If the fan is of the original stock type, then they are stiff to turn, not like a spiral fan.
That is not true (no offense intended). The fan motor has brushes and bronze bushings,so it will not spin as freely as a brushless ball bearing motor, but it should spin easily. You should be able to spin it and not have it stop the instant your finger leaves the blade. It should rotate at LEAST a full turnafter your finger leaves the blade.
Now it IS COMMON for K75s to have stiff fans, it is not because they are supposed to be that way, it is because they are on their way to failure if not addressed.
The early K fan motors are a common source of failure due to a couple factors. They are very light duty. One rumor has it that it was originally designed by Bosch as a Porshe 911 defroster fan motor. As previously stated, they have bushings, not bearings. They run infrequently. They are directly in the dirty airstream behind the radiator. They dirt, grit and moisture pack between the shaft and the bushing for months on end during the coller weather when the fan isn't needed. The in the middle of the summer, when you're in stop and go traffic or stopped at series of long lights, it decides to turn on. The extra drag on the motor shaft causes a rise in current demand. The extra current causes excess heat inside the motor. The extra heat burns out the windings OR melts the PLASTIC brush holders and frys the motor.
You might not notice that the fan is out, other than the horn stops working since it is on the same fuse, until the overheat light comes on (if the bulb is still good and they are paying attention to the gauges). Overheating the engine can do all kinds of damage.
The fan motor, the rear splines, the crankcase vent hose, dissolving fuel pump vibration dampers, dirty starter commutaors, dirty ignition switch contacts, and ignored valve adjustments are the most common failures on early Ks if not monitored.