Stiffy heavy duty tube coupler stiffiners feature a deep spirsl groove for maximum epoxy adhesion. These super heavy duty wall stiffiners are designed to be epoxied inside of our standard tube couplers to greaty increase their lateral strength. Snapping off a payload at the joint under high speed stress (as occurrs when flying at or near Mach speeds) is not a worry when you fly with a Stiffy. NOTE: TC-5.38 is REQUIRED to use this device.