Is the groove clearly visible or do you have issues with the shaft seating properly in the pinion gear? I was told that you should fit the clip up to the slot and with an appropriate small hammer just tap it home. Seems like you might need three hands to use a hammer and drift punch sitting in the "loop" or small hole used to remove the clip earlier in the process. Hope this helps and you are successful, am looking at the same job in a few days!!!

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150