The MB vacuum pump drive is a cam -- sort of a cylinder with a wave cut in it in the long direction. The vacuum pump has a roller than rides on this cam -- the cam pushes the roller forwards, then lets it come back under spring pressure.

What you want to do is have the cam set with the low portion at the bottom so that the roller isn't pushed out -- the pump body will then fit up against the block without you having to compress the spring. If you don't set it this way, you can crack or bend the housing as you tighten the bolts that holds it to the block.

Easy to do -- don't put the bolts in unless the pump housing fits right up tight on the block without pressure. It will be obvious when you take the old one off. You will need to have a socket big enough to put on the crank pulley bolt to turn the engine -- it really isn't very easy to set it with the starter!

The other item means that if the roller was ruined on the old one, and the cam surface is bad, the new one will look like the old one shortly!

I expect it looks very much like the 123 and older diesel vacuum pumps -- I've not had it off on mine!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!