Here is the parts diagram and list for the cutter spindle:
[Linked Image]
[Linked Image]

As you can see, it only shows Item 34, P/N GT13815, Spindle Housing Complete. It sounds to me as if the upper shim, 33, is used to adjust the position of the pulley for belt alignment. The lower shim 33 may be to generate clearance between the cutter plate and some other part of the mower. If it works the way I am guessing, the housing machining sets the distance between the outer rings of the bearings, and a pair of shoulders on the spindle set the distance between the inner rings. Because you probably can't get the machining accurate enough, you will need to also have a shim stack to control the end float of that shaft accurately. The way that was done with the tapered roller pinion bearings on some (maybe most) rear drive cars, was that you had a crushable shim on the pinion shaft in between the inner rings of the bearings, and you tightened up the pinion flange nut exactly the right amount to get zero end float. If you tightened the nut just a whisker too much, you had to pull down the differential again, replace the crushable shim, and try again. Believe me, you soon learned to be careful.

If the Greenfield spindle has a crushable shim, someone has overtightened that nut, and ruined it. One solution would be to pull down the whole spindle assembly and replace the crushable shim, or add a conventional shim to the end of it so you can crush it some more. The other solution would be to buy a new P/N GT13815 spindle housing complete. I'm pretty sure I know what a Greenfield dealer would do in this situation.

I think the next thing you need to do is phone Greenfield's service department and clarify whether my guess is right, or if it's not, how it works.