The instructions would have you install the 24" elastic shock cord under the coupler holding the two body tube pieces together.
I was concerned about the elastic making a bulge under the body tube where it turned under the coupler.
I decided to tie Kevlar under the engine hook and join that to the elastic just under the upper body tube lip.
Looking at the picture at the right, I should have made the loop longer so the knot was above the top of the engine mount tube.
The ST-8 main airframe tube is not much wider than the ST-7 engine tube.
The way I tied it, the knot would press against the inside and make a bulge in the ST-8 body tube. I'm sure this is why Semroc decided to mount the shock cord under the coupler.
I ended up cutting a notch in the engine tube the knot could sit in. This recessed the knot and brought it in flush and inside the diameter of the ST-8 tube.
These CR-78 centering rings are pretty thin. The engine hook did raise them a bit. I sanded down the raised rings for a better fit inside the ST-8 mainframe tube.
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