More Model A valves suffer from being burned due to inadequate clearance than from any other cause. As valves and valve seats wear, the clearance will be reduced. It is therefore wise to check clearance every 5,000 miles. The objective is to have sufficient valve to lifter clearance when the engine is cold so that some clearance remains after the engine and valves have reached their warmest operating temperature.
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To Adjust or Not Adjust May 2013