This works for me - I have a 103 motor.
- The easiest way to drain a 103 radiator is to remove the hose at the passenger-side bottom of the radiator. This drains the radiator and the pressurized expansion tank. There is a red drain plug around the front, but in some cases it's really hard to get to.
- Drain the engine block by removing a 19mm drain plug on passenger side between the #5 and #6 cyl exh. manifold...thereabouts.
- Remove thermostat housing cover(three 10mm bolts) - lift off housing. In order to remove the thermostat, you'll need to get the t-stat gasket out. On some t-stats, it's fitted on to the t-stat and on others, it is merely pressed on to the top of it.
- Leave the t-stat out and temporarily reinstall the t-stat housing cover. You don't need to snug it down all that tight. This is temporary so you can flush with a garden hose.
- Hook up any hoses that are off. I flush thru the expansion tank and thru the block. Some say that flushing with a garden hose doesn't do that much good, but if you change out your coolant often enough, it helps to clean out residual junk.
- Drain out all flushing water from all points that you have drained coolant from. Reconnect all hoses.
- Remove t-stat housing cover and refill with pre-mixed MB coolant and H20. MB says to use a 50/50 mixture. This area often times turns into a heated debate as some say mix 60(water) and 40(coolant). It's your car and your call.
- Install a new t-stat with the air-bleeder hole on the t-stat in the highest position. Install gasket that accompanies it accordingly. Install the t-stat hosuing cover and bolts. DO NOT OVERTIGHTEN these bolts!!! I always coat mine with anti-sieze compound so they're easy to remove the next time.
- Refill your expansion tank until it's at the full mark.
- Leave expansion tank cap off and start car. Let it run 'til mtotor warms up. You do this to bleed air out of the system.
- Reinstall expansion tank cap.
You will likely have to top off the system a couple of times over the next few days. Check the level with the engine cold in the AM on a level surface.
There's a procedure in the 103 engine manual that involves removing a 14mm plug that's just to the left of the t-stat housing. The threads on this plug and the threads in the cyl. head housing can gets striped easily. I do not use this port to refill the system with for this reason. I simply refill thru the t-stat housing as dsecribed above.