I always thought this was the best way to mark the engine tube for the engine hook slot. Set the engine block next to the tube and mark the height with a pencil.
Cut the slot at the pencil line. Using the block for a height gauge will set the upper bend of the hook right below the engine block.
Here's the internal mount, built around a Odd'l Rockets heavy wall BT-20 tube. Everything is standard so far.
This glues into a 4" long BT-50 tube with 3/8" of the BT-20 exposed out the back.
Two more 50/60 centering rings will hold the mount in the BT-60 main airframe tube.
These were spare plywood rings, the lower ring had an engine hook notch. It doesn't matter on this build, this won't be seen under the shroud.
I glued the large centering ring (with the notch) in the lower position, 2 5/16" from the bottom of the BT-50 tube. The shroud will slid on the low end.
The upper ring is glued over the BT-50 tube, 1/16" down from the top edge.
TIP: Lowering the top ring down the tube allows for a glue fillet on both sides of the joint. A flush mount (the ring even with the tube edge) only gives you one side to glue fillet.
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