Top Guide Rail

 450 Guide Rails
Paul M. Prideaux (

Replacement of plastic guide rails for timing chain on 1984 380SL.
(Assumed to be similar to other W107 models)

For this write-up, “right-side” means passenger side, while “left-side” means drivers side. (US)

1. Remove Valve Covers

2. Remove Power Steering pump and set aside with hoses still connected

3. Remove Power Steering pump mounting bracket from left cylinder head

4. Remove Cruise Control module thing from right cylinder head/intake manifold.

5. Remove Alternator, with bracket,from right-side cylinder head/block

6. Remove Distributor (This needs to be put back in the EXACT position in which it came out)

7. Locate and remove the Timing Chain Tensioner on the right side of the block.

8. Locate and clean out(e.g. with WD-40) threaded pins in front of each cylinder head which hold plastic guide rails in place. There are two pressed-in pins holding each guide rail. There are two guide rails on left cylinder head, and one guide rail on right cylinder head.

9. Find or buy a long (approx. 4 inches) 6mm, 1.00 pitch hex head bolt with a matching hex nut. This bolt is designed to screw into these pins, while the nut will be used to slowly “jack” out the pin from its seat. (It is very important that once the bolt is screwed into the pin, the bolt MUST be held still with a wrench while the nut is being turned. This will prevent the bolt from overtightening and breaking off.) The ‘jacking’ process will require a small spacer (I recommend a 13mm socket from a 1/4” drive set) to jack against. This socket will provide space for the pin as it slowly comes out of the cylinder head. The socket will bear directly on the cylinder head while the nut will bear on the back of the socket, and the bolt will transcend through both of them into the threaded pin. (a picture is worth a thousand words).

10. Some of the surfaces that the socket will come into contact with are irregular. Some people custom grind the socket to fit better. I didn’t want to waste a socket, so I used spacers and stuff to prop against to make sure the whole jig stayed straight.

11. Once these pins are out, each guide rail can be lifted out of the head from above (maybe with needle-nose pliers). I don't think the rails can fall down into the head/timing chain cover (out of site), but just be careful anyway. I had my whole engine apart for other reasons, but I don’t think you have to remove the camshaft gears or chain to get the guides out of the top.

12. The new guides can be carefully placed back into the correct position, and the pins can be pushed back into their holes. You will probably need a hammer to tap the pins back into place. Maybe use a soft (brass) bolt to drive them in place (since they will end up being recessed slightly-more-than-flush when completely in place). Make sure nothing is binding as you do this step, you don’t want to break the new guides. The pin needs to be all of the way in so it can ‘seat’ on the inside of the head.

13. This is the time to also replace the Timing Chain Tensioner Rail in the right-side head. This is the longer, curved (banana-shaped) tensioner rail with a metal backing. This rail keeps pressure on the chain via the tensioner device removed in step 7 above. This rail should pivot freely and is held in by one steel pin which should be ‘floating’ in its seat. To remove this pin, you must first remove a large plug-bolt that can be found on the front of the timing chain cover. Once this (approx. 3/4”) plug is unscrewed, the floating pin can be accessed. My shop manual made it sound easy. The hollow pin is not a ‘press-fit’; you should be able to push a long bolt through the hollow pin, kink it slightly, and pull the hollow floating pin out. At this point, the chain tensioner rail can be lifted out by hand. just reverse the process for installation.

14. Of course, I spent 4 hours getting my pin out. For some reason, mine was seized or hung-up in its ‘so-called’ floating seat. The rail would pivot freely, but the pin would not pull out. I ended up machining a bolt to the exact inner-diameter of the pin, gave it a skim coat of epoxy, and tapped it in place, let it sit for 24-hours. I then used the same ‘jacking’ technique described above to remove the pin from its seats. What a pain!

I realize that much of this may be confusing without pictures. I also must admit that I wrote all of this two months after I did my work, and I wrote it from my office. It is quite possible that I have missed some steps or details. I welcome you to call me at home or e-mail me with further questions. I can also mail or fax the sheets from the MB shop manual that pertain to this job. GOOD LUCK !

You should also replace your timing chain if it has not been replaced. You can do this at the same time or come back and do it later. It can be replaced by only removing the spark plugs, fan, valve covers, and tensioner (not the tensioner rail, just the tensioner mounted on the outside of the block.) There is a way to cut the old chain and slowly feed the new chain in by hand cranking the motor over. This would require another detailed set of instructions.