How hard is to replace flex disk
 

Dose anyone know how hard to replace the front flex disk? What do i have to do to remove the flex disk?

You'll need to lower the drive shaft and transmission slightly in order to access the disc properly and tighten securely. The transmission, center carrier and probably rear differential might have to be lowered slightly to do this (or not, depending on your skill - just makes it easier).

Make sure you mark how the drive shaft links to the transmission and differential yokes, so they remain balanced when you reinstall everything back. The flex discs install in a proper way, the yoke clearance will show you how they're oriented.

New discs come with the Allen bolts, if you have a good set of Allens, make sure you have leverage (socket Allens come to mind) but if not then the old bolts should be fine.

Whether they include Allan or Hex bolts depends on the brand. Allan style are superior for multiple reasons. Namely on the front, where you have to go through the balancer disc to reach the bolts, and a hex socket will not fit.

The best brand, Lemfoerder, come with Allan bolts.

Just pulled mine last night. Beforehand, I did a lot of searching through posts on this subject. I found it easiest to jack up just one side of the car, front and rear, with good jack stands and chock the opposite wheels as you will need the car in neutral with the parking brake off.

I believe your driveshaft will have the same index marks on the two portions to reorient to the same position. They appear as a single > on the front and two< on the rear (Look at the flats on the mating ends.) But marking with something more visible would be easiest.

If you're just doing the front one, first you will need to lower the exhaust but it can remain attached at the front end. Then remove the cross brace for the transmission support and the tunnel cover. The center support can remain but loosen it to make it easier. Loosen the clamping nut to enable compression of the drive shaft then the bolts for the disc can be pulled. Note where the bolts pass through the "star" of the balancer. Compress the shaft and swap them out. Also, note that there is a front side of the disc and the ferrules of the disc fit into the transmission flange and driveshaft flange (very important!) Use all consistent hardware.

Unless cost is an issue, you may find it worthwhile to go ahead and do the center support and bearing along with the rear flex disc as you will have done 80% of the work already. It's not a difficult job but can be pretty grimey if you've dealt with fluid leaks in the past. If you decide to clean it up a bit beforehand, let it dry overnite to avoid the constant drip!

Here is an excellent wikka http://www.peachparts.com/Wikka/W123DriveShaftSupportBearing
Also:
http://www.peachparts.com/shopforum/tech-help/235575-flex-disc-orientation-safety-durability-issue.html?highlight=flex+disc
http://www.w124performance.com/docs/mb/W124/driveline_balancing.pdf

I didn't see any markings to line back up too. I just tightened mine back up. I hope I got it lined up...

You have to supply the marks. A little touch of paint or grease pen on either side does the trick.

Answer
 

Bad Flex Disc
http://www.peachparts.com/shopforum/diesel-discussion/121285-bad-flex-disc-post868326.html

The Death of a Flex Disc ... did I kill my 240D?
http://www.peachparts.com/shopforum/diesel-discussion/252017-death-flex-disc-did-i-kill-my-240d-post2194084.html

What happens when you ignore that "little" driveline vibration!
http://www.peachparts.com/shopforum/vintage-mercedes-forum/205055-what-happens-when-you-ignore-little-driveline-vibration.html

Flex Disc Orientation, safety and durability issue.
http://www.peachparts.com/shopforum/tech-help/235575-flex-disc-orientation-safety-durability-issue.html

If your W123 is shaking down the highway. . .
http://www.peachparts.com/shopforum/diesel-discussion/223050-if-your-w123-shaking-down-highway.html

Correct way to install front driveshaft guibo? (FLEX DISC)
http://www.peachparts.com/shopforum/diesel-discussion/187593-correct-way-install-front-driveshaft-guibo.html

I've got one "special" socket for that. 8 sided and thin walled.

Using a pneumatic socket wrench makes the job go alot faster too.

i agree with SD Blue #4, however i didnt see any marks on the drive shafts, made my own, and i didnt have to drop the exhaust.

On the W126, you will need to lower the exhaust. IIRC the W123 is different, as it doesn't have the resonator mufflers.