96 E300D W210, DIY engine fan clutch replace. How hard is it?
 

96 E300D W210,

DIY engine fan clutch replace. How hard is it?

I know this is a bit of a bozo question, but is this a relatively easy job?

Well, I suppose this depends on what vehicle you own.

Doi.
I added it to the initial post

96 E300D W210,

Not that bad, remove upper radit or brace to give you a little more room.

Do I need any special tools?
I saw one pic with an o-ring puller

I use a cut off Allen wrench.

Is it just undo the bolt in the middle to remove the entire fan assembly. Then undo the screws that hold the clutch to the fan?

Yes and as tjts1 said, use a cut down/modified allen wrench.

What prevents the entire fan from rotating when force is applied to the 8 mm Allen Head bolt that secures the fan in place?

See this for the W124, which I think is similar/same to yours for this job:
https://m.youtube.com/watch?v=UwPf_sSTL7s

I'm trying to remember.but it seems I have taken a good, used piece of serpentine belt, wrapped it around the pully and locked it down with a pair of vice grips to secure the fan.

Speed. The bolt isn't in there that right, if shouldn't be. Push down on it hard once and it lets go. After that it just spins out.

I hate that design, it makes accessing anything on the front of the engine a PITA. I ditched the fan clutch, it doesn't do much good on this engine anyway.

How did you ditch the fan clutch?
Doesn't the clutch just keep the fan from self destructing if
obstructed and or avoid severely mangling fingers.
I read one guy said he abandoned this fan entirely and replaced with an electric fan.

I got rid of the whole fan and clutch. The electric fans are more than enough for the non turbo diesel.

Assuming the diesel is like a m103 or m104 its not bad at all. You need to get the plastic fan shroud out of the way and often the radiator too if its been on there a while and you really need to force it. If you want to leave the rad in you may need to cut down a 10mm allen wrench.

The belt usually provides enough grip to keep the pulley from spinning. If its not enough use an old belt and wrap it around the fan pulley and a few others. Then bunch up some extra and run it over itself to lock everything up. This should not the first attempt but as a last resort on a stuck bolt its better than rounding off the 10mm hex bolts on the back of the pulley.