How a Dirt Bike Camshaft Works
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A common camshaft design is the single overhead cam (SOHC). The camshaft is just that, a metal shaft. Rather than being a uniform shaft, though, it features lobes that serve to open and close the valves.
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Engines with camshafts are generally termed "four stroke" engines. The engine's cylinder head is the part that bolts down over the cylinder and forms a seal so the piston can achieve its four strokes. Mounted above the cylinder, inside the cylinder head, is the camshaft. One of these four strokes includes the compression stroke when the engine valves are closed.
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Each time the piston moves up and down in the cylinder, the valves open and close accordingly. The parts that coordinate the valves' opening and closing are cams on the camshaft. The cams are lobed on one side and so they're egg-shaped in cross section. As the camshaft turns, the lobe comes around on every revolution. Each time it does, it pushes against the lifter at the base of the valve stem and forces the valve to open.
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The camshaft has separate lobes for the intake valves and the exhaust valves. Since the intake and exhaust valves must open and close at different times, the camshaft's lobes for the intake valves are on the side of the shaft opposite those for the exhaust.
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The camshaft lobes are in a different combination of positions on each of the four engine strokes. On the intake stroke---when the piston is moving down to draw in the fuel and air mixture---the lobe of the cam for the intake valve is turned so that it is pressing against the lifter at the base of the intake valve stem. This opens the intake valve to allow the piston to draw the fuel and air mixture into the cylinder through the open intake valve. Next, the piston is on its way up on the compression stroke. During this stroke both intake and exhaust valves must be closed, so the cams are in the position in which their lobes are not pushing on the valves. In other words, the more rounded part of the cam is toward the valves. The valves stay in the closed position as the piston is driven down from combustion of the fuel and air mixture on the power stroke.
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The last stroke is the exhaust stroke and the exhaust valve must be open. This means that the lobe on the cam for the exhaust valve is pushing on the exhaust valve and forcing it open. Meanwhile, the lobe for the intake valve is in a position away from the intake valve, allowing the intake valve to remain closed so that the exhaust gasses exit through the exhaust valve and not through the intake.
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A camshaft is really just a metal shaft with sections along its length that are out of round. These out-of-round sections are the actual cams that serve to force the valves open each time the lobed side come around on a turn.
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