Honda CRF250R at the 2009 Seattle International Motorcycle Show. The Honda CRF series is a line of four-stroke motocross and trail motorcycles manufactured and marketed by Honda. The CRF line was launched in 2002, both as a successor to the Honda XR series and a replacement for the two-stroke Honda CR series. The full sized motocross bikes are equipped with liquid-cooled, single-cylinder four-stroke engines that are available from 149 cc to 649 cc. They now have dual-sport motorcycles. The entry-level CRF’s have simple air-cooled engines, and are available from 50 cc to 230 cc. The Honda CRF450R was the first in the series, followed with the CRF250R in 2004. Further down the line, the CRF450X and CRF250X bikes emerged, both designed for mostly off-road use. They are considered among the best motocrossers of their class, and have been a leading seller since their introduction. Engine technology This section does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009) Honda employs advanced four-stroke engine technology in their 250cc and 450cc bikes. The engines in these bikes use technology developed in Formula One racing engines. Honda uses an over-square engine design, this means that the diameter of the cylinder is larger than the stroke of the piston. This allows for higher engine speeds and a reduction of reciprocating mass. Another technology that is used is short piston skirts. The “skirt” area of the piston is the portion on the side of the piston which comes into contact with the cylinder wall and aids piston stability. While the introduction of the shorter skirt on the piston helps to reduce reciprocating mass, it also leads to more “rocking” of the piston, or minute unwanted rotation of the piston around the axis of its wrist pin. This leads to more frequent maintenance intervals for the pistons, piston rings, and cylinder walls. Overall the new high-performance four-stroke engines in Honda’s motocross bikes produce much more power per pound than older generation engines and rev more freely; in the quest for more power, longevity is reduced and maintenance is increased. As of 2008, the CRF X series includes the 250X and the 450X. These bikes are electric start and are still considered race bikes, albeit for off-road rather than motocross. Differences from the R models include lighting, electric start, suspension settings, engine tuning and exhaust tuning for more tourqe, larger fuel tanks and wider-spread gear ratios. As of 2008, the CRF L series includes the 230L. Since this motorcycle is a dual-sport motorbike it is street legal with everything included but can still make its way on the dirt due to Honda’s idea to give it the same engine as the F series, just with a more restrictive exhaust and leaner jetting to meet EPA and DOT standards. Everything else is set up much differently from the F series. Both 230s have electric starts. As of 2008, the XR R series includes the 650R (liquid cooled), 650L (air cooled). The 650L is electric start and is modeled after the old school XR600/XR650L. 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