Hi Jim,
I'll start...
Engine must be cold.
Remove the valve cover (requires removing the main linkage rod, maybe the power brake vacuum line, breather pipe, etc.).
Turn the engine in the normal direction of rotation (never backwards) until one of the cam lobes is pointed straight up; that's the valve you will adjust. Doesn't matter where you start.
Detemine if it's an exhaust or intake valve and select the appropriate feeler gauge (intake 0.003", exhaust 0.007"). Exhaust valves are on the injection pump side, intake valves are on the intake manifold side. The adjusters are on the opposite sides from the valves.
Try to slide the gauge between the base of the cam lobe and the cam follower (rocker arm). If it goes in easily, too loose. If it won't go in at all, too tight. If it goes in with some effort and you feel some drag when you pull it out, it's good (this may be tough to do with the 0.003 feeler, it's easy to wrinkle it if you try too hard).
If adjustment is necessary, take your valve adjusting tool and turn the adjuster - clockwise to loosen, ccw to tighten. If your tool won't turn the adjuster, you may have to remove the cam follower and use a deep socket to break the adjuster loose. After that your valve adjuster tool should work. Some find that a good crowfoot wrench works, but for 230s the factory tool may be best.
If the adjuster turns very easily, it won't hold adjustment very long, and you might want to consider replacement.
The easiest way to adjust is to loosen the clearance so the feeler slips in, then tighten the clearance until you feel drag on the feeler. Not so much drag that it's hard to pull out, but enough drag that you can definitely feel it.
After adjusting that valve, write down which valve you just checked/adjusted. I just number them 1-12 from the front. Keep track of each valve so you don't forget one, and double check that you have all 12 listed when you're done.
Then turn the engine until another cam lobe points straight up, this is the valve you adjust next, using the same procedure. You shouldn't have to turn the engine very far. If you observe the cam lobes, you can see which one is coming up next and have an idea of about how far you have to turn the engine. Sometimes two will come up at almost exactly the same time, but one is always slightly ahead of the other. You don't have to be super exact on "straight up", but you want to be close, a good eyeball "straight up" is close enough.
Turning the engine has several options. If you can get a socket on the crank pulley nut, that's one way. If you have a manual transmission, put it in 4th gear, jack up d/s rear wheel, block the other wheels and release the hand brake, and turn the raised wheel forward while observing the cam sprocket. Strap wrench on the crank pulley may work. "Bump" the engine with the starter. The oh-so-conveniently located camshaft nut must never be used. If you use a tool on the crank pulley, make sure you remove it before trying to start the engine! Taping a reminder to the steering wheel is not a bad idea.
Removing a cam follower requires first removing the little spring that holds the end of the follower, it should just pry off fairly easily. Then push the valve down, and the follower can be lifted off the adjuster ball and pulled out easily. There are special tools available for pushing the valve down, or you can make one, or Benz Dr. Dan uses a heavy screwdriver as a lever, a technique I've never mastered. Note on top of the valve is a little cap with a slot in it that the cam follower sits in. The cap just sits there loose, just be aware of it. Don't forget to reinstall the little spring.
If the valve cover can't be removed because of lack of clearance, jacking up the rear of the engine often helps. If this must be done, you might want to loosen (but not remove) the bolt for the rear motor mount (trans mount) to avoid pulling the mount apart.
When all done, reinstall the valve cover. Don't overtighten the three screws that hold it down or you'll hear horrible noises from the engine when you start it. Screw them in by hand until snug, then maybe another 1/2 to 1 turn. Good idea to have a valve cover gasket on hand in case it needs to be replaced. I don't replace it at every valve adjustment.
Figure 2-4 hours for the first time, depending on what problems you run into and what extra tools you may need. It's also a bit hard on the knees because of all the leaning over the fender that's necessary, but overall a pretty straightforward job.
Good luck!
George Davis
'69 280 SL Euro manual
