Here’s the info.

The length of the upper BT55 is 3 13/16”. The root edge (NOT the slot) of each fin begins at 1/8” from the top of the tube and ends at the 7/8” from the bottom of the tube. I don’t have the dimension of the inner BT20, but if you do indeed want my extra cone assembly you’ll be able to extrapolate the length. I suppose the inner BT20 is only needed if you’re going to slot-mount the fins.

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