On July 31, 1998, at 1830 central daylight time (cdt), a Bell M74-L (a modified Bell 47), N1001N, registered to Northland Helicopters, Inc., of Stacyville, North Dakota, was substantially damaged during a forced landing. The pilot reported a total loss of main rotor rom with an increase of engine rpm. The loss of rotor rpm occurred during a spray run. The commercial pilot was not injured. The 14 CFR Part 137 aerial application flight was not operating on a flight plan. Visual meteorological conditions prevailed at the time of the accident. The flight departed near the accident site. The pilot reported that he was operating the helicopter from a trailer near the accident site. He had just refilled the hopper and was in the middle of a swath run when the engine rpm increased and the main rotor rpm decreased. The helicopter impacted the terrain during the forced landing and rolled over. A teardown of the transmission was performed which was overseen by an inspector from the Federal Aviation Administration Minneapolis, Minnesota, Flight Standards District Office. The inspection revealed that the clutch shoes were worn to the point that there was metal to metal contacted between the clutch shoe and the drum.