I am very happy to report that I was able to purchase a new viscous fan clutch for my '90 420SEL off ebay for $90 plus shipping. I installed it this morning in less than thirty minutes, and it is working very well. My overheating at traffic lights issue is solved. I had read just about every thread relating to the fan clutch, and was prepared for a long day of removing the radiator, having difficulty with the hex-head bolt, all the usual nightmarish stuff. However, once I got the hood up, and the light on the subject, I noticed the fan assembly was held to the water pump pulley with only four 10MM bolts that I could easily reach with a ratcheting boxed-end wrench. By turning the power steering pump bolt, and thus the water pump pulley, I was able to loosen and remove all four bolts quite rapidly. Once the fan assembly was off the pulley, I unclipped the fan shroud from the top, disconnected the top radiator hose, and gently lifted out the shroud and the fan assembly simultaneously. Removing the old clutch was as simple as removing four 10MM bolts from the clutch/fan blade assembly, and bolting on the new clutch. Replacing the fan assembly was a simple reverse of removing it: no 8MM hex-head bolt to be found, no special tool needed to keep the fan from turning, no problemo, and no sweat. I hope someone in need of a clutch fan replacement sees this post, and has the same good luck that I have just experienced.

__________________
1980 300SD 146K miles.