1984 400i water level

For the pre 1982 model, this is specified in the owner manual page 79. You basically open the radiator (not the header tank) and fill the coolant until it is 15mm below the neck of the radiator. There might be a bit of air trapped in the system, so you should review it again once the car has run.

Check level as per the manual: there is a lot of coolant in the system (15l), which means lots of expansion when hot. The system needs space to let expand all this volume of liquid. Excess coolant will be trashed.

 

Unlike most other cars, the expansion of the liquid is taken up entirely in the radiator, NOT in the "expansion tank". Hence the radiator is not filled and Raemin has stated level correctly. The little tank on side should be empty and remain empty unless motor overheats.
Ken

 

My car is a late 82 model (win 43951) facelift, does the same water level in the radiator apply for my car. I am asking because I had an overexpantion from the side tank yesterday and might need to fill ut some water

 

The 412 has a different cooling system, but 400i gen1 and gen2 are basically the same cars (except for the updated interior, front grill, relay and fuse panels, self levelling system, shorter axle ratio, and a few more degrees of inlet camshaft timing). This being said the owner manual is different; that's why I mentioned this was for the older manual so as to be sure that one would not jump to his "400i gen2" manual and could not figure out what I was talking about.

Over-expansion is something you should carefully monitor. Be sure that the coolant is at the proper level otherwise when you stop the engine, your fragile cylinder head could be left "dry" (i.e not filled anymore with coolant). Never refill the engine when hot.

The more glycol in the coolant the less it cools the engine, so no point in selecting -35°C anti-freeze in the desert! Just have a look at the 365 manual for the proper anti-freeze/water amount. If your car does overheat I would even use distilled water and water wetter during summer and revert to anti-freeze before winter.