eylinder engine are given at Fig. 153. In the diagram at the left, which is that of a four cylinder motor having a firing order of 1—3—4—2, the points of valve opening and closing are not only indicated in degrees but also by the equivalent dimensions measured on the flywheel periphery. In the six cylinder engine diagram shown at the right the points are indicated measured in degrees.

In studying the table presented, it should be remembered that there are many conditions upon which valve timing depends, and a formula correct for one motor would not necessarily be satisfactory for another of the same size. In all cases, the inlet valve opens after the piston has reached the end of its exhaust stroke, the lag depending upon the size of the valve, shape of cam and motor speed desired. Most designers do not open the inlet valve until the exhaust is fully closed, but there are some who allow the valves to overlap; that is, the inlet valve starts to open before the exhaust is fully closed. It should be remembered that exhaust closing and inlet opening occur approximately after that point in the cycle where the piston reaches the end of its second up stroke. The inlet valve closes after the bottom of the suction stroke is reached, good practice being a lag of between 30 and 40 degrees crank travel. The exhaust valve opens before the piston reaches the end of its down stroke under the influence of the explosion. The lead may be as high as 60 degrees crank travel before center, but this is an extreme case. Good average practice is covered in the range between 40 and 45 degrees lead.

The reason for having the exhaust valve closing and inlet valve opening overlap is that this takes advantage of the momentum of the exhaust gas and results in clearing the cylinder more positively than if the valve closed absolutely on center. During the final period of valve closing but little space is present for the escape of gases. If the valve closed positively on upper dead center, the area of the port or passage would be small for an appreciable upward movement of the piston when it neared the top of its stroke, and more inert gas would be retained in the cylinder than if the valve was closed after the piston starts down on the intake stroke.

The following table outlines the valve timing of a number of standard American automobile motors.

VALVE TIMING OF SOME TYPICAL AMERICAN CARS
FROM S.A.E. DATA SHEET, No. 53, VOL. I

When the space between the valve stem and the valve lifter is more than it should be there are two methods of compensating for this depreciation. On many small motors no adjustment is provided between the valve stem and the valve stem plunger. The makers of the Ford car advise drawing the valve stems out until the proper space exists between the push rod and the stem. It is important when drawing out the stem or lengthening it not to bend the valve stem, as this will result in the valves sticking, or in any event the bore of the valve stem guide in the cylinder will be worn unevenly. The clearance between the pushrod and the valve

stem should never be greater than 32-inch nor less than 64-inch. If too much clearance is present the valves will open late and close early. If the clearance is less than the minimum there is danger of the valve remaining partially open all of the time because the valve stem lengthens, due to expansion produced by the heat of the explosion. When it is necessary to draw down a valve stem this should be done by peening it for about 3/4-inch above the pinhole or key slot.

It is not a difficult matter to set the clearance exactly as it should be on those types of engines provided with an adjustment screw which may be raised or lowered in the valve plunger or in forms having fiber inserts in the top of the valve plunger. These inserts are utilized to silence the valve action and may be easily removed and replaced with new ones when worn. A simple and cheap accessory that can be obtained on the open market can be used to adjust the clearance on Ford and similar type motors. This consists of a number of stamped steel cups that can be pushed on the lower portion of the valve stem and a number of thin steel washers to be interposed inside of the cup and between the bottom of that member and the end of the valve stem to regulate the clearance as desired.

Sleeve Valve Motors.-A number of automobile manufacturers have adopted the Knight sleeve valve motor as a power plant. The same instructions that have been given regarding the time of opening and closing of the valves of other motors also apply to this form in a limited degree. The only possibility of losing the timing on the sleeve valve motor, which is shown in section at Figs. 154 and 155, is by faulty setting of the sleeve operating crankshaft, in its relation to the main crankshaft. If the timing is altered by depreciation of the bearing in the sleeve operating links they may be refitted; the lower or rotating bearings which would be more apt to wear than the upper or oscillating bearings, are of the same marine pattern as is generally used in the main connecting rod. This means that any depreciation may be remedied by fitting the bearing caps closer to the connecting rod and scraping or reaming to fit.

In order to show clearly the appearance of the various parts