Street machines: 1955-1967
In Japan, the 250cc class of motorcycles was a big step-up from the common 50-125cc models. Moving up in the class required a different driving license and more taxation fees. Honda didn’t create a full-sized 250cc motorcycle until they designed their first OHC engine; the 1955 model SA (later ME). There were larger 350cc versions of these two bikes, called the SB and MF, based upon the 250cc designs. These ungainly thumpers were superseded by the all-new, dry-sump C70/71 Dream twins, released in late 1957. Many of these early 250cc Dreams had rotary gearboxes, but became the forerunners of the 1960 wet-sump C72 Dreams, which were expanded to 305cc C77 models (CA72-77 in the US). Rotary gearboxes were still an option, but slowly phased out and never used for US models, at all. The rare 1959 CE71 Dream Sport (and racing versions RC71/CR71s) was made in very small batches. Less than 500 CE71s, primarily destined for the US market, were built, with many recalled for engine problems.
The following 1960 wet-sump Dreams were models of reliability, for the most part, using new 12v electrical systems and leak-free engines which would spin up to 8.500rpms, which was unheard of at the time for a production street machine. The basic engine architecture of the 250cc Dream was used to create the new twin-carb, 1961 CB72 Super Sports model, also known as the “Hawk 250,” enlarged to 305cc for the “Super Hawk” editions. The CB72s went into production in late 1960, so are referenced, in Japan, as the 1960 CB72, however all the serial numbers indicate the bikes as 1961 models.
The stock 250cc CB72, which really put Honda on the map globally, had a redline of just over 9,000rpms. There were no other 250cc street bikes which were capable of 90+mph in stock form, especially with electric starting and oil-tight engine assemblies. Honda’s sales soared as this new technology and engine design was shown to be reliable and great fun to ride. The CB72-77s were sold from 1961 to 1967. The CB72 engine, shorn of its electric starter system, showed up in the CL72 250cc Scramblers, which evolved into the famous CL77 305cc Scramblers, made from 1965 through 1967.
1960s Roadracing 250cc models
During the 1960s, in the middle of all the mass-production of street-legal motorcycles, Honda somehow found time to develop GP/Roadracing machines of a high-quality, which exhibited outstanding power characteristics. To contest the 1959 All-Japan racing series in the 250cc class, Honda developed the RC160 250cc four, which developed 35 horsepower at 14,000rpms. This bike carried a leading link front suspension, which carried over from the early Dream/Benly and RC70 models, being built and developed at that time. Production racers, available to select racers were DOHC 4-valve, 5 speed CR72s and a CR77 305cc variant.
In 1959, Honda went to the Isle of Man with a batch of RC141/142 bevel-drive, 125cc twins and learned what it took from other manufacturers to make race-winning motorcycles. They returned in 1960 with their first roadracing 250cc four, called the RC161; a 38 horsepower, 6-speed racer which earned one podium finish in its first year of production and racing. In the following years, RC162, RC163 and RC164 models were developed, each with more power and speed and other refinements which took over many race podiums with riders like Mike Hailwood and Jim Redman. It would be another twenty years before Honda built more 250cc fours; the wild and wicked 1986-96 CBR250RR four street bikes, with upwards of 45 horsepower and 19,000rpm redlines. Like the RC race bikes of the 1960s, the CBR250RR engines had gear-driven camshaft drives to ensure accurate valve timing at these astronomical engine speeds.
By the mid-1960s, Honda’s efforts in roadracing were often matched or exceeded by new, lighter and faster machines from Suzuki, Yamaha, MV and other European brands. Honda’s response was a bombshell… The RC166, a DOHC, 4-valve six-cylinder racer with a 7-speed gearbox and an 18,000rpm redline. Honda won 10 out of 10 races in 1966 with this magnificent piece of engineering and design.
New street models 1968-on
New generation 250cc twins appeared in 1968, in Japan and European markets, first as the CB250K0, the base model for the hugely successful CB350/CL350 and SL350 series. These new bikes had the long-awaited 5-speed transmission installed along with more horsepower and sturdy overall construction than their predecessors.
No new 250cc Honda models appeared until the revolutionary XL250K0 Motorsport models, released in 1972. These all-new design singles, featured a SOHC 4-valve cylinder head, 5-speed transmission, light tubular frame with large alloy rims for on-off road riding. Lighting could be quickly disconnected for serious off-road adventuring, reducing overall weight even more. The engine design shared engineering features with the larger XL350, which arrived in the following year, 1974. The XL250-350s were renowned for reliability and off-road prowess, especially when suitable modifications were developed and race-tested in events like the Baja 1000 and other off-road venues. A rare variation was the 1974-75 TL250 Trails bike, specifically engineered for low-speed plonking around on Trials courses. Based upon the XL250K0-2 series engines, many parts were model-specific like the large rotor-flywheel, small carburetor, special cylinder head, camshaft, etc.
