My 82 is filled through the surge tank that is mounted just forward of the brake master cylinder. To drain the radiator it is either take the lower hose loose ortake the temp sensor out. There should be a drain plug somewhere on the block.

82 320i
82 633 CSi needing donor engine

Hi Bruce:

Thank you very much for responding. I did notice that surge/overflow tank, but I've never filled a rad that way. Will the coolant flow freely into the rad through it?
RJ
There is also a bleeder fitting on top, in the thermostat housing. You will have to open it to allow air to escape while you are adding coolant to the expansion tank. If you don't, you will have pockets of air in the system, not good! Also, Bruce mentioned a fitting on the block to drain. It is on the right side below the exhaust manifolds. You should try and crack this fitting and drain the block, but it is a PITA, and coolant will just dribble out for half a day. Anyway, good luck!
Bobbo
1980 633 CSi