in. Most times there are no issues, but Murphy's law can prevail! you simply run a tap of the proper size to clean the threads.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.