Estes Starliner #0863 Build, Part 4, Slotting For The Engine Mount

The older instructions show how to sand the "flats" into the centering rings. I don't remember ever seeing this any other model rocket kit.

The Wing Saddle Template is cut out and wrapped around the BT-20 tube.

Trace around the outside top and mark the arrow points with a pencil. Connect the arrow points with a straight line using an aluminum angle tool.

Using a new sharp knife blade - 

Carefully cut out the slot. I used the aluminum angle again for a blade guide cuts down the long sides. 

The instruction drawing (above) shows a sheet of sandpaper set down on the work table to sand the flats into the centering rings. I find more control using a sanding block.

Notice the flats are sanded directly opposite the engine hook.

Sand a bit and check the centering rings. You don't want to sand into the engine tube. 

Stop when you see the tube starting to show through when the rings are reduced down.