Cooling fan fell off

[egleaves]

I'm positive that they installed a new clutch and used my old fan.  I'll look closer tonight for markings.

So the inside of the clutch has concentric rings instead of vanes or blades, and the viscous fluid transmits power through fluid shear.  That suggests to me that the direction of rotation isn't important to the clutch mechanism.  But if that were true there would be no reason for us to select a different clutch for CW vs. CCW.  What is it about a fan clutch that makes it care which direction it is rotating?  Something about the valving or the fluid path?

[bd]

Rotation controls the migration direction of fluid through the clutch and valve.  AFAIK, a clutch that spins backward will not engage effectively.  A reasonable question might be whether the clutch hub would starve for lubrication in the process.  Even so, I wouldn't expect the spindle to separate from the clutch.  But you would experience overheating for lack of sufficient airflow over the radiator.