I just replaced my the voltage regulator Bosch 120A on my 02 325i.
Some information on testing the alternator diode pack (Rectifier) that might be useful to someone diagnosing the alternator.
1.> Once you remove the cover from the alternator, you will see six soldered terminals around the back of the alternator. These are obviously the stator windings ends.
2.> To test if your diodes are ok, you need to put your DMM (multimeter) in the diode test mode ( -|>|- ) symbol.
3.> Touch one DMM lead to the soldered winding end and the other to the long bolt (The alternator output, B1+) terminal. This will either give you '1' or some number (I got 436) that is the voltage drop across the diode in mV. Reverse the leads, the readings should be reverse of what you got earlier. i.e. if you got a '1' before, you will get some number now and vice a versa.
4.> Repeat this test with all the six exposed soldered ends. In each case the the reading should reverse. if you get same reading in both directions, You have a failed diode and your alternator will be putting out AC voltage.
5.> Now repeat this test with the smaller bolt coming out of the alternator.
6.> Again repeat this test between the alternator body (ground) and the winding ends. You should get the same results. Same reading in both directions from step 4,5,6 indicating a failed diode inside.
DO NOT DO THIS WITH THE CAR RUNNING. DISCONNECT THE BATTERY +ve while doing this.
This will tell you if you can get away with just changing the regulator or not.
Please add/correct if anybody has any further inputs on this.
Also, the screws holding the regulator in place are M4x20 with 0.7 pitch and can be found at Ace Hardware.