the drain tap (on 850) or brass plug on most others does indeed go into the water jacket, but because Minis sit for 30 years without them being drained, bits of rust and gunk plugs the hole.
If you are patient, stick a succession of drill bits in there, and wiggle them by hand, it will eventually run water out.
I've given up bothering, I just pull the bottom hose off the water pump.
If you want more out, pull the pump off too.
I used to drain the radiator, but MiniSpares UK's accountant or Rover's saved £0.02 by deleting the fitting....
<edit> no oil gallery is near it so drill away, carefully!
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
