EMPLOYEE WAS WEARING HARD HAT WHILE RECENTERING TRACK OF KOMATSU FC 220 ROCKBREAKER BACK ONTO FRONT IDLER. HE HAD ATTACHED A CHAIN TO END OF BOOM RUNNING TO TRACK, TO PROVIDE FORWARD PULL & CREATE SLACK. HE WAS OPERATING A JACK ON DOWN SIDE OF TRACK TO GET IT INTO PLACE, WHEN CHAIN DETACHED FROM TRACK, KNOCKING OFF HARD HAD AND STRIKING SCALP.