.I have supertech +1mm oversize valves and the question is do i need to replace the valve seats or could i open the stock just enough to fit.isn't it risky of making the stock valve seat thin and break it?what is the most used way of putting oversize valves?Thanks

 

most people would recommend to stick with stock size valves.
unless you have a full race full built full everything race car.
from other threads I saw here

and I belive tested by 4 piston dyno or flowcharts show better results with stock size valves for most applications