Ford Windstar Owner's Manual page 311

The fuel door
To replace and secure the fuel cap, place it in position and
rotate it clockwise until it clicks (ratchets). The ratcheting
mechanism allows the fuel cap to be sealed without
overtightening.
If you lose the fuel cap, replace it with an authorized Motorcraft
or equivalent part.
If you do not use the proper fuel cap, the pressure in the
fuel tank can damage the fuel system or cause it to work
improperly in a collision.
NOTE: If you replace your fuel cap with an aftermarket fuel
filler cap, the customer warranty may be void for any
damage to the fuel tank and/or fuel system.
Maintenance and Care
WARNING
311