The motor mount is assembled by gluing the two rings onto the motor mount tube. The spacing and alignment of the two rings is critical. If they are rotated relative to each other, the fins won't fit into both rings.

Also, if the fins are spaced apart as per the instructions, I don't think the fins will fit into the space between the rings either. The instructions said to position the rings at 1" and 3 3/16" from the back of the motor mount tube. When I dry fitted the pieces, I couldn't get the fins to go into place. I had to adjust the upper ring at 3 5/16" from the base.

Actually, by gluing on the lower ring first, and using the fins as spacers, the top ring can just be glued into place without measuring it. Just be sure to remove the fins before the epoxy sets.

Also, remove any epoxy from near the mounting point for the fins. I didn't do that on a prior build, and a glob of epoxy prevented the fin from sliding in far enough. Then, I had to grind off a corner of the fin tab. This time, I used a Q tip to clean out the fin mounts.

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