correct piston size

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Nov 14, 2003
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#1
Recently bought 03 yz 125 from friend which had the cylinder re plated i am doing top end job after it has had one new piston in since re plate . What size piston should i use as vertex come in sizes A,B,C,D Shop says C will be fine friend says A. Also what time can you safely leave a piston before change even though i know perforfomance will decrease as friend said that is why it needed re sleeve as left piston too long and wore rings right down, is this a possibilty?