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Replacing Timing Chain Tensioning Rail
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Pelican Technical Article:

Replacing Timing Chain Tensioning Rail

Mike Holloway


1 hour1 hr


$50 to $250




Stud puller or small slide hammer or rail pin puller guide with 6mm extractor pin, needle nose vice grip pliers

Applicable Models:

Mercedes-Benz R107 (1972-89)

Parts Required:

Timing chain tensioning rail

Hot Tip:

Remove any component to provide more room

Performance Gain:

Proper chain tension and increased wear resistance

Complementary Modification:

Timing chain, chain tensioner, cam sprocket replacement

The Mercedes Benz 450SL is considered by many to be one of the most attractive cars Mercedes produced. It is a stylized combination of power, grace, and comfort and was sold under the model designation R107. At over 3,600 pounds and designed to meet (and actually exceed) strict safety regulations, the 450SL was nicknamed "der Panzerwagen", which means "the armored car", by the engineers who designed it. The 450SL was produced from 1973 through 1980, after which the R107 became known as the 280, 380 and 500 SL.

The SL variant was a two-seat convertible with a standard soft top and optional hardtop and optional folding seats for the rear bench. The designation SL derives from the German Sport Leicht, or Sport Lightweight and was first applied to the infamous Gull Wing 300SL. The 450SL was the third generation SL. The SLC (C107) derivative was a two-door hardtop coupe with normal rear seats. The SLC is commonly referred to as a 'SL coupe'. This was the first time that Mercedes-Benz had based a coupe on an SL convertible platform rather than on a sedan, replacing the 280 and 300 SE coupé.

The robust, V-8 powered SL is a joy to drive. Many drivers test the power of the 450SL. While the speed off the line is questionable, the top end performs smooth and responsive, giving the driver a sense of security. The 450SL cruises comfortably at speeds in excess of 75 mph.

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 sloppy movement as the chain passes over the sprocket surfaces. This creates an even greater opportunity for chain breakage.

The guide rails will be the first order of business to address. In order to change the guide rails in preparation for the replacement of the timing chain, all the components in front will have to be removed. This includes the belts, pulleys, alternator, the distributor and the power steering pump were all removed. Please refer to these articles should you need edification.

I also removed the radiator. Normally you wouldn't have to do that but it was already out due to the development of another article. Refer to removing the radiator article for help in that regard.

Also, the distributor should be set at top dead center (TDC) and marked. This is needed when you put everything back. The distributor was also removed earlier. Hence, you can refer to that 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.

After the chain guide rails, the chain tensioner, and the timing chain sprocket on the right side have been removed, it is now time to change the chain tensioner rail.

The tensioner rail is held in place by bearing bolts. These devilish devices provide hours of frustration if not properly dealt with. There are several ways in which they can be removed. The easiest way is to use a tool offered up by Pelican Parts called a rail pin puller guide with a 6mm extractor.

The tensioner rail can be found after the chain tensioner has been removed.
Figure 1

The tensioner rail can be found after the chain tensioner has been removed. Refer to the chain tensioner removal article for assistance.

There are two closing plugs located towards the bottom right side of the timing chain cover.
Figure 2

There are two closing plugs located towards the bottom right side of the timing chain cover. The top-closing plug will have to be removed to access the bearing bolt that holds the tensioner rail in place.

Once the closing bolt is removed, a 6mm bolt is screwed into the bearing bolt.
Figure 3

Once the closing bolt is removed, a 6mm bolt is screwed into the bearing bolt. You can use a slide hammer or a stud puller, but most slide hammers will not fit due to the fact the slide is too long. I was able to find a small one but I still had to cut off 4 inches to make it work. A 6mm bolt is screwed into the bearing bolt and the slide hammer fitting fits over the bolt shaft. Slide the weight to the back creating force to pull on the bolt. While this is the method the Mercedes Benz service manual calls out, I didn't have any luck. I also noticed that I would have had to cut the slide even shorter if the radiator was still in. You run the risk of slipping off the bolt and sending the end of the slide hammer into the fins of the radiator. A better method is to use a stud puller and a 6mm bolt, with a washer as a spacer. 

I used the stud puller.
Figure 4

I used the stud puller. The stud puller has a reverse thread. The base is held in place with an open-ended wrench. I used an adjustable wrench. The socket wrench is turned clockwise to back the bolt out. It is counter intuitive. You have to make sure that the bolt that is screwed into the bearing bolt will not turn when you are backing it out or you will shear it. If you use a washer between the bolt, it will reduce the chances of the bolt shearing. If I was to going to do this job again, I would use the Rail Pin Puller guide with 6mm extractor pin Pelican Part # TOL-M0044-SIR. Once the bearing bolt is free it can be unscrewed from the bolt. 

Once the bearing bolt is removed, the tensioning rail can be slipped out.
Figure 5

Once the bearing bolt is removed, the tensioning rail can be slipped out. Installation is the opposite of removal. 

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