As long as the correct seat angle and width are kept, worn-out or burned valve seats can be reworked, but if they change, the whole cylinder head must go. Always ensure you have the right size valve guide pilot when refacing valves and valve seats so the reseating stones give you a true and complete surface. The starting point is to measure the center of the valve seat with a proper dial indicator, so that the total run out is no more than 0.051 mm (0.002 inch). Then, touch a bit of Prussian blue to the valve seat, fit the valve into the cylinder head, gently press on it while turning it slowly. Should the blue color mark end up below the top of the valve face, use a 15 degree stone to lower the valve seat; if the mark is at the bottom, use a 65 degree stone to raise it. With the seat in the proper place, the intake seat should be 0.0689 - 0.0928 inches wide and the exhaust seat should be 0.0673 - 0.0911 inches wide. Once the valves and seats are refaced, check that neither the intake nor exhaust valve springs has a height over 40.74 mm (1.6039 inch). The bottom line is you must maintain a face angle between 44.5 and 45°.