The YZ 125 cylinders come stock in four different sizes, which are microscopic in size difference. If you look at the back of the cylinder there will be an A, B, C, or D stamped on it, unless it's been worn off. If you're buying an aftermarket piston (such as Wiseco) there's only one size that fits all four cylinders. However, if you're buying a stock piston make sure and specify which letter cylinder you have.