I just used a tap from my metric set. Hardly any carbon came out on it and the pre-chambers looked clean when I have had the injectors out, so doesn't seem like reaming is needed on my two OM617 engines.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans