Cross head pins should be kept eased away on the two parts of their circumference which are within the connecting rod brasses or boxes and near the joint faces of the same. This is necessary because the wear is greatest on the crowns of the boxes, and the pins are apt to wear oval. In some engines, the surface of the pin is cut away, but if it is not, and the pin can be revolved in the cross head, it is a good plan to give it half a turn occasionally, which will keep it round.
THE GUIDE BARS.
The guide bars of an engine require to be set exactly in line with the axis of the cylinder bore, so that they may guide the piston to travel in a straight line. They should be an easy sliding fit to the cross-head guide.
The top bar is more difficult to lubricate than the bottom one, especially when it receives the most pressure, as is the case when the top of the fly-wheel runs towards the cylinder.
Cast iron guide bars wear better than either brass, iron, or steel ones, so long as they are properly lubricated. The face of each guide bar should be cut away, so that the ends of the cross head guides will travel past it. This will prevent a shoulder forming at the ends of the bar as the face wears away. Such shoulders are apt to cause a knock as the connecting rods are lined up, because in the lining the connecting rod is restored to its original length, and the path of the cross-head guides along the bars may be altered.
THE CONNECTING ROD.
There are two principal kinds of connecting rods, the “strap ended” and the “solid ended.” The solid ended wear the best, but are more difficult to get on and off the engine.
Connecting rod straps are secured to the stub ends (as the ends of the rod are called), either by bolts or by one or two gibs, and the brasses are set up by a taper key or wedge.
The taper for connecting rod keys is about an inch per foot.
The angularity of a connecting rod is a term that applies to its path of motion, which is (during all parts of the stroke except on the dead centre) at an angle to the line of engine centres. The effect of this angularity is to cause the piston motion to be accelerated at one part of the stroke and retarded at another, thus causing the point of cut-off to occur at different points of the two strokes.