Engine Valve Inspection
After removing the valve from the cylinder head and cleaning, it should be inspected for:
No margin: The margin of the valve will get pounded away in time. If the margin is to thin the valve will heat up and burn prematurely.
Pitting, Burning, Cracks: This usually happens to the exhaust valve because they run hotter than the intake. Exhaust valves are usually made out of high quality stainless steel, but don't benefit from the cool air and fuel the intake valve receives on the intake stroke. Valves usually burn when they are left open during combustion and exposed to extreme temperatures. This usually happens because the valve is left open al little longer as the spring weakens. Keep in mind that older engines with more wear are naturally more susceptible to burned valves if not repaired.
Cupped Heads: Once the head is cupped or mushroomed the valve must be replaced.
Stretching: Stretching is the narrowing of the neck above the head of the valve. This effects the length of the valve.
Worn Keeper Grooves or Valve Stem Tip: Look for worn keeper grooves, rotators, and retainers and replace if any of these are worn. If a keeper or retainer pops out of place the valve will drop in the cylinder causing engine damage. A worn valve stem tip will 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.
Valve Springs should be checked for their installed height which is from the base or pocket in the head to the bottom of the retainer. If this height is excessive a shim must be added to bring the spring back to its proper tension. When placed on a flat surface the springs should remain square and any short ones (*1/16 inch) should be replaced. Tension may also be checked with a valve spring tester.