Why is the exhaust valve clearance larger than the intake valve clearance?
2 Answers
Because of the difference in thermal expansion coefficients. Here are the relevant explanations: 1. Intake: It relies on vacuum suction to draw air in, while exhaust is expelled through compression. Secondly, to allow the engine interior to obtain more fresh air, thereby enhancing the vehicle's power performance, naturally more intake is required. Therefore, the intake valve is undoubtedly made larger, while the exhaust valve does not have this requirement. 2. Clearance: The intake valve clearance is generally within the range of 0.20mm~0.25mm. The exhaust valve clearance, due to thermal expansion, is larger than that of the intake valve side, so naturally a larger clearance is needed, typically between 0.29mm~0.35mm.
I've been working in auto repair for over 20 years. The reason why exhaust valve clearance is larger than intake valve clearance is mainly because the exhaust side gets extremely hot, constantly exposed to high-temperature exhaust gases. When metal heats up, it expands. Exhaust valves expand more, so if the clearance is too small, they'll expand excessively when hot and won't seal properly. This leads to compressor air leaks, reduced engine power, and potential valve head burn-out. Intake valves are on the cooler air side with lower temperatures and less expansion, so smaller clearance is fine. I've encountered many vehicles with problems caused by improper clearance adjustment, such as hard starting or black smoke emission. Regular inspection is crucial, especially for older vehicles where clearance changes rapidly under high temperatures - don't risk bigger issues by trying to save money. Pay attention to abnormal engine noises while driving; a rattling sound might indicate clearance problems.
