Since the least common result of valve failure is the valve itself, it is always necessary to determine the cause of failure prior to replacement of the damaged valves (Figure 1).
The illustration in Figure 2 shows the characteristic appearance of an impact failure.
NOTE: Breakage by impact is a sudden failure. Example: piston to valve collision.
The illustration in Figure 3 shows the characteristic appearance of a fatigue failure.
NOTE: Breakage by material fatigue happens after thousands of cycles, starting in a small defect and advancing until breakage occurs.
Source: Supertech Performance