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A four-lobe cam may be used on either a seven-cylinder or nine-cylinder engine. [Figure 1-27]  On the seven cylinder, it rotates in the same direction as the crankshaft, and on the nine cylinder, opposite the crankshaft rotation. On the nine- cylinder engine, the spacing between cylinders is 40° and the firing order is 1-3-5-7-9-2-4-6-8. This means that there is a space of 80° between firing impulses. The spacing on the four lobes of the cam ring is 90°, which is greater than the spacing between impulses. Therefore, to obtain proper relation of valve operations and firing order, it is necessary to drive the cam opposite the crankshaft rotation. Using the four-lobe cam on the seven-cylinder engine, the spacing between the firing of the cylinders is greater than the spacing of the cam lobes. Therefore, it is necessary for the cam to rotate in the same direction as the crankshaft. 

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## Camshaft 

The valve mechanism of an opposed engine is operated by a camshaft. The camshaft is driven by a gear that mates with another gear attached to the crankshaft. [Figure 1-28]  The camshaft always rotates at one-half the crankshaft speed. As the camshaft revolves, the lobes cause the tappet assembly to rise in the tappet guide, transmitting the force through the push rod and rocker arm to open the valve. [Figure 1-29] 

## Tappet Assembly 

The tappet assembly consists of: 

1. A cylindrical tappet, which slides in and out in a tappet guide installed in one of the crankcase sections around the cam ring; 

2. A tappet roller, which follows the contour of the cam ring and lobes; 

3. A tappet ball socket or push rod socket; and 

4. A tappet spring. 

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