I have read the replies to your question and they are very informative. Finding that little hole in the back of the pulley is tedious. My car is an 87 260E.
The socket head cap screw uses the 8mm hex Allen wrench.
I made a tool similar to that described by another post. The short end of the "L" fits into the hole in the back side of the pulley and fits betweeen two flanges or "ears" on the water pump. With that tool in place, the pulley will not turn. Of course you will have to turn the pulley with the 8mm hex key wrench to find the hole in the back side of the pulley.
My socket head cap screw was a monster to remove. All of the internal hex edges rounded off. I tried to reshape even larger Allen wrenches to no avail. I finally purchased a short stubby multisplie screw extractor from MSC Industrial supply to remove the cap head screw.
Believe it or not, I had to use a piece of pipe 2 feet long on the end of the wrench attached to the screw extractor. I felt sure I would break the bolt or water pump or both. I had to pull with all my might to loosen that bolt. After I got it out, the bolt was not corroded or rusty. It appeared to have a little bit of Lock Tite on the threads.
I hope you have better luck than I did. It is a standard right-handed thread. That is, standing in front of the engine and looking toward the engine, you turn the socket head cap screw counter-clockwise to loosen.
Best of Luck.