1.6" - 1.1" (ID 38 mm - 29 mm) centering ring holds 29mm motor tube inside 38mm airframe. 1/4" plywood.
Fits into any 38mm airframe. Works best with Rocketarium and Aerotech paper, LOC paper, fiberglass, Semroc 29mm motor tube. (1.22 - 1.23" motor tube OD)

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