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Negative-Maximum7830 posted on r/motorizedbicycles5w
Some YD100 engines were built with a 12 mm wrist pin to or their standard 50 mm bore. Most have a 10 mm pin. Windowed piston example attached. https://www.amazon.com/ZZRSDAN-Yd100-LD100-Windowed-Piston/dp/B0C96N7GQM