These days, four-stroke motocross machines are phasing out two-strokes, which have dominated the sport since the 1960s.
But there was a time, long ago, when two-strokes did the same to four-strokes. And the last of the old guard they replaced were thumpers like this one.
Back in the early '60s, off-road bikes didn't get much more exclusive than this hand-built Manx Norton 500cc off-roader, powered by a road-racing engine. The odd combination was the brainchild of British racer Les Archer, framebuilder Ron Hankins and engine tuner Ray Petty.
Engine: Double-overhead cam, 500cc single
Compression Ratio: 11:1
Ignition: Twin plug
Weight: 285 pounds
Owner: Fred Mork
The collaboration had already proven successful, with Archer using a Hankins-built Norton to win the 1956 European Motocross Championship, which became the World Motocross Championship a year later. But by 1962, Archer was ready to experiment with something new.
Instead of the tried-and-true long-stroke Norton engine he had used for years, Archer bolted a more-powerful double-overhead-cam, short-stroke road-racing engine, prepared by Petty, into Hankins' custom frame.
Despite its road-racing origins, the bike made tractable power, says current owner Fred Mork. Plus, it was a screamer.
"During that time, there was probably nothing that made more power than that bike," Mork says. "But it was built at the very end of a golden era, when CZ, Greeves and Husqvarna were about to take over."
And take over they did. By the second half of the '60s, even exotic four-strokes like this one, with its aluminum tank and titanium axles, couldn't compete with the explosive power of lighter two-stroke bikes.
Instead, the four-strokes were sidelined—and butchered. When Mork bought this bike, many parts had been replaced, and the beautiful aluminum tank had been painted red, which made for a challenging restoration job.
"It probably had a couple of pounds of Bondo on it—and I mean literally a couple of pounds," Mork says.