I took the shredder apart again. Unfortunately, ALL the flails were broken - with exception of the one I had welded. I believe that they hit the output gate, possibly because of the crankshaft being bent, or the engine being pushed out of alignment. I believe that the flails
actually interlace a bit with the rails of the output gate, so proper alignment is important.
At this point, things were looking grim for the unit. A new impeller costs about $250 from
Sears, and I can buy local chippers all day on Craigslist for around $300. But since I was in that far, I figured I'd try to balance it anyway without the flails.
So I removed the flails and built a stand out of scrap bits of 2x4 and plywood. Pictures below. I had made up a balancing tool consisting of a 7/16NF20 bolt drilled right down the middle on my lathe. Poked a piece of string through the hole, tied a knot on the bottom, and
hung it from the frame. I hung the impeller just high enough so I could get my carpenters level under it and see the balance visually.
Perfect balance was achieved with a 2-inch bead of MIG weld. I put the whole thing together without the flails, and started it up. It ran smooth as silk, and actually chipped and
shredded surprisingly well. The bits aren't as small as with the flails, but still small enough to make good compost. Actually, specifically for composting - larger shreds might be superior because really tiny ones might tend to mat up and exclude oxygen.
BTW, my shredder isn't the exact model being discussed. It's the 7.5HP one.