William:

I own a '91 300-SEL which has the smaller 6 cyl motor vs. your 4.2 litre V-8, but...no matter.

Your idea is fine and is how alot of folks refill the 6 cyl motor.

Fill the overflow tank and also fill thru the thermostat area if possible. I'm assuming the 420's is horizontal, thereby allowing for refill thru the t-start area. Once it's full in the t-stat area, reinstall the t-stat and cover. Leave the cap off the overflow tank and start the car. As it warms up, you may see bubbles in the tank - a sign air is escaping the system...good thing. Reinstall pressure cap after "burping the air.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"