A fan penetration fixture consisting of a threaded shaft and a lock nut design. The fixture is designed to take the place of the fan clutch on a fan drive assembly in the vehicle being tested for a desired cooling system performance characteristic at each possible fan penetration. The threaded shaft portion of the fixture bolts to the fan hub. The desired fan is then bolted to the lock nut portion of the fixture. The lock nut can be positioned at any point along the threaded shaft to achieve a desired fan penetration.