the most common place is under/next to the coil pack. their is a torque bit plug that I believe is the right thread pitch and type that is needed. This is also a port that is often used for oil feed lines for a turbo.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)