I'm about to perform a coolant flush in my R53. Don't want to say how long it's overdue, but it's overdue.
Anyway, is there a way to "add" a drain plug to the system? For example, tap the bottom of the radiator, to make coolant flushes and exchanges easier?
Wondering if someone had modified or done something like this.

 

Not sure about adding, but to drain any remaining coolant from the system (R50/R53), there is a 14mm drain plug located at the rear face base of the cylinder block adjacent to the starter motor underneath the exhaust manifold heat shield (if looking down at the engine). We have pictures and go over this in our tech article if needed.