being slightly larger than the bore 17 thus forward forces on the bit 11 along the axial direction are resisted and the bit 11 tends to remain in the bore 17 the primary compressive force on the cap 15 is in the area where the cylindrical wall 29 joins the bottom 31 in this area the bottom 31 resists radial compressive forces against the wall 29 while at the open end of the cap 15 the wall 29 tends to spring inwardly due to a compressive force and the radial holding forces are not as great the retention forces exerted by the cap 15 on the bit 11 should not be sufficient to permit withdrawal of a bit 11 during use but permit withdrawal of the bit 11 for installation of new bits during typical use the bit holders 13 are mounted about the periphery of a cylinder and the cylinder is rotated into work so that the primary forces tend to urge the bit into holder 13 or block such use may include mining or planning asphalt or concrete roads typically the bit may be pryed from the block with a tool the cap 15 of the present invention tends to seal the bore from the entry of the dirt which might cause undue wear or make extraction of the bit more difficult the ridge 33 engages complimentary ridge 27 to create a force hereinafter called shank retention force relative to the cap 15 the cap 15 engages the inner surface of the bit holder 15 in the bore 17 to create a force hereinafter called cap retention force the resist the forward movement of the cap relative to the bit holder 13 either or both forces may act to retain the bit in the bit holder.