Depreciation in Valve Operating Systems
There are a number of points to be watched in the valve operating system because valve timing may be seriously interfered with if there is much lost motion at the various bearing points in the valve lift mechanism. The two conventional methods of opening valves are shown at Fig. 185. That at A is the type employed when the valve cages are mounted directly in the head, while the form at B is the system used when the valves are located in a pocket or extension of the cylinder casting as is the case if an L, or T-head cylinder is used. It will be evident that there are several points where depreciation may take place. The simplest form is that shown at B, and even on this there are five points where lost motion may be noted. The periphery of the valve opening cam or roller may be worn, though this is not likely unless the roller or cam has been inadvertently left soft. The pin which acts as a bearing for the roller may become worn, this occurring quite often. Looseness may materialize between the bearing surfaces of the valve lift plunger and the plunger guide casting, and there may also be excessive clearance between the top of the plunger and the valve stem.
On the form shown at A, there are several parts added to those indicated at B. A walking beam or rocker lever is necessary to transform the upward motion of the tappet rod to a downward motion of the valve stem. The pin on which this member fulcrums may wear as will also the other pin acting as a hinge or bearing for the yoke end of the tappet rod. It will be apparent that if slight play existed at each of the points mentioned it might result in a serious diminution of valve opening. Suppose, for example, that there were.005-inch lost motion at each of three bearing points, the total lost motion would be.015-inch or sufficient to produce noisy action of the valve mechanism. When valve plungers of the adjustable form, such as shown at B, are used, the hardened bolt head in contact with the end of the valve stem may become hollowed out on account of the hammering action at that point. It is imperative that the top of this member be ground off true and the clearance between the valve stem and plunger properly adjusted. If the plunger is a non-adjustable type it will be necessary to lengthen the valve stem by some means in order to reduce the excessive clearance. The only remedy for wear at the various hinges and bearing pins is to bore the holes out slightly larger and to fit new hardened steel pins of larger diameter. Depreciation between the valve plunger guide and the valve plunger is usually remedied by fitting new plunger guides in place of the worn ones. If there is sufficient stock in the plunger guide casting as is sometimes the case when these members are not separable from the cylinder casting, the guide may be bored out and bushed with a light bronze bushing.
A common cause of irregular engine operation is due to a sticking valve. This may be owing to a bent valve stem, a weak or broken valve spring or an accumulation of burnt or gummed oil between the valve stem and the valve stem guide. In order to prevent this the valve stem must be smoothed with fine emery cloth and no burrs or shoulders allowed to remain on it, and the stem must also be straight and at right angles to the valve head. If the spring is weak it may be strengthened in some cases by stretching it out after annealing so that a larger space will exist between the coils and re-hardening. Obviously if a spring is broken the only remedy is replacement of the defective member.
Mention has been made of wear in the valve stem guide and its influence on engine action. When these members are an integral part of the cylinder the only method of compensating for this wear is to drill the guide out and fit a bushing, which may be made of steel tube.
In some engines, especially those of recent development, the valve stem guide is driven or screwed into the cylinder casting and is a separate member which may be removed when worn and replaced with a new one. When the guides become enlarged to such a point that considerable play exists between them and the valve stems, they may be easily knocked out or unscrewed.