At 1645 central time, a Piper PA-28R-180 and an amateur-built WAR F4V collided in the pattern at St. Clair County Airport. Both aircraft sustained substantial damage but neither pilot was injured. A witness waiting at the hold short line observed the Piper turning left base for runway 20 and again on final, well past the tree line and descending. At that time the witness saw the F4V approaching the Piper from inside the pattern and banking left. He then saw the right wingtip of the F4V collide with the left side of the Piper. Both planes yawed and then the F4V continued under the Piper, landing and nosing over in the grass to the right of the runway. The Piper landed on the runway. The witness pilot heard pattern position reports from the Piper but could not recall any radio transmissions from the F4V.