Estes Airborne Surveillance Missile Build, #0867, Part 5, Engine Mount

Usually the instructions have you glue on the rings and blocks to one side of the pencil marks. This kit has you set the rings on either sides of the marks.

The smaller middle ring is very close to the dowel notched ring. 

Set a line of glue around the tube and slide the notched ring into the glue. Smooth out the fillet using a Q-tip. You'd never be able to get a fingertip in there.

Here's the finished engine mount - 

Be sure the small notches in the dowel notch ring are directly over the fin position lines already drawn on the tube.

TIP: This is not my idea, but it's a good one - Fliskits used to sell an accessory called the "Block and Tackle". 

The 1/4" wraps of  masking tape at the low end of the spacer tube work as a depth "stop" preventing the engine block from ending up too high up inside the tube.

Place a line of glue high up in the tube. Slip in the engine block. Slide the spacer tube in the tube pushing the engine block up. Keep sliding in the engine block until the spacer tube stops when it hits the tape ring at the bottom of the BT-5 tube. Remove the spacer tube. 

On the right side, the tape wrapped spacer tube is almost butted up against the bottom of the BT-5 tube.

Keep the taped spacer tube for future use. Make up and use BT-20 and BT-50 "Block & Tackle" tools as needed. Most rockets have the engine extend 1/4" out the back of the engine mount tube.