Instead of buying a reamer, try a 6mm allen wrench. Anything that has hard edges to it and that fits should work. I never thought to crank the engine over, without glowplugs to blow the carbon out. I'll have to try that when ever it comes time to change my plugs out.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member