Exhaust valves run hotter than intakes so the valve stems stretch more. Every time the valve opens it gets exposed to the hot exhaust gas, while the intake is cooled by the incoming mixture. The long screwdriver is a field expedient stethoscope- - - -they're listening for a rattle that indicates the valve adjustment is loose.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!