Thread size

[benfield4]

I have searched in the forum for this and can't find it. Does any one know the thread size for the bolt that goes on the coolant vent hole on the M103 head? This is the hole closest to the front of the engine on the cylinder head next to the purge valve actuator. these are both right next to the valve cover. I don't have any metric taps to find the size myself. it looks to be a fine thread and is definitly smaller than a M14x1.5.

Its on my M103.981 in my 89 300SE.

I am trying to add and extra temperature switch for a fan and would like to put it in this position or in the Thrermostat cover vent hole.

Thanks!