It is something like 24mm (or 22mm), but normally you can untighten them with a "combinatie tang" as well (don't know the english word for it, but it is a common tool to hold (pull/bend) things with).

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)