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CR125 Piston Type?

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I have an 02 CR125, I recently blew the top end and I'm clueless about what size piston to get, when I got the bike the previous owner had a receipt to show he had just put new rings in. The rings were 54mm, would that mean I need to order a 54mm piston? Also do I have a B or A piston?

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the clyinder will be stamped with a letter to tell you what size piston your cylinder is for 

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Before you go and order any new parts you should first pull the cylinder off and inspect everything so you know exactly what you need to get. If your cylinder needs replated then you can get any size and brand of piston you want. If the cylinder is fine then you need to first measure it to determine what size piston you need. The fact that its an 11 year old bike you never know exactly what size the bore is until you measure. A or b means nothing at this point. Your service manual will have all the measurements so you can determine if it is still in spec first off and what size piston to get secondly. Either take the cylinder to a local machine shop and have them measure it for you or you can buy a set of telescoping bore gauges and a digital micrometer. Harbor freight has them for very cheap. They aren't the greatest or most accurate out there by any means, but they are good enough for what you need. If you plan on using them alot then I'd get something of better quality.

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