it's a measurement of tdc to head clearance. I'm just learning all of this too. I've got a yz250 completely apart right now and have learned just enough to know I don't know crud! Age has nothing to do with it. you know the BAP drill!!

Replacing factory with factory is a 'safe' bet with me for certain things such as gaskets. I do replace some items with proven aftermarket parts. ie pistons,reeds

I'm pretty sure that once you clean the PV, change your pilot and make sure you don't have a leak, you will find a big difference in the bike.