Mercedes-Benz Parts Catalog Mercedes-Benz Accessories Catalog Mercedes-Benz Technical Articles Mercedes-Benz Tech Forums
 
Follow Pelican Parts on Facebook Follow Pelican Parts on Twitter Follow Pelican Parts on Instagram Follow Pelican Parts on YouTube Follow Pelican Parts on Pinterest Follow Pelican Parts on Tumblr
  Search our site:    
View Recent Cars  |   Cart  | Project List | Order Status | Help    
Bookmark and Share

Pelican Technical Article:

Replacing the Timing Chain

Mike Holloway

Tools:

27mm socket wrench, needle nose pliers, flathead screwdriver

Applicable Models:

Mercedes-Benz R107 (1972-89)

Parts Required:

Right timing chain top guide rail, left timing chain top guide rail, timing chain

Hot Tip:

Remove any component to provide more room

Performance Gain:

Access to guides and rails as well as chain

Complementary Modification:

Replace chain tensioner, tensioner rail, and cam sprocket

One would think that when the engine is at speed there is a higher incidence of wear and eventual failure.  This isn't necessarily true in the case of certain components, namely the timing chain.  Chains, sprockets, gears and even bearings experience the most wear upon startup. The timing chain and the various components (guide rails, tensioning rail, chain tensioner) wear out after 100,000 miles and should be replaced.  The sprockets should also be replaced.  The chains can stretch but they also wear.  As each rolling element of the link begins to experience wear it sets up for increased vibration and movement of the chain links and pins against the sprocket surface.  As many teeth as the sprocket has, the chain has many more links.  The sprocket surface wears down contributing to more movement as the chain passes over the sprocket surfaces. This creates an even greater opportunity for chain breakage.  

The 450SL engine was considered robust yet like so many engines, there are issues that must be addressed.   The engines can accumulate enormous mileage given proper maintenance and with careful attention to the timing chain health. A chain or chain guide problem is the usual culprit that will destroy these engines.  This issue is not exclusive to the 450SL but crosses all engine technology that uses timing chains.  Timing belts are far more susceptible to breaking but chains can stretch leading to other issues.   In many examples there are unacceptable degrees of chain slack and worn or broken guides.  These need to be replaced. Neglecting the chain can lead to catastrophic results. The usual scenario involves a broken guide being dragged up the chain and wedging between the cam gear leading to chain slippage or breakage. This will allow the pistons to contact valve heads.  In the early days, the condition intensified due in part to Mercedes wanting to lighten the valve train, as M-B reduced the timing chain width by about half with the introduction of the 380SL. This design flaw increased the failure rate of the chains, which had a direct impact on warranty. Mercedes phased the double width timing chain back. 

Replacing the chain can be done so with greater ease if the timing chain cover is removed.  Many folks claim the only way to completely remove the cover is to pull the engine. This may not be the case.  It is stated in the service manuals that the oil pan has to be loosened in order for the cover to be completely removed.  While this may be the case, for the purposes of this article, the timing chain will be replaced without taking off the cover.

In order to change the timing chain, all the components in front will have to be removed. This includes the belts, pulleysalternator, the distributor and the power steering pump were all removed. Please refer to these articles should you need assistance.

Also, the distributor should be set at top dead center (TDC) and marked.  This is needed when you put everything back together. The distributor was also removed earlier. Hence, you can refer to the distributor removal article if you need assistance.  The valve cover will have to be removed as well.  In order to do that you will have to remove the air box. Refer to the valve cover removal article for help on how to do that project.

Before you actually change the chain, it's a good idea to see just how much it has stretched. As long as you're at it, look to change the valve cover head gasket.  It's a simple change and inexpensive.  Another thing to consider is the camshaft oiler tubes. They have plastic connectors that will need to be replaced. These get brittle with time, and if they fail it will have a dramatic effect on the cam. Cams require oil.  These tubes provide the cams with a continual oil bath to the cam surface. 

Replacing the chain guide rails is also a very good idea if you haven't done so.  I suggest that you replace these prior to the chain replacement.  There is an article on this, which should prove helpful. It's always a good idea to be careful you do not drop any debris or parts into the opening. The chain can get away from you so it's a good tip to secure the chain with a wire.  A wire coat hanger is an easy fix.  Hook wires to the ends of the chains so you can keep them under control. Do not start the engine until you turn the engine a couple of revolutions by hand to make sure both cams and the crankshaft are in sync. You will have to take off the valve cover to expose the chain.  This is covered in the article on replacing the valve cover gasket. 


When the crankshaft is at TDC 0, the timing marks on the cam stands must never be early, however they should be right on or slightly late.
Figure 1

When the crankshaft is at TDC "0", the timing marks on the cam stands must never be early, however they should be right on or slightly late. To properly measure timing chain stretch, using the 27mm socket, read the crankshaft timing indicator markings. The spot where it lines up with the pointer is approximately how much stretch your chain has. However, this can be off by up to 3 degrees. Therefore, if you are reading 5 degrees of stretch, you may actually have up to 8 degrees and you need to replace your chain. If you have 12 degrees or so, you should notice an increase in fuel efficiency and power.

After you determine that the chain must be replaced, use the box that the new chain comes in as a dispensing tool.
Figure 2

After you determine that the chain must be replaced, use the box that the new chain comes in as a dispensing tool. This will allow the chain to be kept clean as well as being easy to work with. Cut or tear a small part of the box's end flap off, and pull the chain through it. Then tape up both ends of the box.  Take the master link parts bag out first.  

