An improved expansion assembly for a mine roof bolt for use in a nominal one inch diameter bore hole with a nominal 5/8" diameter roof bolt is provided. The expansion assembly has a tapered plug with four tapered surfaces, an expansion shell that has four leaves extending axially upwardly from a support ring and into contact with the four tapered surfaces on the tapered plug, and a threaded support washer to support the expansion shell on the threaded end of a mine roof bolt. The tapered plug has a threaded bore therethrough for threading onto the mine roof bolt. The tapered plug tapered surfaces have an angle of taper of at least 6.5.degree. to the axis of the mine roof bolt. The expansion leaves each have seven serrations formed on the surface such that the serrations are formed by the intersection of a planar surface perpendicular to the axis of the mine roof bolt and a frusto-conical surface. The frusto-conical surface is at an angle of 72.3.degree. to the planar surface. The serrations are equally spaced from each other and have a peak to peak distance of 0.186". The expansion shell has a maximum length of 2.35 inches. The tapered plug has a rounded upper surface and channels running longitudinally between the tapered surfaces on the plug for facilitating use of the expansion assembly with resin. The improved expansion assembly may be utilized with or without resin bonding.