The cut bucket is not completely cut out. The bottom of the bucket is cut out, leaving a flange. That flange is where the bolt circle on the 8.75 chunk is resting. By sliding the cut bucket in the whole one, the cut bucket will get stuck in the uncut bucket before the ring gear bottoms out. He's using buckets because , they are common and come with a sealed lid.
Anyway, that's the way I see it.
