Although the tapping trick may have worked with some early design knock sensor systems, you won't likely notice any timing change when tapping on the block. The knock sensor systems are designed only to respond to misfiring noises at higher engine speeds only and to ignore other noises -such things as tapping, rattling parts and road vibrations. See the 700-900 FAQ section under Engine Sensors for more info on knock sensors and tips on diagnosing knock sensor problems.
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Dave -not to be confused with a real expert