DMC-12, definitely replace the back piece and O-ring. I guess you meant to say "camshaft adapter", obviously the distributor rotor doesn't come with the camshaft attached. From your pictures it appears you removed the old distributor rotor from the distributor rotor adapter so I don't understand your problem with the three 3mm allen head bolts. At this point best to buy parts you need from a MB dealer. Avoid buying a Uro distributor rotor adapter as Uro parts are inferior Chinese parts.

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Fred Hoelzle