The FLXM13 has a 1/4" shaft so it will fit into place. Also, because of the thin shaft it twists alot and feels like it is going to break. This twisting action makes the shaft act as a torsion bar, storing energy. Once the bolt starts to move a little bit, the stored energy keeps it moving and breaks it loose when a thicker extension would not.

It sounds weird, but it works.

Have a great day,