300tdi valve clearance

I'm gradually going over everything on my latest landie, yesterday I checked the valve clearances and was expecting to have to take up slack on the adjsters and was amazed to find 4 of the valves too tight. I've no idea when they were last done or by who, my son reckons someone set the gap and just tightened the lock nut without being carefull to hold the screw in place, sounds a reasonable theory. Anyway I took it out for a good run this morning and found the engine sound not as harsh and possibly running a bit better, my concern is do valves being set too tight cause any long term damage.
Alan.

yes it will allow hot gasses to burn grooves through them and seats , check all nuts have about the same thread poking out the top it indicates poor setting or issues , set using rule of 9 ,8 thou/0.2 mm,