If you get lucky, you will have access to the master pin, which will allow you to remove the link clasp and easily hook up the new chain, turn the cam and feed the chain through.
Figure 3

If you get lucky, you will have access to the master pin, which will allow you to remove the link clasp and easily hook up the new chain, turn the cam and feed the chain through. This may not be the case. This is what the master pin clasp looks like.

The next step you need to do is clamp the chain to the sprocket with two vice-grips.
Figure 4

The next step you need to do is clamp the chain to the sprocket with two vice-grips. Place a rag down below the sprocket to avoid grindings from falling into the engine. Then cut the link with a bolt cutter or grind off one of the link's ends. Be very careful not to bend the other links or they will get caught up in the chain sprocket teeth. As soon as the link is cut off you can pry off the link plate. 

This is what the master linkage looks like taken apart.
Figure 5

This is what the master linkage looks like taken apart.

As soon as you have the link off, take the trailing end of the chain (the side closest to the tensioner) and tie it off using a stiff wire such as a clothes hanger.
Figure 6

As soon as you have the link off, take the trailing end of the chain (the side closest to the tensioner) and tie it off using a stiff wire such as a clothes hanger. This will help prevent the chain from falling into the engine should your clamp come off in the process.

Attach the master link of the new chain and attach it to the old chain.
Figure 7

Attach the master link of the new chain and attach it to the old chain. Making sure that the vice grips are secure, slowly turn the engine in the direction of revolution (clockwise as viewed from the front, the ratchet wrench set to "Tighten" or "On"). Do not turn the engine too far, but only so that the cam sprocket rotates approximately 90°.

Attach the vice grip to the sprocket on the trailing end freshly exposed.
Figure 8

Attach the vice grip to the sprocket on the trailing end freshly exposed. Attach it on the back end of the sprocket.

Take the old chain off the sprocket down to the clamp.
Figure 9

Take the old chain off the sprocket down to the clamp. Then put the new chain on the sprocket in its place. Make sure you don't skip any links.

Now, you need to clamp on the new chain at the top middle - the same spot you removed the clamp from earlier.
Figure 10

Now, you need to clamp on the new chain at the top middle - the same spot you removed the clamp from earlier.

Making sure the clamps are on tightly, turn the engine clockwise again until the camshaft sprocket moves 90°.
Figure 11

Making sure the clamps are on tightly, turn the engine clockwise again until the camshaft sprocket moves 90°. Your engine should look like it did in the previous picture, with more chain out than before.
Feed the chain according to the following steps:
Add the clamp on to the back of the sprocket's left side.
Then remove the top clamp.
Remove the old chain to the left clamp, and then put the clamp back on the top of the sprocket, to hold the new chain on.
Then remove the right side clamp.
Continue to roll the new chain in by rotating the engine so the camshaft moves 90°.
The steps include adding the third clamp, removing the top clamp, pulling the old chain off the sprocket to that third clamp, putting the new chain on, then re-attaching the top clamp, removing the right clamp, and rotating the engine until you have reached the end of the old chain/beginning of the new one.

Once you have reached the end of the old chain, your chain ends should line up perfectly on the sprocket.
Figure 12

Once you have reached the end of the old chain, your chain ends should line up perfectly on the sprocket. However, it is very possible that if you were in a rush, you skipped a tooth on the camshaft. This also happens if one of your clamps slipped off. If you did skip a tooth, it will look like the picture.

To resolve this issue, you will need to do this: Remove the wire tying the old chain to the hood and attach it to the leading end of the new chain.
Figure 13

To resolve this issue, you will need to do this: Remove the wire tying the old chain to the hood and attach it to the leading end of the new chain. Now, CAREFULLY rotate the camshaft CLOCKWISE until you move it forward the EXACT amount of teeth you need. As an example, if you were off by one tooth, move it just ONE. Now be warned - rotating the camshaft may also rotate the engine slightly as the new chain, being much tighter, will pull the crankshaft. Re-clamp the chain to the sprocket, and check the chain to make sure that the two ends meet perfectly. If so, remove the old chain from the new chain. BE CAREFUL not to lose any of the pieces of the master link. Then, attach the two ends of the new chain together with the master link. You're almost done!

Now, even if you are CERTAIN that you DIDN'T skip a tooth, you WILL want to check to make sure that your camshaft and crankshaft are properly aligned to prevent certain valve train damage. The camshaft gear has 36 teeth, ONE tooth on the camshaft: 10 degrees is 20degrees at the crankshaft! That's the same as 20 degrees of chain stretch. It will almost certainly cause fatal valve-piston interference!
Turn the engine clockwise until the camshaft-timing indicator lines up with the marking on the tower, as you did in step 2. If your markings are within 3 degrees of TDC (top dead center) then you did the job well and you deserve a pat on the back. Now, it is important to note that the guide rails wear with age (especially the solid plastic ones) and can cause the chain to read more stretch. It is VITALLY IMPORTANT that you change your top three rails if they are of the plastic variety when you do the chain.











Bookmark and Share

  Search our site:    

View Cart & CheckOut | Project List | Order Status |  Help    

 

[Home] [Customer Service] [Shopping Cart] [Project/Wish List]
  [Privacy Statement]  [Contact Us] [About Us] [Shipping] [Careers]

Copyright © Pelican Parts Inc. -    DMCA Registered Agent Contact Page

Page last updated: Wed 12/7/2016 02:49:35 AM