Honda CRF series
From Honda Wiki
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 Honda CRF250R in 2004. Further down the line, the Honda CRF450X and Honda 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.
The smaller, air-cooled versions of the CRF-series are intended for the casual off-road user, or younger riders. The larger, liquid-cooled bikes feature a SOHC engine design and four valves (two intake, two exhaust). Earlier models of the bike have been plagued with valve train issues: especially pertaining to the intake valves. Honda uses titanium intake valves for reduced reciprocating mass; however titanium is naturally a soft metal which lends itself to easy deformation. To combat this issue, Honda employs a manufacturing process in which a hard, wear-resistant coating is applied to the intake valves. When this microscopic coating has worn off, the valves wear out very quickly. Infrequent cleaning of the air filter or improper oil change intervals are known to decrease the valve train life. A commonly-employed fix for the valve train reliability issue is to install stainless steel intake valves. This requires new (heavier) valve springs and other associated parts. This fix has been known to increase the valve durability significantly.
Like their other leading competitors, Honda employs advanced four-stroke engine technology in their 250 cc and 450 cc bikes. The high-revving, high-output engines in these bikes use technology developed in Formula One racing engines. Of the primary differences from a normal four-stroke engine, Honda uses an "over-square" engine design. This means that the diameter of the cylinder is larger than the stroke, or, swept volume, 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 F series includes the 50F, 70F, 80F, 100F, 150F, 230F. The 150F, 230F come with electric start.
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, and larger fuel tanks.
As of 2008, the CRF L series includes the 230L. Since this motorcycle is a Dual-sport 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 CRF R series (racing series) includes the 150R, 150R Expert (big wheel), 250R, and 450R. All bikes are kick 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.