A technician is performing a valve adjustment on an overhead valve (OHV) engine with solid lifters. The service manual specifies a valve lash of 0.008 inches (0.20 mm) for the intake valves and 0.010 inches (0.25 mm) for the exhaust valves. After setting the intake valve on cylinder #1 to 0.008 inches, the technician notes that the exhaust valve on the same cylinder is significantly tighter than specified. What is the MOST likely reason for this discrepancy?
- AThe engine was not correctly positioned on the base circle for that exhaust valve.
- BThe camshaft lobe for that exhaust valve is worn.
- CThe engine was not at the correct operating temperature.
- DThe valve spring for that exhaust valve is weak.
Show answer & explanationAnswer & explanation
Correct answer: A. The engine was not correctly positioned on the base circle for that exhaust valve.
For solid lifter valve adjustments, it is absolutely critical that the cam lobe for the valve being adjusted is on its base circle (the lowest point of the lobe, where the valve is fully closed and there is no lift). If the engine was not correctly positioned, the exhaust valve might be partially open, or the lifter might be on the ramp of the lobe, leading to an artificially tight reading. While other factors can influence lash, improper positioning is the most common cause of significant discrepancies during adjustment.
Why the other options are wrong
- B. A worn camshaft lobe would typically result in a larger-than-specified lash (looser), not tighter.
- C. Incorrect temperature affects all valves somewhat, but typically wouldn't cause one valve to be 'significantly tighter' if the intake was set correctly.
- D. A weak valve spring would not directly cause a tighter valve lash; it would more likely lead to valve float or poor valve seating.
Camshaft Base Circle
The circular, non-lifting portion of a camshaft lobe where the valve is fully closed and there is zero valve lift, which is the correct position for measuring and adjusting valve lash.
- Essential for accurate valve lash adjustment.
- Ensures the valve is fully closed and not being lifted.
- Incorrect positioning leads to false lash readings.
Memory trick: Base circle's key, or lash won't agree.