At the heart of the CRF250R lies a four-stroke single-cylinder engine designed to run at speeds in excess of 13,000 rpm- faster than the majority of multi-cylinder super sport street machines. Cradling this stellar powerplant is a new, fourth-generation aluminum frame equipped with the best suspension and chassis components available. With such stellar credentials, the CRF250R's formula for success is all but a given.
Engine development engineer Akifumi Nomura, who worked on the creation of both the CRF250R and the CRF450R engines said, "If the DOHC four-valve head configuration is considered prototypical road racing technology, then the CRF's Unicam configuration can be linked to the design of these two motocrossers. 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."
Bright-think rocker arms for a new era in four-strokes
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 CRF250R's 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 250's displacement is almost half that of the 450's, 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. Honda's use of small plugs is not new: Honda's '60s-era GP road racers made common use of 8mm spark plugs. Both the 1964 RC113 (50cc) and the1965 RC164 six-cylinder (250) used them.
To achieve the required durability at such ultra-high engine speeds, the CRF250R's 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.
A more complex piston shape for a higher-revving engine
Tsutsumi also explained, "If you compare the piston crowns of the CRF250R and the CRF450R, you immediately notice significant differences. The CRF250R's 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. Actually, it looks a lot like just a piston crown with a wrist pin attached."
Look closely, and you'll see the 250's 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 250's 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.
Make it light, make it tough
The themes of lightness of design and engine durability echo throughout the quarter-liter CRF powerplant. In fact, this 249cc four-stroke engine weighs only 52.7 pounds, a mere 1.87 pounds more than the CR250R powerplant-that's less weight than a quart of your favorite beverage. High-tech touches include the use of lightweight magnesium to reduce the weight of the ACG cover, clutch cover and cylinder head cover; and the vertically split crankcases feature a built-in oil pump, which reduces weight compared to a conventional design.
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 counterbalancer system does double duty by quelling vibration and driving the water pump-again, another testimony to efficiency in design by Honda's engineers. Fuel/air mixing chores are handled in thoroughly modern fashion by a 37mm Keihin FCR flat-slide carburetor featuring a throttle position sensor that helps maintain a linear throttle response throughout the rev range. The flat slide even features four rollers that ensure smooth operation while keeping throttle effort commendably light-older-generation flat-slide carbs that were subject to sluggish slide movement due to the high engine vacuum.
A world-class chassis for a world-class engine
The CRF250R's cutting-edge engine is encased within a rolling chassis no less impressive. As explained elsewhere, the CRF's fourth-generation aluminum frame takes a big jump forward in design; it actually weighs almost 2 pounds less than the CR250R frame. And weighing in at a feathery 206 pounds, the CRF250R scales a full nine pounds lighter than its two-stroke 250 cousin!
As befitting a next-generation Honda motocrosser, the 250 boasts a wealth of high-quality chassis components. The fork is a lightweight 47mm inverted Showa® twin-chamber cartridge unit, fully adjustable with 12.4 inches of travel. And now the inner surface of the fork's outer tubes receive the same honing treatment as works bikes for ultra-low-friction operation-a first for a production MX bike. Honda's much-respected Pro-Link® suspension system graces the rear end, and it likewise delivers 12.4 inches of fully adjustable travel.
Large, 240mm disc brakes have been mounted front and rear, with a dual-piston caliper up front. The aluminum swingarm features a dual-axis, double-taper design, with a stout cast aluminum cross-member for rigidity. HRC works-type lightweight aluminum spoke nipples help reduce unsprung weight.
Other Team Honda-inspired touches include a durable Renthal handlebar, a quick-adjust clutch perch for easy on-the-fly adjustments, and newly designed plastic bodywork for a more aggressive look plus lighter weight. Then there's the trick non- slip seat cover, plus the new-generation Dunlop D742 front tire, complimenting the D756 rear-the list of trick bits just keeps going on and on.
Simply put, Honda's 2004 CRF250R defines state-of-the art four-stroke technology-along with cutting-edge performance to match.