عربى 
Valves and valve seats are a pair of critical components in the engine's valve train (the system that controls the engine's "breathing"). They work together to precisely open and close the intake and exhaust passages at the right times. Think of them as the engine's "valves and their doorframes."
Specifically:
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Valve – It consists of a disc-shaped head (commonly called the "valve head" or "mushroom head") and a slender stem. The head seals or opens the gas passage, while the stem provides guidance and transmits the driving force.
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Valve Seat – It is a ring-shaped metal piece embedded directly in the cylinder head (or cylinder block), which mates tightly with the tapered surface of the valve head. It serves as the "doorframe," providing a precise and durable sealing surface for the valve.
How do they work?
When the camshaft presses down on the rocker arm or tappet, the valve is pushed downward, lifting off the valve seat and opening the intake/exhaust passage. When the camshaft rotates and releases the pressure, the valve springs back rapidly under strong spring force, pressing its tapered head firmly against the valve seat to seal the passage. The sealing quality of this mating surface directly determines the engine's cylinder pressure and power output.
Their harsh operating environment
This pair works under extremely tough conditions:
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Intense heat – The exhaust valve is directly exposed to combustion gases at temperatures exceeding 1000°C.
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High-frequency impact – At thousands of engine revolutions per minute, the valves open and close dozens of times per second, constantly striking the valve seats.
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Severe wear – Continuous impact and high-temperature gas flow cause rapid wear on the contact surfaces.
Materials and maintenance tips
Because of these harsh conditions, the materials used are carefully chosen:
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Valves – Intake valves are typically made of heat-resistant steel (e.g., 40Cr), while exhaust valves must use more advanced heat-resistant alloys (such as silicon-chromium steel or even nickel-based alloys) to resist high-temperature corrosion and burning.
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Valve Seats – Usually made of alloy cast iron, powder metallurgy, or cemented carbide, offering good wear resistance and thermal conductivity.
In routine maintenance, it is essential to periodically inspect the sealing taper between the valve and valve seat. If burning, pitting, or deformation occurs, causing leakage, the engine will suffer from power loss, difficult starting, or even backfiring ("popping"). In such cases, the valve seat typically needs to be refinished with a special reamer, and the valve must be lapped (ground) against the seat to restore a tight seal.
