A 4-stroke camshaft turns at half crank speed

Because four strokes take two full crankshaft revolutions, the camshaft that opens each valve exactly once per cycle must turn at exactly half the crankshaft\’s speed — never the same speed.

Why it works

Intake, compression, power and exhaust each take one piston pass (a half-revolution); four halves make two revolutions, but each valve still only needs to open once, so the cam driving it is geared or chained down 2:1 from the crank.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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