Could it maybe be part# 58 isn't torqued enough?

Good idea, but since both ends of the v-belt pulley shaft are turning, I am assuming the clutch is providing (at least some of) the torque transfer needed. Also, with the engine shut down and PTO disengaged, I can turn the v-belt pulley (which also turns the whole input shaft assembly) by hand, with no gear-reduction drag nor any rotation of the tiller, so I think the problem is internal to the Chain Case Assembly. I probably oughta tell you the rest of the story too: The tiller stopped working all at once last fall, while I was unintentially digging up some pretty bid rocks. Gut feel says I busted something internal. I welcome your concurrence or debate to my theory! Also, I assume that due to the gear reduction of the Chain Case Assembly, I probably shouldn't be able to turn the tiller by hand, right? I cannot right now, so it's either due to gear reduction or something is actually jammed in the CCA.

Sounds like maybe a busted chain or cog then.
Carefuk with the "big" gaskets, # 24! Nearly $14 ea.

http://www.m-and-d.com/MTD-721-0132.html

Roller chain should be available at most any "power transmission" type store. Look under bearings in the yellow pages. Bike chain is way too narrow.

http://www.azusaeng.com/chain/rollerchain/sngle.html

Man, you know your stuff! Thank you very much. Now all I need to do is get that CCA off the chassis and dive in. I'll let you know what I find out. Thanks again!