In his workshop in Chehalis, Washington, Gary Klein makes a bicycle -- out of specially-treated aircraft aluminum alloy and an elegant batch of extras -- that costs a cool $6,000. "That may not be overpriced -- his workmanship is gorgeous -- but it's just too expensive," scoffs Tom Kellogg, who builds bicycle frames in Wescoville, Pa.

Kellogg's bargain-basement model -- wrought of lightweight alloy, joined by silver solder and fit precisely to the customer's body -- goes for a mere $725. That's just for the frame: he never bothers with the rest of the machine.

The casual rider would be hard-pressed to tell the difference between a Klein and a Kellogg, though he'd find them somewhat nicer than his reliable $139.95 Schwinn World 10-speed. Schwinn also makes the Paramount, a custom- built number priced at $2,500.

The quest for the Ultimate Bicycle, that gleam in the middle distance, is fraught with cabalistic mysticism and Diamond-Jim-Brady frugality: the legendary dandy, so the story goes, having presented a favorite mistress with a diamond-studded bike. There are also conundrums involved, and not a little vanity.

"My daughter tells me, 'Harold, you're trying to find God', and in a sense, that's true," said Harold Wooster of Bethesda, a senior scientist at the National Library of Medicine, who's been searching for the Ultimate Bicycle since he started riding, at age 10, half a century ago. "It's a very limited section of the universe that I can control. I don't have to go to my editorial review committee or to the board of regents to get permission to change my bike. If I want to change things, I change them."

Wooster, who owns six cycles, ranging from a "minimalist bike" built from castoff parts to a pricey Puch Austro- Daimler, tinkers endlessly in his basement. "One of the more delightful things I do is build my own wheels; the tension-spoked wheel is the height of mechanical art." He's also technical editor of American Wheelmen magazine, the hobby's official journal, and often lectures on the subject.

"When I lecture, I like to draw a cost-effectiveness curve. It's sort of an upside-down hyperbola -- the same curve that the Department of Defense should know about," he said. "It rises steeply at first, levels off in almost a right- angle turn, and stays almost horizontal out to infinity."

The Wooster Curve starts to level off at about $500 -- at which price, he says, "Mr. Raleigh or Mr. Fuji or Motobecane or any number of others will sell you a bike that you'd have no reason to be ashamed of." Beyond comes the airy region of Ultimate.

Folks aspiring to such heights, whether in a racing or touring bike (the geometries differ a bit), must first consider the frame. It should be lightweight, strong and, if for racing, stiff -- which means, in biking lingo, that there's little flex in the joints, permitting the most efficient energy transfer from human being to machine.

Gary Klein, a graduate of the Massachusetts Institute of Technology, uses heat-treated, boron-fiber-reinforced aluminum tubing to make the stiffest frame around. Some people argue that it's too stiff -- "dead" or not "resilient" enough -- but that's a controversy too arcane to explore.

More than likely, the Ultimate frame will be constructed (by hand, of course) from alloy tubing, chrome molybdenum or some other such wonder, which is double-butted -- that is, thicker at the ends than in the middle -- for added strength at the joints.

Tom Kellogg uses a mix of Columbus tubing from Italy and Reynolds from England, as well as Ishiwata and Tange from Japan. Proteus Design, a framebuilder in College Park, mainly uses Reynolds; Georgetown Cycle Sport, which makes the Alpine frame, uses Reynolds or Columbus. In either case they start at around $500.

It goes without saying that frames this expensive should be mitered to perfection, sport investment-cast lugs at the joints -- Kellogg uses Henry James lugs, which go for $25 a set -- and brazed with silver solder. The finishing and painting should be artful indeed.

There's a lot more, of course, to a first-class frame -- the niceties of design, for instance, and how it fits your build -- but choosing the Ultimate components seems even a more Byzantine business. A bicycle has about a hundred different parts -- not counting bolts, screws and ball bearings, which may also deserve your attention. Such components as derailleurs, brakes, cranks, shift levers, rims, hubs, spokes . . . can be installed in near-infinite combinations.

The Italian firm Campagnolo, whose American marketing representative, Amos York, boasts that "our components are priced 15 to 20 percent higher than the competition's," sets the standard, at least in racing gear, by which all other makers are judged. Campagnolo's Super Record line -- which features most everything but rims and spokes, chains, handlebars, saddles and a few other accessories -- sports the company's classic design and famous precision, plus several parts (like pedal spindles) in lightweight titanium.

Shimano and Maeda-SunTour, two Japanese firms, supply comparable equipment, while Shimano offers aerodynamic components in its Dura-Ace AX line. The October issue of Bicycling magazine devotes 18 pages to comparing Campagnolo, Shimano and SunTour parts. There are, of course, many other respected brands, including Cinelli, especially for handlebars, and Clement for tires.

The Ultimate Bicycle could cost you more than $2,000. The only other thing you'd need would be the Ultimate Lock. CAPTION: Pictures 1 through 4, THE ULTIMATE BICYCLE? -- The racy little Italian number pictured here, the Bianchi Super Corsa, boasts a handmade frame of lightweight alloy Columbus SL tubing and a blend of top-of-the-line components: mostly Campagnolo Super Record, the priciest -- many say the best -- that money can buy. Also featured are Ambrosio Montreal Crono Rims ($110 a pair) and Clement Criterium silk tubular tires ($30 each). At Metropolis Bike & Scooter Inc., which supplied the bike for these photos, the whold can be had for about $2,400. At 18 1/2 pounds, that comes to about $130 a pound. Close-ups show: 1. Campagnolo SR rear derailleur ($95), OMAS titanium sealed-bearing hub ($30 a pair), Regina Futura freewheel system ($130), Regina Everest chain ($25) and DT Swiss Stainless spokes ($10 a set); 2. Campagnolo SR pedals and crankset -- the crankset spiders cut out for weight reduction -- plus Christophe toeclips, featuring lightweight titanium spindles in pedals and bottom bracket, and crank arms pantographed with the Bianchi logo ($345); 3. Campagnolo SR steel alloy brakes ($145). Photos by James Thresher