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See this is where I get into trouble explaining it. They say "Well if the pushrod is expanding and getting larger and the roller tip is expanding then the lash will become smaller(tighter) as the two metals "grow' towards one another as they expand!!!
Mike

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The pushrod is expanding at a smaller rate per material and it only see ambient temperature inside the valley. The block and heads see a much higher temp from combustion and higher thermal expansion rates. The roller tip expansion is almost nil.

Edit: short answer the block/heads are expanding at a greater rate than the cam/pushrods/roller tip.

ask someone who has an aluminum block and aluminum head Hemi about setting lash cold.