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Dirt Bike Discussions By Brand
Honda MX & Off-Road Dirt Bikes
2002 CR250 Piston A or B
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[QUOTE="NWDirt, post: 1198179, member: 72763"] The gap between the piston and cylinder wall is an engineered distance that needs to be maintained. A Japanese manufactured A cylinder with a D piston will seize. Ask my riding partner. If you have ever honed a press fit hole on a mill, or bored an accurate ID on a decent lathe, than you realize how little it takes to open up an ID. An extra half a second on the up stroke of a hone can open up an ID more than you wanted. The machines used for mass machining have tool wear etc to deal with so being perfect is pretty cost prohibitive. If you only had one tolerance range for the ID of the cylinder, you would be scraping a lot of cylinders. The answer is to have 3 or four tolerance ranges and 3 or 4 different diameter pistons manufactured for each range. They machine the bores and nikosil the cylinder wall. They measure the bore accurately and depending on the bore diameter, they mark the cylinder A (tightest bore diameter) through C or D (largest bore diameter). That way they can match it to a piston that will allow the designed gap between bore and cylinder. You can measure the piston with a dial caliper (digital is better) just above the intake opening (largest diameter) and if that falls within the piston dia tolerance given in your manual for an A cylinder, its good. chances are it was not marked B for no reason though. Eric Gorrs book does a nice job of describing all of this. Working in Metric is also easier as a system. It was designed way later than empirical system and thought through so going from volume to weight as an example, is less painful. It doesn't matter what system you use, as long as anyone else wanting to use a different system (machining, measuring, designing around) understands how to convert tolerances correctly. Many talented machinists will get a metric print that says 10.5 +/- .02 and the first thing they do is convert 10.5 to .4134" and then accidentally us .02 (20 thou) as the tolerance. Oops. Converting measurments can cost a lot of scrap parts. 2 cents. Dave [/QUOTE]
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Dirt Bike Discussions By Brand
Honda MX & Off-Road Dirt Bikes
2002 CR250 Piston A or B
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