Drive block keeps slipping.

Have a Whirlpool, direct drive washer. Basket was binding up on spin cycle. Took it apart and saw that the drive block had slid down the spin tube and was touching the top of the support tube. Cleaned the spin tube and the inside of the drive block, reassembled and tightened the heck out of the spanner nut with a pipe wrench and hammer & punch (don’t have a spanner wrench).

Worked fine for a few weeks, then the drive block slowly slipped back down the tube, and bound the basket up again.

Does anyone have any tricks for keeping that drive block from slipping down the tube?

NOTE: I’ve attached two jpeg’s showing the drive block and spin tube (maybe I’m missing a part or something?), and also showing that the drive block is pretty much flush after I’ve assembled it (maybe it’s supposed to be set differently?).

Frustrating…thanks for ANY ideas!!!

nick