4.0" - 1.1" (4" - 29 mm) centering ring holds 29mm motor tube inside 4" airframe. 1/4" plywood.
Fits into any 4" airframe. Works best with Rocketarium TW Paper, Aerotech paper. (1.22 - 1.23" motor tube OD)

Please see next item for same ring cut for different 29mm tubes.