1.3.2.7.4. Parts of valve mechanism The valves in some engines rub against hardened inserts called the valve guide and valve seal. Most combustion heat is passed from the valve to the water passages via the seat when the valve’s closed. A worn valve guide will allow oil to be sucked into the combustion chamber. A damaged seal may lower the cylinder's compression (Figure 1.28). Rocker arms on many cars are made of pressed steel. These GM arms pivot on ball studs instead shafts. Valve Operation: As the cam rotates, it lifts the lappet and pushrod, pivots the rocker arm, and opens the valve (Figure 1.29-A). Further cam rotation allows the tappet and pushrod to fall and the spring to close the valve (Figure 1.29-B). This design with the camshaft in the cylinder block is called an overhead-valve pushrod engine. 1.28: Parts of valve system
Figure 1.29: Valve operation
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