A sickle cutting system is mounted on a header for forward travel over ground having a standing crop thereon and includes a cutter bar with a plurality of knife guards and two opposed sickle bars driven in opposite phase with a drive system for driving the sickle bar through repeated cycles of reciprocating movement from start-up of the system through to a shut-down. The drive system includes a control device responsive to the sensor signals from both of the first and second drive systems for advancing or retarding one of the first and second drive systems so that a number of sensor signals obtained from the first drive system is continually maintained so as to be substantially equal to a number of sensor signals obtained from the second drive system to maintain the sickle bars in opposite phase.