Lycoming recommends this procedure every 400hrs for airplanes and every 300hrs for helicopters. The intent is to prevent exhaust valves from sticking 'i.e. morning sickness' by making sure the valve has ample clearance within the guide where exhaust residue can build up and can cause the valve to get stuck. leaning in flight and on the ground can reduce the likeliness of a sticky valve.
Mechanics who are familiar and proficient with this test sometimes do an abbreviated version of it where they can tell by feel how far the valve moves side-to-side, saving much time.
Fixture tool info is attached as well.
This procedure is particularly useful for high time engines where valve sticking can be more likely.
Lycoming388CProcedureWobbleTest)20Determine20Value20Guide%20Condition Page
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