Honda’s Throwback: 1978 Honda CX500

In the late 1970s, Honda decided to return to its roots and produce a reliable motorcycle aimed at the get-to-work crowd. Enter the 1978 Honda CX500.

| March/April 2015

1978 Honda CX500
Claimed power: 48hp @ 9,000rpm
Top speed: 111.9mph (period test)
Engine: 496cc liquid-cooled OHV 80-degree V-twin, 78mm x 52mm bore and stroke, 10:1 compression ratio
Weight (wet): 481lb (219kg)
Fuel capacity/MPG: 5gal (19ltr)/40-50mpg
Price then/now: $1,898 (1978)/$1,500-$5,000

Once upon a time, a motorcycle was transportation. Faster than a streetcar and more reliable (and cheaper) than a horse, motorcycles were bought by thousands of people who used them to get to school or work. Delivery services and police departments relied on motorcycles to get where they wanted to go quickly and economically.

Most motorcycle manufacturers got started building reliable, economical get-to-work bikes. Honda was no exception. Yet as the 1950s gave way to the 1960s, in developed countries like the U.S., England and Europe, motorcycles stopped being transportation, morphing into a Lifestyle Statement instead. Bikes sprouted exotic bodywork, multiple cylinders, overhead cams, high handlebars and fancy seats. Advertisements stopped mentioning miles per gallon and started crowing about quarter-mile times.

Yet in the late 1970s, Honda, then (and still) the largest motorcycle manufacturer in the world, decided to return to its roots and produce a reliable motorcycle aimed at the get-to-work crowd. Honda’s brief called for reliability and low maintenance, and up-to-date technology. The CX project was headed up by Shoichiro Irimajiri, who designed Honda’s 6-cylinder road racer in the mid-Sixties. After two prototypes were designed and built, one displacing 350cc and another 500cc, the decision was made to make the new model a 500. The CX was given liquid cooling, a 5-speed gearbox, constant velocity carburetors and shaft drive. Surprisingly, it was not given an overhead cam engine.

Design considerations

The decision to use an overhead valve design with pushrods on an otherwise very modern machine may seem a little strange, but was compelled by the longitudinally-mounted, 80-degree V-twin design, which had the crankshaft in line with the frame as on a Moto Guzzi. To accommodate this, the 4-valve heads were skewed 22 degrees to keep the carburetors from banging the rider’s knees. That skew would have made it very difficult to use overhead cams, so it was back to pushrods. The pushrods were short and hollow, and an exceptionally (for then) over-square bore and stroke of 78mm x 52mm let the engine rev to 10,000rpm. Mated with a free-flowing intake tract and large carburetors, the engine had good volumetric efficiency.

The crankcase and transmission housing were a single casting, with the cylinders cast in with the crankcase. Crankshaft, camshaft and gearbox removal was accomplished by removing individual end plates. The arrangement is reminiscent of very early motorcycle construction, and creates some repair complications. For instance, you have to remove the gearbox internals before removing the left connecting rod bolts, and you have to pull the engine out of the frame before removing the gearbox.

4/9/2015 4:08:42 PM

I was working at a Honda/Yamaha dealer when the CX500 'Plastic Pig' was launched. There were at least 3 cam chain tensioner updates. Even the final fully automatic tensioner only worked for about 18,000 miles if bike wasn't ridden properly. It was (is) an engine that not only likes a lot of RPM, it needs high rpm to survive. Crankcases without numbers are from the early model, the tensioner pivot bolt boss would snap off the case when people tried to ride them like a V-Twin double the capacity (slogging around in too high a gear at too low an rpm) If Harley wasn't such a we don't care what they do company, they would have known the load reversals destroy chains and tensioners. Twin cam should have been designed with gears from the ge tgo, I doubt one could rev high enough to prevent the load reversal that destroy the tensioner

Ride 'Em, Don't Hide 'Em Getaway

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