Sun Aug 17 20:48:06 2003
From: Mike Frank 

Despite the blackout, I finished this repair yesterday. A few tips and 
observations:

First of all, Derrick was correct: it's a poor design. This mechanism might 
be ok in a Dodge, which you expect to last for maybe 100K, but in a 
long-lived engine like a Mercedes Diesel, it's inappropriate, due to the 
design of the bearing.  There's no way to substitute a roller bearing, 
because the reciprocating motion would wear pockets in the race. So they 
used a plain bushing, but there's no provision for lubrication. When the 
factory lubrication dries up, it wears out. They COULD have installed a 
grease fitting and cut a groove in the bushing, but Mercedes seems to never 
install grease fittings.  Once the bushing dries up, the end is pretty 
quick. The rocker starts to vibrate, and potentially will crack the timing 
chain cover. Stinky design.

The thing is to keep an eye on it. There simply shouldn't  be any visible 
or audible vibration. If there is, then replace it.

Replacement is fairly straight forward, with a few caveats. Here's the 
procedure, since it's poorly documented in the book:

1) There are a few unique fasteners involved. Lose nothing, or it's a trip 
to the dealer.

2) Remove the radiator shroud (may vary depending on year):
      - Unclip the thingy that holds the wiring to the top. You must do 
this, since it holds the two halves of the shroud together.
      - Undo the inner half of the shroud by twisting the whole thing, 
directions are molded into the piece.
      - Remove the outer half of the shroud by removing the two clips and 
pulling up. Slide out the inner half of the shroud.

3) Remove the fan blade by undoing the three bolts that hold it to the 
clutch (may vary depending on year).

4) Undo the nut on the plastic rocker. Release tension on the serp belt by 
inserting a bar in the plastic rocker, and levering until you can get the 
bolt out. Then release the spring.

5) Unbolt the shock absorber. Be sure to note the positions of the bolts, 
which have different height shoulders.

6) The serp belt should now be removed and inspected. May as well replace it.

7) Pop the cap off the tensioner bearing.

8) Using a 12mm Allen socket (this is a difficult tool to find, be sure you 
can get one), unbolt the rocker arm. It's held on with considerable force, 
but don't use an impact tool: use a long breaker bar and steady pressure to 
unseat it. There is a large washer behind the rocker arm, don't lose it, 
it's a spacer with a specific thickness. Note: older engines used a 
different bolting arrangement, but all replacement rocker arms use the 
newer (12mm) cap screw.

9) Transfer the jockey wheel to the new rocker arm. Although it uses the 
same type of bushing, it's less prone to wear, so probably ok. But replace 
it if there's ANY play.

10) Install the new rocker arm assembly. Be VERY careful not to cross 
thread the bolt, it's easy to damage the timing chain cover. Use Locktite 
on the threads (the kind that can be removed later, not the deadly stuff 
that can't be undone without heat). Torque the bolt to exactly 100NM (74 ft 
lbs). This is more precise than most specs, and reflects the engineer's 
concern with vibration.

11) Install the dust cover. Make sure it's snapped on tightly all the way 
around, it's very awkward.

12) Replace the shock...if the rocker was bad, so is the lower shock 
bushing. Be sure to use the correct bolts.

13) Replace the belt, setting the tension with the plastic rocker.

14) Replace fan, shroud, etc.

Mike Frank