3.0" - 1.1" (3" - 29 mm) centering ring holds 29mm motor tube inside 3" airframe. 1/4" plywood.
Fits into any 3" airframe. Works best with 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.