110 Engine Valve Adjustment

The venerable M110 twin cam 6-cylinder gas engine was available in Mercedes-Benz automobiles from 1974 through 1985 model year, although few were imported after 1982. The 280S/SE (116 chassis), 280E/CE (123 chassis) are the models most seen in the U.S.  A number of gray market cars also have this engine, including the 280SL (107 chassis) and the 280SE (126 chassis).

My most recent experience with one of these unique engines was my daughter's 1980 280SL, formerly my car. The engine needed valve adjustment after running some 600 miles following head rework and valve job.

My daughter, Kim, has a keen interest in learning how to take care of her bright red roadster, and willingly donned her paint-spattered coveralls and rubber gloves to dive into the engine and learn about valve adjustment.... so here's how it's done - this procedure is also shown in the Mercedes-Benz Maintenance Manual Section 0560.10.

Do this job with a cold engine. A fully warmed up engine is HOT - making it difficult to handle the various parts. Leaning on hot radiators is no fun, either. And technically, the engine will cool during valve adjustment to the extent that the gaps will change, making it difficult to get them just right.

First, carefully pull each spark plug wire off its spark plug by pulling on the metal shield only, not the wire.  Mark each wire with its cylinder number on a piece of masking tape - it makes re-connecting much easier. Use an air gun to blow out any debris in the spark plug recesses, otherwise whatever is lurking down there will fall into the combustion chamber when the spark plug is removed. Remove the transmission throttle link that crosses over the cam cover and remove the cam cover.

No special adjusting tool is needed, only a 17mm open end wrench and a set of feeler gauges - one .10mm (intake) and one .25mm (exhaust)(cold settings).

If you don't have a special starter button wire harness, hook up a wire to positive post on the battery. Locate the terminal strip on the right fender well - one of those terminals (Terminal 30) will "jog" the engine starter when the hot wire is touched to it. Clip a second wire to this terminal and clip the other end to a non-metallic component nearby. (I use one of the plastic tie straps on the windshield washer bottle that secures the washer pump wire). When you want to jog the engine over slightly, simply touch the two wires together momentarily. That's how you turn the engine over to get the cam lobe 180 degrees opposite the rocker. Don't worry about getting a specific cam lobe where you want it - look and see if ANY cam lobe is at 180 degrees opposite the rocker.  If not, jog engine momentarily to bring another lobe into position.

(Here refer to the picture in the shop manual)

When you get a cam lobe positioned where it needs to be, remove the spring clip (1) using a small screwdriver to pop it off. Use the feeler gauge to measure the valve gap (arrow). Use the 17mm wrench to rotate the valve adjusting screw (2). The valve clearance is correctly set if the feeler gauge requires a tight pull. Leave the spring clip off to indicate that that valve has been set. Then jog the engine again until another yet-to-be-set cam lobe is aligned opposite its rocker - then remove that spring and set the valve. Leave the spring off.

In this manner you can tell at a glance which valves have been set (no spring clip) and which ones have not (spring clip still there).

When all the clips are gone, all your valves should be set. If you want to make sure all gaps are correct, re-check (and adjust again if necessary) each valve as above.

When you finish checking each valve, install the spring clip to indicate that the valve has been checked. I find that a plastic-faced hammer works well to tap the spring clip back into place on the rocker pivot stud. Simply tap on the lower part of the clip where it snaps on to the adjustment nut.

Note: If any valve adjusting screw turns too easily - less than 20 Nm, it will have to be replaced together with its threaded bushing (3).

When all the tension springs are in place - you're finished. Look closely at each tension spring to make sure it is seated properly. Install the cam cover and transmission link rod. Now is the time to install new spark plugs (gap to .8mm) and re-connect spark plug wires...