Jaw Crushers MJ 90, MJ 110 and MJ 130 Technical Specification Jaw Crushers reduce large size rocks or ore by placing the rock into compression. This mechanical pressure is achieved by the two jaws of the crusher of which one is fixed while the other reciprocates. There are also primary and secondary types of jaw crushers.Jaw Crushers are sized by the top opening of the crushing chamber. For example, a 32 x 54 Jaw Crusher measures 32 from jaw die to jaw die at the top opening or gape opening and 54” across the width of the two jaw dies. The narrower bottom opening of the crushing chamber is used to size the discharge material.