K, here goes. I've been at the local Ford dealer for 10 1/2 years (parts mgr) and the f1sf and f1sz are indeed one and the same. The f on the end denotes the engineering #, the one that the manufacturer used the designate the part. The z is used by Ford to denote the part #. It will work just fine. -Side note, part # called for by Ford is f1sz, # on old part is f1sf