While the night mechanic was assisting senior mechanic in removing chain driven sprocket from mud pump, a puller and jack were being used for this job. Everybody was clear during the operation then it was decided to release pressure from the jack to check progress of the job. The puller and jack were secured by rope and chain to avoid being dropped, while removing the puller, which has 2 long studs; the end of one stud struck the injured person on left cheek. The injured person suffered a small cut (2cm long) 5cm from his left eye. The injured person took a medical treatment and returned to work.