Having an interesting problem and can't imagine I'm alone.
I have a 54" mower deck with aluminum spindle housings. I had a hard time getting them removed from the deck because the nut had rusted and the bolt would spin in the square hole of the housing. Only 4 bolts had this problem.
Went to the dealer and ran into two mysteries.
#1 The JDParts Catalog specifies 03M7186 which is an M8 x 30 carriage bolt with a 3mm neck! That's a sheetmetal bolt. The old bolts have a 5.6mm neck. So either the part number is wrong in the catalog or the engineers decided that a 5.6mm long neck was a problem and a 3mm long neck was the solution.
#2 As a sanity check we pulled a brand new spindle and bolt and the slop in the square hole in the spindle and bolt is that same as my old spindle and bolt. So it's by design.
The slop is so bad if I over tighten the nut, or re-use the old nuts the bolt just spins.
I'm planning on tightening up the slop somehow. Probably just drop a couple of 3mm split lock washers in the gap between the bolt shoulder and the square hole in the spindle as I have these on hand.
So the question to the forum is, why is there so much slop in the square hole in the spindle? Is it for thermal expansion? Is it so the spindle can withstand shock loads? It sure makes regular service a giant PITA.