Valve Spring Free Length
Check valve spring free length with a caliper, as shown in the illustration. Checked the springs installed height; from the base or pocket in the head to the bottom of the retainer. If this height is excessive, add the appropriate shim to bring the spring back to its proper tension. Any short springs (~1/16 inch) should be replaced. Tension can be checked with a valve spring tester.
Valve Spring Squareness
Use a square and a flat surface to check valve spring squareness. When placed on a flat surface, the springs should remain square. Rotate the valve and make sure all the coils contact the square when it's rotated. Use a feeler gauge to measure tolerance. Check the manufacturer's manual for any specifications or special procedures. Notice the space between the top coil and the square.
Cylinder Head Valve Inspection
After removing the valve from the cylinder head cleaning it, and inspected for:
No margin: The margin of the valve gets pounded. If the margin is to thin, the valve heats up and burn prematurely.
Pitting, Burning, and Cracks: This usually happens to exhaust valves, because they run hotter than the intake valves do. Exhaust valves are typically made out of high-quality stainless steel. But they don't benefit from the fresh air /fuel charge the intake valve receives on the intake stroke. As the spring weakens, the valve sticks open a little longer. Valves burn when they're left open during combustion and exposed to extreme temperatures.
Cupped Heads: Replace the valve if the head is cupped or mushroomed.
Stretching: Stretching is the narrowing of the neck above the head. This affects the length of the valve.
Worn Keeper Grooves or Valve Stem Tip: Inspect keeper grooves, rotators, and retainers for wear and replace if necessary. If a keeper or retainer pops out of place, the valve will drop in the cylinder, causing engine damage. Also, a worn valve stem tip can cause retainer and rocker arm damage.
Valve Stem: The valve stem should be measured with an outside micrometer in several places to check for uneven or excessive wear.