Spark Ignition Aircraft Engine Endurance Test of Aviation ... - FAA

The intake valve stem diameter, guide diameters, and valve guide clearance were all within the
manufacturer’s new limit specifications at the end of the test. Both the intake and exhaust seats
showed a hammered effect and were out of the allowable service limits at the end of the test.
This may have been partly due to having to retard the ignition timing by 5°. Figures 61 and 62
show the intake and exhaust valve deposit buildup and the damage to the exhaust valve faces.
Figure 62 shows the hammered effect on the exhaust valves with ridges formed on the valve
faces.
Figure 61. Posttest Intake Valves 1 Through 4 (Left to Right)
Figure 62. Posttest Exhaust Valves 1 Through 4 (Left to Right)
The exhaust valve stems showed no wear, but the valve guides showed significant wear. All the
valve guides were out of the manufacturer’s service limits at the end of the test, as were the
valve-to-valve guide clearance measurements. This also may have been a consequence of
retarding the ignition timing by 5°.
The valve rocker arms showed normal wear. Figure 63 shows a typical rocker arm at the
completion of the test.
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