134 Auto Engine Repair Copyright by Goodheart-Willcox Co., Inc. Head Cam drive Crankshaft Camshaft Valve in block Figure 6-18. The L-head engine is no longer used as an auto- motive engine. The valves are contained in the block in this design. (Ford) Valve adjustment screw Locknut Push rod Rocker arm shaft Rocker arm A B Camshaft Cam follower Valve clearance Figure 6-19. Compare cam-in-block (overhead valve) and overhead camshaft configurations. A—When the camshaft is in the block, push rods and rocker arms transfer the camshaft motion to the valves. B—An overhead camshaft acts directly on the valve. (Federal Mogul) type engine is a refinement of the overhead valve engine. The industry trend is toward the OHC engine since it offers performance advantages over the cam-in-block engine. With the camshaft in the head, the number of valve train parts is reduced. This cuts the weight of the valve train. Also, the valves can be placed at more of an angle to improve airflow through cylinder head ports. OHC engines were first used in racing cars because of their efficiency at high engine speeds. Now, they are common in nearly all makes and models of vehicle. Without push rods to flex coupled with less valve train weight and improved valve positioning, the OHC increases high-speed efficiency and power output. Single Overhead Camshaft Engine A single overhead camshaft (SOHC) engine has one camshaft per cylinder head. The camshaft lobe may act directly on the valves or rocker arms may transfer motion to the valves. This is a very common design found on today’s cars. See Figure 6-20. The single camshaft in a SOHC head operates both exhaust and intake valves. Rocker arms are needed to transfer camshaft lobe action to the valve stems. Note: A V-type, W-type, or opposed engine may have two camshafts, but one per cylinder head. These are classified as SOHC engines. Dual Overhead Camshaft Engine A dual overhead camshaft (DOHC) engine has two camshafts per cylinder head. See Figure 6-21. One camshaft operates the intake valves and the other camshaft operates the exhaust valves. This design can be used with a two- valve, three-valve, or four-valve combustion chamber and is common in many late-model engines. A DOHC engine, although normally more powerful and efficient than an OHV or SOHC engine, is more complex and costly to repair. Camshaft Drive A camshaft belt drive uses a cogged rubber belt to turn the camshaft, Figure 6-22. A gear is mounted to the crank- shaft. A larger gear is mounted to the camshaft. The timing belt connects the two gears. This type of drive is normally used when the camshaft is located in the cylinder head because it can span the long distance from the crankshaft to the camshaft. A belt drive provides very smooth, quiet, and trouble-free operation. A camshaft chain drive is another common method of driving the camshaft. A gear is attached to the crankshaft. A larger gear is attached to the camshaft. A heavy-duty timing chain connects the two gears. This type of drive is normally used when the camshaft is located in the engine