Abstract: The clearance measurement value of some batches of engine valves was abnormal,higher or lower than the practical value.In order to find out the measurement error,the senor,measurement location of sensor,tappet level algorithm and PLC logic procedure were checked.The results show that the deficiency of the tappet level algorithm,senor,measurement location of sensor,and PLC logic is the main cause for the abnormal measurement value.By modifying the calculation method,replacing the sensor,adjusting the location,and optimizing the measurement logic program,the measurement error of valve clearance has been sharply reduced,and the qualification rate of the measurement value stays stable at 97%.