I thought I let everyone know of the problems I had with the Stens TCA24880 replacement units.
So far ordered five of these 285-251 spindles and installed three of them. Actually actually the last two were replacement of two the failed under warranty.
Diagnostics of the two failed unit revealed assembly problems. The first one had a lower spacer that was too thin causing the unit to bind up damaging the housing. The second one it was the M20-1.50 Flanged nut being designed wrong. The JD nuts are countersinked where the threads are. The Stens nuts are not. This cause the nut to bottom out on the spindle hex for the pulley adapter preventing application of full clamping pressure.
At least Stens refunded me the cost of the spindles.
So far I have yet to find these nuts other than at JD; all others are like the Stens nut.