Abstract: An adaptor for converting a chain saw into a circular saw by means of a collar secured to both the clutch housing and the hub of the existing chain saw, the collar being held by a washer and a nut from the chain saw, said collar being cylindrical with a concentric opening through it, the concentric opening having a smaller diameter for a minor portion of the length of the collar and a larger diameter for a major portion of the length of the collar, the portion of the opening with a larger diameter being threaded and portion of the opening having a smaller diameter being ribbed, the adaptor further including a shaft extension plug threaded into the threaded opening of the collar, the shaft extension plug having a face plate and a concentric threaded shaft which extends concentrically from the face plate, for mounting a circular saw blade with a nut being threaded on the concentric shaft to hold the circular saw blade.