well i have my kid helping me so it goes quick, yes if you cant shim the valves it is because the outer edges start to curl towards the piston, mine looked kinda like a coffee saucer, they call this tuliping it is from the edge of the valves getting hammered against the valve seats, i replaced mine cause the didnt seal anymore and i had my jugg of putting in a new piston, but yes sooner or later they will be tuliped so much they wont seal then they need to be replaced, but if you can grind your shims and get the clearance and it starts good maybe you can make it till winter before having to do it, when it wont start when it is cold out is a good sign your valves are not seating and out of adjustment