After the success (?) of acid-flushing the SDL's cooling system, I decided that my wife's '76 450 SL needed it too. It had green stuff in it, of indeterminate age, and was starting to heat up more than it used to, especially with the air-conditioning running. As I still had 2.5# of citric acid left, I used it on the car.
Getting the thermostat out is always fun. I ended up just removing the alternator. This is easy, and makes the rest of the process very easy. I just left the belt loose in there, sitting on the cross member. (It didn't move during all this.) I hooked up a battery charger to the car to make up for the lack of charging, since it was going to be running for half an hour or so.
I still had the car's old, defective, thermostat. (It opened too cold.) It was a lot easier to wedge open than the SDL's thermostat, I didn't need the valve spring compressor. Just needlenose pliers, and a suitable length of .357 shell casing. (Or other spacer.) With the pliers you can separate the two halves of the thermostat. Then you just slip the brass collar over the pin and reassemble. Voila! Instant flushing tool. (You can't just take the thermostat out, because the bypass passage will be left open and you won't get circulation through the radiator.)
The other tool you need is an upper radiator hose that's teed into a garden hose. From before I already had something, which was a suitable length of hose from a junker, with a fitting poked through the side. Leaky, but who cares?
At this point, you follow MB's procedure. Run the engine with the heater on defrost and fresh water running through the engine, for five minutes. (I left the radiator drain open, and the cap off the expansion tank. Water ran out of both at all times.) Then, drain and close the cooling system and put in the 2.5# of citric acid, dissolved in a gallon of water. Top off with water. Run for 15 minutes. Then, drain and repeat the fresh-water flush for 5 minutes. Drain and close her up and fill with fresh coolant mixture. (MB stuff preferred, of course.) Don't forget to log this procedure, so you can tell when you need new coolant again. (Hopefully on schedule, before you would need the acid again to clear out corrosion, or even to flush.)
After all this, I also hosed out the radiator and condensor too, as I was there. They didn't look bad, though. A new radiator cap was supplied too, as the rubber seal on the old one was tearing.