I attempted today to change the thermostat on my 1.9 twin cam Z3 and managed to get the radiator out and the viscous fan off. I then used a quarter drive socket set to undo the four thermostat housing bolts. One came out at the top but the other snapped off at the bolt head with hardly any pressure on it! The bottom two are solid and feel like they will break off as well. I soaked them thoroughly with penetrating oil before I started and again as I was doing it. Does anyone know whether I can drill them out in situ. and are they blind holes ? I put everything back together and started it up and the thermostat housing is not leaking and I am left with the same problem as before with the heater either blowing hot or more likely cold when the engine is warmed up.