To line up the camshaft marks, turn the engine in direction of rotation (never turn backward) using a 27mm deep socket wrench on nut in center of vibration damper(balancer). This is the "wheel" on the front of the engine that has the belt pullies attached to it. This is best done by 2 people: one turns, one watches for the marks to line up. If alone, turn only 3 clicks of the socket wrench at a time and check for alignment each time engine is turned.
The marks are lined up when the notch in the spacer behind the cam drive gear lines up with a short line on the cam tower. There is a more accurate way of doing this by checking valve movement with a dial indicator
but I don't deem this necessary.
After the marks are lined up,look down between the radiator and engine on driver's side, you should see a pointer that will point to the degrees stamped into the balancer.
If the engine is perfect, when the notch is lined up with the line, the pointer will be at 0 degrees on the balancer. Normal wear will increase the chain length and the pointer will be at higher numbers. Although not specified in MB manuals, there seems to be a consensus that 3 degrees is a maximum before the chain is replaced or an off set woodruf key is put in camshaft gear.
The chain should be tight at all times. If not, the chain tensioner is bad and must be replaced. It is available from parts shop.
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