Sean,
If you are doing the same job as I am tonight...replacing the plastic shaft
with a 1/4" bolt, then I figured we might compare notes as we go along. I am
generally following the advice of Will O'Brien athttp://biobug.org/coffee/turbo-crazy/sc-mod/.  He did not make it clear (to
me) the distances or the reasons for some of his washers. Surely sockets
have different depths, bolt heads come in different thicknesses, etc., so
this is to be expected. Here is what I am noticing on an original Stir Crazy
(Cat No. 82306 0146):
The original plastic shaft was a many stepped thing. The uppermost smooth
diameter (excluding the threaded portion) acted as a bearing to keep the
thing centered. Looks to me as if we need a metal or Teflon bushing or
something of the sort to duplicate this centering function. The OD would
need to be about 11/16". I found an old 1/4" drive 1/2" socket with that OD
and am presently trying to work that into the design while messing with the
height adjustment at the motor. You may have noticed by now that the motor's
square shaft arises from a slight boss requiring that the 7/16" socket be
recessed to compensate when assembled. Just fiddly fitting stuff.
How's it going with yours?
Paul
-- 
"There is nothing- absolutely nothing-
half so much worth doing as simply
messing about in boats."
---Ratty to Mole in Kenneth Grahame's
  The Wind in the Willows
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