To check the gap on a crusher a procedure called LEADING is done. A lump of lead, whose dimensions are greater than the gap of the crusher, is lowered through the crusher while the crusher is running. The lead is pulled back through the crusher very slowly to be sure that it is flattened as far as it can be. The narrowest point on the lead is then measured. This will be the SET POINT of the crusher. If the liner and the mantle is at all worn then it will be necessary to lead the crusher at three or four different spots around the crushing surface. This is because as mentioned earlier the working surfaces do not wear evenly, when this is the case the narrowest point will be your set point.
The size that the crusher will reduce the rock to is determined by the gap between the liner and the mantle. This gap is adjusted by moving the bowl liner away from the mantle. The closer to the mantle the smaller the gap. The bowl liner is adjusted the same way that a nut is put onto a bolt. The liner is turned on a set of threads. By turning the entire bowl assembly clockwise the threads will cause the liner to move closer to the mantle.