The Estes Cosmic Explorer (BT-55) has through the wall fins, the root edge of the fins glue to the engine mount tube.
Looking inside the pre-cut fin slot you can see the black electrical tape I used around the engine hook. The black tape also holds the tube for the replaceable Kevlar mount.
In the above left picture the fin doesn't quite seat against the outside body tube. The electrical tape is wider than the slot recess that would have fit the Mylar retention ring.
I've marked the tab with pencil where the notch needs to be wider. I carefully shaved off the pencil area to fit over the tape wrap.
The right side of the first picture (above) shows a better fit of the root edge.
Back to the larger Centurion:
These fins are surface glued onto the body tube - no fin slots.
After the fin lines are drawn, the primer paint is scraped off a little wider than the 3/32" thick fins.
The pencil line is scraped off, but redrawn before gluing the fins on.
From the back end -
At the edge of the fin you can see a pencil line drawn around the end of the body tube.
These fins totally cover the pencil line at the back. The small pencil "tic" mark gives you something to center the root edge on when you can't see the line.
Inside is the engine lock strip, centered between two fins.
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