Do all the pump heads rotate counterclockwise?

There is clockwise and counterclockwise; it’s not to prevent the nut from coming loose, as nuts have right-hand and left-hand threads

From the motor side, centrifugal pumps generally rotate clockwise.

Both the impeller and bearing lock nuts of centrifugal pumps have orientation requirements. Why is the locknut right-handed? That’s because, from the motor side, the centrifugal pump rotates clockwise, so the lock nut must be right-handed. The locking direction is opposite to the rotation direction; this is also why reversing the motor causes the impeller to fall off. If it rotates counterclockwise when viewed from the motor side, then the lock nut must rotate in the opposite direction. With double locknuts, the one near the motor side is right-handed, while the one far from the motor side is left-handed. The bearing tightening nuts are similar in this regard; they are not always right-handed, as it depends on the specific model of the centrifugal pump as well as the required rotation direction. One thing that must be confirmed is that the rotation direction is opposite to the locking direction.

Actually, this is just a theoretical concept; in practice it seems to make no difference. I’ve seen a pump where it took half a year before someone realized that the impeller cover hadn’t come loose either

Centrifugal pump impeller back caps come in both right-hand and left-hand threads, depending on the design, that is, the rotation direction of the impeller. The rotation direction of the impeller back cap is opposite to that of the impeller. In this way, the cap will tighten as the rotor rotates, preventing loosening. Based on experience, the back cover of the oil pump impeller is usually left-handed, while that of the water pump is right-handed.