Abstract: An expandable key for being received in registering channels which form a keyway in a hub member and a shaft member. The key includes an expansion member composed of a plurality of parallel fillets which collectively have a rectangular cross sectional peripheral outline and a bore extending longitudinally therebetween, and at least one clamp body positioned within the bore. A threaded tensioning member extends through the bore of the fillets and threadedly engages a bore in the clamp body, such that upon rotation of the tensioning member the clamp body is drawn into the bore to expand the fillets. The clamp body and the bore of the fillets have interengaging generally flat tapered surfaces which are in the form of a truncated four sided pyramid.