one of the first repairs i attempted on my 1972 220d was to renew the thermostat. what i didn't know at the time was the housing is pot metal and the wrong coolant (green) only accelerates the corrosion. two of the four bolts snapped off.

after removing the housing i heated the other two bolts and got them out fine. the housing itself was too corroded and could not be saved.

ever since, when removing the bolts on thermostats, i generously apply penetrating fluid (kroil) and heat. with any luck you'll be able to salvage the housing and drill out the broken bolt. good luck!

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