This drive arm transmits motion from the header gearbox to the cutterbar knife assembly. It connects the rotating drive to the reciprocating knife, converting rotary input into the linear stroke needed for the cutting action. The arm has three mounting holes and a 16-spline interface that matches the output shaft of the drive housing. A worn or elongated hole in the arm introduces play, which reduces knife stroke and causes uneven crop cutting. The manufacturer machines the spline profile to match the original shaft tolerance, so the arm seats without additional shimming. On reinstallation, verify that the retaining bolt is torqued to specification and that the arm rotates freely before engaging the cutterbar.