Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

Valve Timing



See Figures 1, 2 and 3

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Fig. Fig. 1: Offset Woodruff key specifications

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Fig. Fig. 2: Offset Woodruff key installation combinations 6-cylinder engines

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Fig. Fig. 3: Offset Woodruff key installation combinationsV8 engines

Ideally, this operation should be performed by a dealer, who is equipped with the necessary tools and knowledge to do the job properly.

Checking valve timing is too inaccurate at the standard tappet clearance, therefore timing values are given for an assumed tappet clearance of 0.4mm. The engines are not measured at 0.4mm but rather at 2mm.

  1. To check timing, remove the rocker arm cover and spark plugs. Remove the tensioning springs. On the 6-cylinder engine install the testing thrust pieces. Eliminate all valve clearance.
  3. Set up a degree wheel on the crankshaft pulley or cam shaft pulley.

If the degree wheel is attached to the camshaft as shown, valves read from it must be doubled.

  1. A pointer must be made out of a bent section of 3 /16 in. brazing rod or coathanger wire, and attached to the engine.
  3. With a 22mm wrench on the crankshaft pulley, turn the engine, in the direction of rotation, until the TDC mark on the vibration damper registers with the pointer and the distributor rotor points to the No. 1 cylinder mark on the housing. The camshaft timing marks should also align at this point.

Due to design of the chain tensioner on V-8 engines, the right side of the chain travels farther than the left side. This means the right-side camshaft is approximately 7- retarded compared to the left-side, and both marks on each side will not simultaneously align.

  1. Turn the loosened degree wheel until the pointer lines up with the 0- (OT) mark, then tighten it in this position.
  3. Continue turning the crankshaft in the direction of rotation until the camshaft lobe of the associated valve is vertical (i.e., points away from the rocker arm surface). To take up tappet clearance, insert a feeler gauge thick enough to raise the valve slightly from its seat between the rocker arm cone and the pressure piece.
  5. Attach the indicator to the cylinder head so that the feeler rests against the valve spring retainer of No. 1 cylinder intake valve. Preload the indicator at least 0.008 inch then set to zero, making sure the feeler is exactly perpendicular on the valve spring retainer. It may be necessary to bleed down the chain tensioner at this time to facilitate readings.
  7. Turn the crankshaft in the normal direction of rotation, again using a wrench on the crankshaft pulley, until the indicator reads 0.016 inch less than zero reading.
  9. Note the reading of the degree wheel at this time, remembering to double the reading if the wheel is mounted to the camshaft sprocket.
  11. Again turn the crankshaft until the valve is closing and the indicator again reads 0.016 inch less than zero reading. Make sure, at this time, that preload has remained constant, then note the reading of the degree wheel. The difference between the two degree wheel readings is the timing angle (number of degrees the valve is open) for that valve.
  13. The other valves may be checked in the same manner, comparing them against each other and the opening values given in the "Specifications." It must be remembered that turning the crankshaft contrary to the normal direction of rotation results in inaccurate readings and damage to the engine.
  15. If valve timing is not to specification, the easiest way of bringing it in line is to install an offset Woodruff key in the camshaft sprocket. This is far simpler than replacing the entire timing chain, and it is the factory-recommended way of changing valve timing provided the timing chain is not stretched too far or worn out. Offset keys are available in the following sizes:
  17. The Woodruff key must be installed with the offset toward the "right", in the normal direction of rotation, to effect advanced valve opening; toward the "left" to retard.
  19. Advancing the intake valve opening too much can result in piston and/or valve damage (the valve will hit the piston). To check the clearance between the valve head and the piston, the crankshaft must be positioned at 5- ATDC (on intake stroke). The procedure is essentially the same as for measuring valve timing.
  21. As before, the dial indicator is set to zero after being preloaded, then the valve is depressed until it touches the top of the piston. As the normal valve head-to-piston clearance is approximately 0.035 inch, you can see that the dial indicator must be preloaded at least 0.042 inch so there will be enough movement for the feeler.

If the clearance is much less than 0.035 in., the cylinder head must be removed and checked for carbon deposits. If none exist, the valve seat must be cut deeper into the head. Always set the ignition timing after installing an offset key.