Honda showed off their new 250cc four-stroke motocross machine at the Melbourne Supercross event staged on Saturday night (October 4).

Yamaha have had things all their own way in this class for a number of years after the release of their pioneering YZ250F. It has taken the others a while to catch up with Kawasaki and Suzuki only joining the fray with machines going on sale only a couple of weeks ago.

Big Red have been planning their entry in to the class for some time but their machines will not go on sale in Australia until January.

Honda aficionados are hoping the wait will be worthwhile. After a recent quick spurt on one of two pre-production models in Australia it is safe to say the Honda Australia boys are quietly confident that their new machine has the goods to shoot straight to the top.

A featherweight 93.5 kilograms is the weight Honda are claiming for the new machine. Three kilograms lighter than the two-stroke CR250R.

Honda’s CRF450R is an awesome unit and the new 250 shares the same basic Unicam engine configuration with big brother. But CRF250R project leader Yasuhiro Nakayama, “The final CRF250R is quite different from the CRF450R and the initial 250 prototype we built.

“Obviously, the 250 is smaller than the 450, but it also has a significantly higher rev ceiling. We wanted the 250 to have strong acceleration and good punch coming out of the corners, but we also knew that giving the engine too much punch would lead to jerkiness that could spoil the good traction that is one of the inherent benefits of four-stroke power. It could also make it harder to effectively use the machines engine braking characteristics.

“We also wanted the CRF250R to appeal to a broad spectrum of riders, and that meant paying attention to a number of new elements that hadn’t been required with the CRF450R. Striking the best balance between all these elements wasn’t easy”.

While the 450 revs to nearly 11,000rpm the new 250 has been designed to be able to safely exceed 13,000rpm.

Engine development engineer Akifumi Nomura, who worked on the creation of both the CRF250R and the CRF450R engines said, “The Unicam layout helped us to run four valves in a lighter cylinder head and it also offers the advantage of a narrower included valve angle than is possible in a DOHC configuration”.

Nomura continues, “Until now, common wisdom held that rocker arm systems are not well suited for engine speeds above 10,000 rpm. Since the CRF250R was designed to rev to speeds greater than 13,000 rpm, its rocker arms had to be both lightweight and rugged. Of course, because everything is smaller, you have to deal with size issues as well.

“Designing the CRF250Rs rocker arms turned out to be even more difficult than it was with the CRF450R, given our goal of making it not only lightweight, but also as small as possible. For example, the spark plug is located between the two actuating arms of the forked roller rocker-arms in both the CRF250R and CRF450R engines.

“However, even though the 250s displacement is almost half that of the 450s, it doesn’t mean the size of the spark plug can simply be cut in half. Instead, the CRF250R utilizes an NGK C-type spark plug with a 10mm thread diameter. The CRF450R, in contrast, incorporates a 14mm BK-type NGK plug.”

To achieve the required durability at such ultra-high engine speeds, the CRF250Rs cylinder head and valve train had to be exceptionally strong. And that required some new tricks. Engineer Koichi Tsutsumi, who also worked on development of the new 250 four-stroke engine, said, “The very lightweight and very tough titanium intake valves, for example, feature a special surface processing, while the exhaust valves are made of high-temperature steel with heat-resistant Inconel alloy used in the neck area just before the flare of the valve head”.

Inconel alloys contain high levels of nickel and can be thought of as super-stainless steels that have exceptional anti-corrosion and heat-resistance properties. As in the CRF450R, valve stem diameters are 5mm and the stem seals are the same as those used in the 450 as well. These features were carried over to ensure sufficient durability in the smaller but higher-revving engine.

“Tsutsumi also went in to further detail, “If you compare the piston crowns of the CRF250R and the CRF450R, you immediately notice significant differences. The CRF250Rs compression ratio is set extremely high, at 12.5:1, (the CRF450R features a compression ratio of 12.0:1 in comparison) so its piston uses a much higher dome. But overall the 250 piston height is very low and its skirt is ultra-short, so it doesn’t look much like the pistons generally used in production four-stroke engines.

“Look closely, and you’ll see the 250s wrist pin bosses stand out starkly indeed, with just mere vestigial traces of a piston skirt remaining in two places to prevent the piston from rocking in the cylinder. These amazingly short and narrow piston skirts come from the factory with a low-friction molybdenum coating to ensure a smooth piston-to-cylinder fit beginning with first use. The forged piston runs in a cylinder impregnated with a tough, low-friction Nikasil lining, a setup that also enhances cooling and reduces weight because it eliminates use of a cylinder liner.

“In addition, the 250s connecting rod is double-carburized for extra toughness and it utilizes a needle bearing in the big end to maximize high-rpm performance and increase durability. Likewise, the high-strength, low-carbon steel crankshaft features carburized main journals for maximum durability under high engine speeds.”

The 249cc four-stroke engine weighs only 24kgs, a mere .7 kgs more than the two-stroke CR250R powerplant.

Like its big-brother CRF450R, the 250 incorporates a twin-sump lubrication system that separates the engine oil from the oil bathing the clutch and the close-ratio five-speed transmission. This dual-supply system ensures a cooler environment for the clutch while also isolating the engine from any potential contamination caused by clutch and transmission material. In addition, because the total volume of oil circulating to the crankshaft, piston and valve train has been reduced, the oil pump supplying these vital parts can be made commensurately smaller and lighter.

A lightweight, compact internal automatic decompressor system provides easy start-up whether the engine is hot or cold, and a gear-driven counter-balancer system does double duty by quelling vibration and driving the water pump again, another testimony to efficiency in design by Hondas engineers.

Fuel/air mixing chores are handled by a 37mm Keihin FCR flat-slide carburetor featuring a throttle position sensor.