I have been trying to get a straight and correct answer on this topic, off and on, for freakin years!! It seems like I continually encountered self-professed experts around every corner who continually contradicted each other. Frustrating!! I finally got my hands on an authentic Lexus dealership shop repair manual and began combing the pages. I finally found this statement on page EM-4 (Engine Mechanical-4), word for word, from the actual creator of my 1993 1UZFE. "Pistons are made of high temperature-resistant aluminum alloy, and a depression is built into the piston head to prevent interference with valves". BAMMMM!! :) At least I know I'm in the clear with my '93 SC.