2.6" - 2.2" (2.6" - 54 mm) centering ring holds 54mm motor tube inside 2.6" airframe. 1/4" plywood.
Fits into any 2.6" airframe including BT-80 tube. Works best with Madcow kraft, GLR fiberglass, some LOC Paper 54mm motor tubes. (2.23-2.24" motor tube OD)

Please see next item for same ring cut for different 54 motor tubes