Just managed to shear one of these whilst reassembling after a duo valve clean and heater matrix flush on a 1994 E300D estate. It's a M3 x 45mm and like a wood screw, but engaging in plastic. Torx/star pan head. Does anyone have a duo valve which is u/s which might provide one ? The duo valve p/n is A0018303884 according to the EPC, for heating only autos (no air con). I would rather keep it original than cobble something with threaded bar or similar.