If sitting in the combine, the gearbox on the left hand side of the feeder house. Where the attachment chain wraps around the two sprockets to drive a head. The sprocket on the feeder house keeps moving back toward the feeder house and the four bolts holding the spring plate are cutting into the chain. I've took the sheave apart, but cannot figure out why the sprocket keeps moving closer to the bolts. Any advise would be appreciated.