(FORTUNE Magazine) – Nestled in the lush pastureland outside tiny Viola, Wis., is an industrial building where 160 people work three shifts a day, using precision machine tools to crank out big, fire-breathing motorcycle engines. At first glance--and second and third--these aluminum and chrome V-twin powerplants look for the world like the distinctive motors that power Harley-Davidsons.

But no. The building belongs to S&S Cycle, a company that over four decades has carved out a profitable niche selling the hot-rod parts that make Harleys go faster. S&S is now the biggest supplier of complete engines and major motor parts to more than 15 companies that build several thousand Harley-like cruiser bikes each year. These clonemakers charge as much as $30,000 for their precustomized creations, which have names like American Eagle, Titan, and Big Dog.

S&S has built a name for itself on its ability to improve on Harley's handiwork. Clone bikes are sort of like the Model-A Ford cars that hobbyists assemble from mail-order parts, all of which are new and none of which are made by Ford. Some of the clonemakers' customers are would-be Harley buyers frustrated by long waiting lists at the dealer; others are horsepower junkies who want their bikes to go like stink from day one. Available in displacement sizes ranging up to 113 cubic inches--which makes Harley's 80-inch Big Twin motor seem rather effete--S&S's engines, which can cost $5,000 to $6,000, are the choice of true speed fiends.

The company began one-upping Harley-Davidson in 1958, when founder George Smith started fiddling with Harley motors, applying a simple formula: More cubic inches equals more speed. His creations distinguished themselves at drag strips and in speed-record runs at Utah's fabled Bonneville Salt Flats, and so Smith went into business selling them. This racing heritage makes S&S parts acceptable to even the most hard-boiled Harley traditionalists--and given the vast number of Harleys rumbling around the country and their owners' tinkersome nature, the hop-up after-market is a considerable one. (The privately held S&S doesn't discuss sales figures.)

S&S stays abreast of its evolving market by ordering a new Harley bike every year and taking apart the engine to see what it can improve. Curiosity is huge right now about what's going on inside Harley's brand-new Twin Cam 88 motor, says S&S's president, Sam Scaletta. The new engine is bigger, more powerful, and more rugged than its predecessors. And Harley has caught on to the enthusiasm for hopping up its bikes: It has its own line of Twin Cam 88 speed parts.

With Harley improving its own wares, can S&S still play a role? "We will try to do better than whatever they're doing," vows Scaletta. S&S is also looking to a possible new market: developing go-go motor stuff for the new cruiser bikes now being built by Victory Motorcycles and promised soon from Excelsior-Henderson, both in Minnesota. Scaletta thinks he can continue to beat the big manufacturers at their own game. "We're not very big on red tape," he says, "and we can move pretty quickly." Same goes for S&S's customers.

--Stuart F. Brown