TIP: Don't just glue the centering ring onto the engine mount. Use the black retainer ring to perfectly align the centering rings on the engine mount tube.
Set the ring on the pencil mark then slide the black retainer ring down to the centering ring.
Press the ring against the black retainer. The black retainer will hold the centering ring edges perpendicular to the engine mount tube.
Apply a fillet to the opposite side of the black ring, don't glue the black ring in place yet. When the glue fillet starts to set, slide the black ring up and away from the centering ring. When the first fillet is dry, apply a fillet to the other side of the ring.
The inset picture shows the ring slid up for upper ring positioning. It's taped in place.
Slid the upper centering ring down onto the black retainer and apply a glue fillet to the top.
Again, when the glue starts to set, slide the ring down away from the upper ring and glue.
Run a line of glue above the lower ring.
Slide the black retainer down into the glue and against the lower ring. Turn the black retainer to spread out the glue. Smooth out any glue that is squeezed out into a fillet.
The engine block is set pretty far down inside the top of the engine mount tube.
Push it in with an old D engine casing to be sure it sits square inside the tube.