Piston Tech needed

You measure them perpendicular to the pin bore with the pins out (in case one is tight - it might distort the skirt), about halfway between the pin centerline and the bottom of the skirt. Don't squeeze the skirts, they will flex.

Modern pistons have the clearance built in, so a .040" over 340 piston that needs .004" clearance will measure 4.076", give or take bit. Every piston should be measured before the boring bar is turned on!

Hope this helps.