How are inlet and exhaust valves held firmly closed?

• A. By tight bolts.
• B. By friction alone.
• C. Held closed against their seats by tight springs. Gas tight seal.
• D. By magnetic attraction.

Answer: (C)

Valves stay closed because the valve springs push the valve against its seat, creating a gas-tight seal. When the camshaft lobe pushes a valve open, it compresses the spring; once the lobe moves away, the spring drives the valve back onto its seat, sealing the combustion chamber against pressure. This spring force must be strong enough to overcome the pressure trying to push the valve open, ensuring the seal remains intact. Bolts or merely relying on friction wouldn’t provide the reliable, repeatable closing force needed under operating conditions, and magnetic attraction isn’t used here.