Hi-
I saw your post and wanted to check mine to see how that pulley is setup.
I have a DMM60 which is a 60" side discharge deck. In the manual the exploded parts diagram shows a cone shaped spacer type thing (taper lock bushing in the parts list) w/ slot for woodruff key and "plug" (it says plug in the parts list, but I imagine it functions as a set screw). The taper lock bushing's smaller diameter is at the top (engages bottom of pulley) and larger diameter towards bottom. In the parts list, it says it requires 1 taper lock bushing, 1 woodruff key, and 2 "plugs" which I think are set screws. So even though the picture shows one plug, the parts list says two are required. So I imagine my center pulley is held w/ two set screws and hence the reason I probably haven't experienced problems -- although my deck has only about 10 mowings on it. (purchased tractor a few months ago -- it has about 23 hours on it).
Perhaps you could check if the bushing, key, and "plugs" for my deck will work on your deck. Part #'s:
W3021-34171 - Taper lock bushing (1 required)
W3021-33141 - Plug (2 required)
05811-00519 - Key (1 required)
Other numbers of interest:
W3021-33111 - Bevel Gear box - the entire deck transmission
W3021-34162 - Big pulley that's involved with the 3 parts above
If these are compatible with your deck, you may be able to switch to the parts.
Good luck
-Eric M.
CK20 HST
60" MMM Side Discharge
KL120 loader
23 hours