the 2nd ring will never see the same amount of heat as the top ring, especially in a street application. that combined with the accumulator/ channel/ groove, is the reason for leaving the 2nd ring a small amount tighter in the bore. (less expansion}

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that groove is called an accumulator groove, btw.

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it's also called a pressure balance channel, a pressure balance groove and a few other things. different names from different piston manufacturers.