WARNING: This page is about a different car, the 2003 Mercury Sable and 2003 Ford Taurus. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Identifying Special Tool
Courtesy of FORD MOTOR CO.
Use a straight edge and a feeler gauge to inspect the cylinder head for flatness. If the cylinder head is distorted, install a new cylinder head.