ASE A1 Engine RepairCylinder Head and Valve TrainEasy
A technician is performing a valve adjustment on an overhead valve (OHV) engine with solid lifters. The service manual specifies a valve lash of 0.010 inches (0.25 mm) for the intake valves and 0.012 inches (0.30 mm) for the exhaust valves, measured with the engine cold. What is the primary reason for having a slightly larger lash specification for exhaust valves compared to intake valves?
- ATo compensate for the higher spring pressure on exhaust valves.
- BTo allow for greater heat expansion of the exhaust valve components.
- CTo reduce noise levels from the exhaust side of the valve train.
- DTo ensure proper scavenging of exhaust gases from the cylinder.
Show answer & explanationAnswer & explanation
Correct answer: B. To allow for greater heat expansion of the exhaust valve components.
Exhaust valves are exposed to much higher temperatures than intake valves. This causes greater thermal expansion of the exhaust valve stem and related components. A slightly larger cold lash ensures that the valve does not lose all its clearance and 'hang open' when the engine reaches operating temperature.
Why the other options are wrong
- A. Spring pressure is designed to close the valve and is not the primary reason for differing lash specifications between intake and exhaust valves.
- C. Lash adjustment can affect noise, but the primary reason for the difference is functional (thermal expansion), not noise reduction.
- D. While valve timing affects scavenging, the lash adjustment specifically compensates for thermal expansion, not directly for gas flow.
Exhaust Valve Thermal Expansion
Exhaust valves operate at higher temperatures than intake valves, leading to greater thermal expansion. A larger cold valve lash compensates for this to prevent valve hang-open.
- Exhaust valves get hotter
- More heat causes more expansion
- Larger cold lash prevents valve hang-open
Memory trick: Hot exhaust wants more space, cold lash gives it grace.