I beleive you are speaking of the timing gear wheel on the camshaft and the timing chain, MB were incredibly stupid to use a bicycle chain for timing chain on this engine. Its a terrible design and requires a complete redo of the engine, You cannot remove the timing case with the head or sump in place, one has to be removed.
MB has updated the timing chain to a silent style more bulkier chain in the new cars but its still single row. I dont beleive you can use it in the old engines, I checked the EPC and it does not show any updated part nos.
btw the timing gear wheels for the camshaft are shockingly expensive - best find some used ones and get a new genuine MB star timing chain and tensioner, If you cannot find a MB chain then dont settle for anything other than IWIS brand. Such failures are usually linked with bad oil used in the engine.
