As JoeR said, most auto parts stores sell a pack of 4-5 12-point bits that fit in a 1/2" socket. Has 8mm for the OM603 and 10mm for the M103. Costs $12-15. No clearance problem in my 300SE which has the same engine as your 300CE. Holds up to the higher torque spec in the W126 CD than what's in the W124 CD for the same engine (what's up with that?).

Sixto
95 S420
91 300SE
87 300SDL
83 300SD