A cutter guard assembly for a knife drive of an agricultural farm implement is provided. The cutter guard assembly includes a cutter bar assembly, a window frame, and a floor sheet. The cutter bar assembly has a cutter bar and a reciprocating knife assembly operatively connected to a knife drive. The window frame is rigidly attached to the cutter bar. The window frame includes an opening through which a knife arm of the reciprocating knife assembly extends through. The floor sheet extends rearwardly from the cutter bar and pivots relative to the window frame.