An employee was reinstalling a track on a bulldozer. A chain had been used to position the track; with the track in position, the power had been shut off and the track had been secured with a block. As the employee was preparing to correct a slight misalignment using a pry bar, he took hold of the loose end of the chain, when the overlap of the chain released unexpectedly. The employee's thumb was pulled up and caught under the chain, where the thumb tip was compressed and split. The employee suffered an amputation to the thumb.