OK, back now, yes the spacer rings are also called shim rings, they are different thickness to make up for slight differences in the housings, I found this out the hard way after buying a diff off an SD to put in my 123 300D. I had to get some rings from a friend in another shop to fit the half shafts otherwise they wouldn't lock the clip inside because they were too thick. I was told that MB used to sell a kit of these spacers, you will find one on each side on the outside of the housing where the axles go in. They probably throw them out at wrecking yards.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!