A word of caution to those six cylinder BMW owners who have drilled out the holes in their friction fan couplings to lock the mechanism up with M6 x 30 mm screws, thereby allowing the fan to run continuously: If the screws have been over-torqued, there is a possibility that one or more of the four aluminum bosses around the bearing of the clutch drum may have cracked. If this occurs, it will cant the fan blades at an angle and cause an imbalance that can be detected as fore and aft "run-out" just a few thousandths of an inch off at the center can mean up to a quarter of an inch or more variation at the end of the blades.
The condition can be observed easily with the aid of a strobe light or, if severe, can be seen at idle without too much trouble. If this defect is not corrected immediately, the fan will develop a mind of its own and decide to rapidly bore a hole through the radiator. The remedy required is to replace the clutch (expensive) or to substitute an aluminum plate in its stead. Anyone who would like a drawing of this adapter may obtain same by sending me a request with a stamped, self-addressed envelope.
By all means, if you suspect that this may be the case with your fan clutch, check it or have it checked immediately! This is a potentially dangerous situation.
Author: Stan Simm