An employee was changing a preheater feed screw. He was using a 4-foot-long piece of pipe to bump the blow-back cylinder and make it retract so that the feed screw could be removed. The cylinder then moved forward instead of retracting, which forced the pipe back. The pipe pinned the employee's hand between the end of the pipe and a very heavy flange that was part of the machine frame. The employee's right index finger was amputated.