Hello,
I made some progress with my M104.98x top end rebuild.
Today I took care of all the components from the bottom of the car. All went fine up to the point where I tried to drain the coolant from the block. Using a 19mm socket and a 13” long 3/8 ratchet I tried to loosen it but it is froozen, I sprayed some kroil and it started to move (very very hard) I worked it out 1/2 turn when it got completely froozen again, antifreeze drops came out.. I tightened agin as I didn’t want to risk it. I’ve done a few M104s overhauls but I never encountered a drain plug that stuck..
Did anyone experience something similar?

The drain on my SL only rotates a turn or so. Drained coolant and closed it again.

I need to pull the engine for a head gasket and refresh so I'll address it then. Will probably have to heat or drill it out. ( the drain is just like a brake bleeder )

Given you are pulling the engine, work the drain back and forth to break rust. Just forcing it can break the drain.

mine broke off,I just remove bottom hose to drain.Hope to buy one soon,as trans cooler can rot in the stock ones.

__________________
1999 w140, quit voting to old, and to old to fight, a god damned veteran

Turn the drain tap anti clockwise a little., poke the hole in the centre it may be full of rust and dont remove it .