educated guess at M14x1.5
Tried look it up on realoem but they have the bolt removed.
http://www.realoem.com/bmw/enUS/show...diagId=11_6072
EDIT
Here we go
put in the M50 details and it shows the size ( has to be the same ???? )
Item 12
http://www.realoem.com/bmw/enUS/show...diagId=11_1282