2.1" - 1.1" (ID 54 mm - 29 mm) centering ring holds 29mm motor tube inside 54mm airframe. 1/4" plywood.
Fits into any 54mm airframe. Works best with Aerotech paper, LOC paper, Rocketarium thick-wall, fiberglass, Semroc 29mm motor tube. (1.22 - 1.23" motor tube OD)

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