Abstract: A circular saw having a driven shaft and mounting a blade-receiving arbor are adapted to carry a blade having a circular inner margin by the use of an arbor adaptor. The arbor adaptor is provided with a central aperture and a first engagement side having a central recess and a blade positioning ring. The central recess is sized and positioned to receive a mounting lug of the arbor therein with the blade positioning ring in radially surrounding relationship, the blade being received on the ring. A friction surface on the arbor adaptor serves to frictionally clamp the blade to the arbor. In some embodiments, a drive pin may be provided on the adaptor to fit within a complemental hole in the blade for rotatably driving the blade. A second engagement side opposite the first engagement side may be provided for using the arbor adaptor with a second, different saw.