This is an old revision of the document!
REF: Engine Mechanicals - Sub-02B
Testing the Clearances in Piston Valve Pockets (reliefs)
Clay pressed into the valve pockets can used to test how far the valves come down toward the piston (when installed).
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photo by kwbikr of the XLFORUM http://xlforum.net/forums/showthread.php?t=1674723&page=25