On October 5, 1997, about 1230 eastern daylight time, a Hiller UH-12C, N49983, was substantially damaged when it rolled over while hovering at the William H. Morse State Airport (5B5), Bennington, Vermont. The certificated flight instructor and student pilot were not injured. Visual meteorological conditions prevailed for the local instructional flight that originated at 5B5, about 1210. No flight plan had been filed for the flight conducted under 14 CFR Part 91. In the NTSB Form 6120.1/2, the certificated flight instructor (CFI)/owner stated that they had been hovering over a grass area, when the loss of control occurred and the helicopter rolled right onto it's side. According to a Federal Aviation Administration (FAA) Inspector, the CFI had been demonstrating how to hover a helicopter to a student pilot. While the student pilot was attempting to hover the helicopter, it began to roll right. When the CFI took hold of his controls, the top of his cyclic stick detached. The helicopter then rolled right and collided with the ground. Examination of the helicopter by the FAA Inspector revealed the helicopter's cyclic stick had been replaced with a military style cyclic. According to the FAA Inspector, the owner reported that the cyclic had been cut down by removing a section of the cyclic. The top of the cyclic detached at the repaired joint. A review of the maintenance records did not reveal who or when the modification was performed.