The following pertains to the W123 300D with the 617.952 engine.

Having just completed this repair (due to failing alternator bearings)
I thought I'd write up a short narrative on this. The upper and lower mount bolts/nuts are 17mm, and the adjustment nut takes a 12mm socket. The nut on the alternator pulley is 22mm.

Note: If you want to remove the upper alternator bracket (for cleaning, etc) you will want to remove the U shaped air intake boot - otherwise you'll have a really tough time getting at the head of the 17mm bolt.

Removing the alternator from the vehicle:
1. disconnect negative battery cable
2. disconnect the alternator from its wire harness (be gentle), you will have to displace the spring clip securing it in place.
3. loosen alternator belt tension adjustment bolt, remove from alternator and bracket ( you will have to play with this a bit to make it clear the water pump pulley and fan blades) At this point the two belts should be loose, but probably still won't come off the pulley
4. Remove the lower alternator mount bolt ( using a socket wrench with a short extension makes this happen more quickly)
5. Remove alternator, bracing it with your free hand as you remove the bolt, otherwise the alternator will fall on you.

Removing 22mm nut holding the pulley on:
1. place the lower mount bolt you just removed back thru the mount hole in the alternator itself. Place an eight inch or so screwdriver thru the one of the spaces in the cooling vanes. Allow the screwdriver to rest against the bolt - this will secure the pulley from turning when you remove the nut on the end.
2. Secure the body of the alternator so that it won't move. Using an impact wrench, remove the nut from the alternator shaft. It may be possible to use a breaker bar to do this, but I haven't tried.
3. Removing the pulley: If your pulley doesn't just slide off - use a rubber mallet to deliver blows to the face of the pulley around its circumference - this will free the pulley from the rust that will have formed after 20+ years. You will have to work the pulley back and forth to get it to come free.
DO NOT LOSE THE KEY THAT POSITIONS THE PULLEY.

4. Remove the pulley, spacer, and cooling vanes from the dead alternator.
5. Place key into new alternator shaft. Place cooling vanes on next, followed by the spacer, and then the pulley.
6. Place split washer on the shaft, followed by the nut.
7. Immobilize pulley ( I simply placed a cloth in my hand and held the pulley edges). Use impact wrench to tighten.
8. Reinstall alternator, placing the lower mount bolt in and hand tightening
9. Reinstall the upper belt tensioning bolt ( be patient, as getting everything to line up and feeding the bolt thru the mount hole in the alternator may take time)
10. Tension the belts to between 25-30 KG of tension- using a Krikit gage helps tremendously. If you don't have a gauge, just tighten the belt until it won't slip on the alternator pulley - don't go overboard - or you'll shorten the life of the bearings.
11. Attach wire harness, attach negative battery cable, check for any extraneous objects under the hood, and then starter up. This repair took me about 2.5 hours from start to finish- It probably could be done in two hours or less by someone who's done this repair before.