On the earlier gas engines the interference between adjustment threads was not as tight . So the valve adjustment would creep over time more. Always closing the clearance. Then they made a much greater interference or tighter thread fit on the adjuster.

Even the so called loose ones could be hard to adjust at times. There was a special offset tool you used with a small power bar. It has an open end with a forged loop that placed the socket to put the power handle into right above the adjustment over the arm.

The later ones can be so tight you will have to remove the rocker arm and get a real socket on some of them. Especially if you do not own the right 14mm tool if I remember the size correctly. It too may have changed size over the years. Any adjuster that is fairly easy to move in comparison to others should be checked with a torque wrench and replaced if below specs. Not doing so can result in a burnt valve pretty easy. Some of the later ones where extremely tight.

Expect to have to apply signifigant pressure to move them. A wrench in the hand is pretty much useless to adjust them. Incidentally it is very unusual to have excess clearance occur with that setup. Better have a look at the cam lobe carefully for wear. Mercedes made a lot of cams with the first lobe area quite soft in comparison to the rest of the cam. Or it was an oiling problem. Either way I had to have the lobe built up with weld and the cam reprofiled on more than one car.

If this turns out to be an issue there are probably good used cams out there cheap today for that engine. About the only way you can encounter excess valve adjustment clearance.

If the engine is running well is the lobe is worn unfortunately. Checking it against other lobes with a digital gauge is not a bad ideal. Or someone went at the adjustment with inadequate tools earlier.

In this design I have never seen a valve clearance increase. It always tightens the gap. If one is found the need to replace a cam with a used one might exist. You need the arms that were on that cam and reinstalled in the exact same places on the engine the used cam goes into.

Also I forgot to mention. The arms or cam followers themselves have a thin layer of really hardened surface wear material. On that one cylinder you want to pull the arm or cam follower and it is easy. Look at the area that contacts the cam to make sure that has not worn through. The visual difference between the surface hardness is so great compared to the remaining softer metal underneath it. It is obvious when the hardness is worn through. This is another thing that can cause the excess clearance you ran across. It can get so bad it in effect it machines the cam lobe it rides against.

My point is look for the reason the excess clearance exists in the valve clearance. Not simply adjust it. If it looks like only a very slight wear on the lobe adjusting it is probably okay. Only if you establish the wear is not being caused by the surface arm hardness being worn through.. If it is just replacing the arm or cam follower with a new will do no more harm usually. Do not use a used arm ever in a situation like that. There are existing surface wear patterns to consider.