In 1978, a more refined XL250S engine became available, becoming the XL250R, XR250R and the basis for the XR350 thumpers. Variations in bore/stroke dimensions yield the established engine displacements and electric starters have been included in most current models. In the years 1988-90, Honda created the NX-series of dual-sport machines and the NX250 was one of those refinements. The NX250 is a liquid-cooled, 249 cc, single-cylinder, four-valve, DOHC, four-stroke engine with electric starting. It has a bore and stroke of 70.0 mm × 64.8 mm (2.76 in × 2.55 in), 11 to 1 compression ratio, and a six-speed transmission. The XR250L series, built in the mid 1980s, had a RVFC radial valve cylinder head (all four valves canted into each other) with dual carburetion, one primary and a secondary carburetor throat feeding the twin inlet, twin exhaust cylinder head design. The dual carburetor models made good power, however were complex and difficult to service in the field, so were dropped after 1987.
In the US, new CB/CL/CJ360 twins, based upon the CB250T engine design, were introduced in 1974, hot on the heels of the wildly successful CB250-350K-series bikes. Only the 360cc machines were seen on US shores, but the 250s were sold in Japan, Asian countries and in Europe. These bikes were the first equipped with a 6-speed transmission for Honda street machines.
By 1978, the little twins evolved once again, as the radically new CB250-CB400T models, with 3 valve heads, CDI ignition systems, 6 speed transmissions, with high-pressure oiling systems which fed automotive-style plain bearing crankshafts. A chain-driven counter-balancer system quelled the vibrations from the 360 degree firing crankshaft powerplants, yielding a smooth and powerful street machine. Again, the US market only saw the 400-450cc engines in various models like the CB400A (automatic), CM400, CB400T, CM400C and eventually some 450cc versions, late in production. The 250s were known as CB250N Super Dreams in Japan, the UK and elsewhere.
In 1977, Honda created a new line of engine designs, which had vertically split crankcases, Morse Hy-Vo link-belt camchains and a very robust crankshaft bearing support system. From the initial CB125T starting point, the engine grew to 185cc, then 200cc; finally bored and stroked out to 250cc, which became the famous CMX250 Rebel/Nighthawk 250 street bike series, which has endured from 1985 to this very day.
In 1980, starting with the basic Honda XL250S engine, a simple SOHC 4-valve single cylinder, dual-port exhaust head engine with counter-balancers, a CDI ignition and simple slide-type carburetor, the engineers at Honda transformed the XL250S-based bike into a lovely, low-priced, CB250RS-A sports machine. At 275 lbs dry and sporting an upgraded 32mm carburetor, camshaft and high-compression piston, the little bike became a champion for UK couriers and a solid transportation device for many which yielded 60+mpg and a 90mph top speed. The first generation bikes were kick-start only, but the later “C” models were equipped with an electric starter system, similar to that of the FT500 singles, sold in the US. The CB250RS engine had a 9.2:1 compression engine with 74x57.8 bore/stroke.
The only other 250cc street bikes to come to the US in the 1980s, were the VT250R “Miniceptor” V-twins, which were made from one half of a VF500F V-4 Interceptor engine. Popular in Japan and elsewhere, starting in the early 1980s, the little 250s had a tough time gaining sales traction in America. Sold here in 1988-90, the first two year editions had an “inboard” front disc brake, which was difficult to service and not highly effective. The 1990 models had a conventional front brake, but sales had been lagging all along, so Honda pulled the plug that year. The bikes continued production until 2009, under various model names in other countries. These tiny twin engines had a 13,500 rpm redline and were capable of nearly 100mph, in US trim.
Now, in the 21st Century, the 250cc options for today’s riders are the new CBR250R sports machine and its cousin the CRF250L, a dual-sport design. These bikes’ engines feature modern 4-valve, fuel injected, liquid-cooling, with 6 speed transmissions and electronic ignition/engine management.
Honda’s 250cc engines have spanned almost 60 years in all configurations, applications and model designs. From the 1955 SA to the 2014 CRF250L dual sport models, Honda’s care and quality engineering have always shined a bright light on this quarter-liter engine class.
Bill “MrHonda” Silver