Abstract: A retainer for mounting and retaining a rotatable cutting tool or bit in a block bore. The cutting tool contains an annular channel. The retainer has an unstressed diameter greater than the diameter of the bore so that when the cutting tool is inserted into the bore, the retainer expands against the bore so as to be held therein along with the cutting tool. The retainer comprises a cylindrical sleeve having an engaging member extending circumferentially around the sleeve and having a constant cross section shape which projects into the annular channel of the cutting tool and which annular channel has an inward rectangular surface corresponding to the inward engaging member of the retainer for receiving and retaining the retainer to the cutting tool.