Checking the timing chain

If you have just purchased an older V-8 gas Mercedes-Benz, removing the valve cover, checking the chain, chain tensioner, chain guides and cam oiler tube kits should be the first thing on your preventive maintenance list. After 1973, all M116 and M117 engines used plastic chain guides. The three in the cylinder head are the ones that wear out. There is also a tensioner rail with a plastic skin, and two in the inside of the timing case, but these rarely break or fail. On pre-1973 engines, the chain guides were made of rubber and aluminum. The chain guides can still wear out, and on many examples, they have been replaced with the updated plastic guides. 

If the guides are brown and not yellowish in color, they are brittle and should be replaced ASAP. The inner left guide fractures most frequently. The right inner guide is next, while the left outer guide rarely fails. The guides are held in with pins that are internally threaded and can be extracted with a puller, such as a Baum Tools 915-0734 or similar. 

Next, the timing chain needs to be checked for stretch. There are two measurements that need to be taken during this check. First, the degrees of stretch between the camshaft and the crankshaft must be measured. To do this, remove both valve covers. Using either the crankshaft 27mm bolt or power steering pump 22mm nut, rotate the engine until the crankshaft pointer is at exactly top dead center. Then look for the timing mark on the right camshaft. Rotate the engine until the timing marks line up on the right cam spacer and the cam support. Next, check the crankshaft. How many degrees has it rotated? If it is 8 degrees or fewer, then your chain might be ok. If it is greater than this more investigation is needed. 

The next test involves the tightness of the chain. As you rotate the engine, does the chain become slack on its own? If so, is it so loose that it is resting on the chain guides? Can you pick it up and does it feel loose? If so, your chain is worn out. A new chain or a healthy chain will not do this. If you see that the chain is limp and floppy, you are running on borrowed time. While this can sometimes be corrected with a new tensioner, if the chain is old and worn, replace it. 

Installing the new chain

This is a rough overview and isn’t intended as a guide to the job itself. Here are the basic steps. Remove the rocker arms and spark plugs from both cylinder banks using a correct lever type valve spring compressor, such as a Baum 123-0361. While many mechanics use a chain feeder, I dislike these on alloy block engines because it presents the possibility of stripping a head bolt thread. Not only this, but removing the rocker arms takes the pressure off having to time everything perfectly. It is hard enough feeding a chain in.  

After removing the rocker arms, return the engine to TDC and remove the chain tensioner. Cover the right cam sprocket area with protective materials and use an angle grinder to grind off the pressed ends of one of the chain links, unless you have a master link, and using the new master link, couple the new chain to the old. Have a partner hold the new and old chains, while you rotate the engine. Keep the new chain tight as you feed it in, and make sure the old one does not get bunched up. When you are done, decouple the old chain and use the master link to join the two parts of the new chain. Make sure the open side of the c-clip on the master link points to the passenger side of the car. Some difficulty may be expected when you join both ends of the chain on the right side sprocket. You may have to walk the crankshaft a little to allow the right side of the chain to extend all the way. 

After you have completed the chain install, it is vitally important to take the new tensioner and inject oil into the fill hole using a syringe. Verify your cam timing at top dead center. Be sure to rotate the right cam gear counter clockwise to place all the slack on the tensioner side. 

It is in this position that the timing marks must be checked. If they are off, remove the cam gear where there is a timing issue and set the cam to top dead center, rotate the gear inside the chain to top dead center and slide the gear back on the end of the cam. DO NOT uncouple the chain.  After you install the tensioner, turn the engine through several full rotations and check timing again. If this checks out, install the rockers, and again, rotate the engine through several times. It should not hang up, especially with the plugs removed. If all is well, you can continue reassembly and drive away with confidence. Be careful not to pinch a valve cover gasket!