Camshaft Valve spring Push rod Tappet Figure 1-25. Valve-operating mechanism (opposed engine).  Camshaft 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. 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] The tappet assembly consists of: Figure 1-27. Radial engines, cam ring table. Figure 1-26. A typical set of valve springs used to dampen  oscillations. Multiple springs are used to protect against breakage.  Tappet Assembly 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. 1-20 Number of Lobes Speed Number of Lobes Speed Number of Lobes Speed 5 Cylinders 3 2 1/6 1/4 1/8 1/6 1/10 1/8 with crankshaft opposite crankshaft 4 3 5 4 7 Cylinders 9 Cylinders Direction of Rotation