Yamaha initiated the movement with the TT500 in 1976.  For two years, the big Yamaha remained basically unchanged, okay from a marketing standpoint because it was the only 500cc four-stroke dirt bike widely available. If you've got the only art around, then you're the sole processor of the state of it. With the exception of the aftermarket engine and chassis people and a whole bunch of garage engineers, Yamaha's competition was scarce.

Not for long, though. In 1979 Honda filled out the four-stroke market with the introduction of its XR series. By constantly refining the XR500 and incorporating the Pro-Link rear suspension system in 1981, Honda took lead of the class. Honda's refinement did something else: it made a "playbike" something a racer would consider owning. The XRs handled well enough to allow near-competitive speeds. In the meantime, Yamaha continued to improve the TT500, and Suzuki bumped its four-stroke single from a 370 to a 400 to a full 500 in 1981. Even so, Honda continued to offer the best off the-shelf thumper.

Enter 1982 and our friends from Canada—Can-Am. Can-Am's long-awaited thumper appeared as a 1982 model in the form of an off-road playbike / enduro racer. The Sonic was built along the guidelines thumper freaks had set down for their one-off playbikes. Equipped with the latest in 500-single power (a Rotax fourvalver), the Can-Am had a stout chassis which provided good handling in almost all conditions, and it sported some of the best suspension components in the business—a Marzocchi fork and Ohlins shocks. Though the Can-Am didn't have much power over the XR500, it performed a little to a lot better in nearly every area and so became the new thumper king.

Which brings us to the latest four-stroke goodie on the market—the 1983 KTM 504 MXC Pro-Lever. Although the KTM is powered by a stroked version of the same basic Rotax engine the Can-Am uses, the MXC powerplant features first-rate engineering and some innovative designs which may extend its service life considerably. The single overhead camshaft, for instance, rides on roller bearings instead of the usual plain bearings. A toothed Gilmer belt drives the cam, taking its power from the left side of the crankshaft, and the composite belt is guarded in its own compartment. To maintain constant tension and prevent resonant flapping, two rollers tension and guide the belt along its path from crank to cam.

The old maxim says you get what you pay for. True enough: the KTM is the most expensive and best thumper you can buy.

The cam-follower/rocker-arms are other high-quality pieces. The intake and exhaust rockers are each forked (to open the four valves); the cam operates them via roller-bearing cam-followers (replacing the flat plain-surface shoe-type). To operate properly, both types require an oil shield between the working surfaces of the cam and follower; however, because the roller-type rolls rather than slides over the cam lobes, it generates much less friction than the shoe-type. Less friction results in less heat and, consequently, less wear. Screw-type adjusters regulate valve lash, and the good-sized rocker cover plates allow easy access to the adjuster screws.

The Rotax engine is stout from top to bottom. The crankcase is well ribbed, making it strong yet light. Although the cases are compact (an advantage allowed by the dry-sump lubrication system), don't assume that it's a short engine—the powerplant measures 18 inches from drain plug to cam cover, average for a 500cc four-stroke single.

The 504's big end differs from the Can-Am Sonic's by featuring its own connecting rod and crankshaft, both necessary changes to bump the stroke 1.6mm over the 494cc Can-Am's.

In the rest of its particulars, the engine is straightforward. The nearly flat piston has a wrist pin which rides on a plain-type bronze bushing. The big end relies on needle bearings, while caged ball bearings support the crankshaft. Straight-cut gears run the primary gears and counterbalancer, and the five-speed transmission delivers power through an eight-plate wet clutch. Since the dry-sump system precludes splash lubrication, a pressurized spray bar lubricates the transmission.

The five ratios are fairly widely spaced, which you'd expect with a big-bore four-stroke single, but the overall gearing is pretty short. Although short gearing can be a bonus for bopping through a tight woods section, on fast fireroads you might wish for taller overall gearing.

The KTM/Rotax powerplant breathes through a 36mm round-slide Bing carburetor. It mounts on a rubber-elbow type manifold, needed to angle the carburetor and airbox away from the centrally located shock absorber. The air filter, a K&N wire-mesh/oiled-gauze element, resides in a small, maze-like airbox. An intricate array of channels prevents water from reaching the filter, but the airbox cover may make the precautionary channels useless. Two small rubber bands hold the cover in place, and one of them disappeared before we had used the first two gallons of gas.

The twin head-pipe exhaust system winds its way from the right side of the motorcycle to the left between the cylinder and the rear shock, then exits into a Super Trapp silencer/spark arrester. The head pipe skirts the shock reservoir, which straps to the frame on the right side. The reservoir heated up even while we cruised on a smooth road, which is why most manufacturers position shock reservoirs as far as possible from engine heat.

The 504 MXC chassis follows modern, up-to-date design guidelines. The chrome-moly frame produces a 58.6-inch wheelbase, almost identical to the Can-Am Sonic's and 2.7 inches longer than Honda's XR500. Of these three big-bore singles, the KTM has the steepest fork angle (27.5 degrees). The '82 XR has 28.0 degrees and the Sonic a stretched-out 29.4 degrees. Trail figures for the three are within 0.4 inch.

The frame's front downtube and backbone sections double as the oil reservoir for the dry-sump engine. The single front downtube splits at the top of the engine case where it forms a full-cradle back to the swing-arm pivot. At this point, the engine cases act as a stressed member; the swing-arm pivot bolt runs through the rear of the engine. The rear subframe bolts on at two points: behind the swing-arm pivot and where the backbone meets the lower frame section at the tank-seat junction. This setup gives two benefits. Removing the rear frame section exposes the shock completely; without the breakaway section, access to the shock would be very restricted. Second, if you crunch the rear subframe, you can buy only the part you need.

We can find little fault with the KTM's handling. It tracks along any line the rider chooses, regardless of terrain, and still retains excellent steering characteristics. Very often, Something Stable becomes Something Unable when the going gets tight and you're still trying to maintain a high-speed average.

KTM's Pro-Lever rear-suspension system resembles Honda's Pro-Link setup. The top of the shock bolts to the frame beneath the rear of the fuel tank. The bottom of the shock mounts on a steel link which bolts to the frame at its front, and to the swing arm via two alloy links (one left, one right). This arrangement allows for a floating lower shock mount, enabling the shock shaft to accelerate at a ratio other than one-to-one with the swing arm. Both the swing arm and the bottom steel link pivot on caged needle bearings; the two alloy links pivot on bushings. Field maintenance is limited to two external zerk fittings—one on each of the two alloy links.

The Pro-Lever shock, a remote reservoir Fox Shox unit, has independently adjustable compression and rebound damping. Rotating a knob at the base of the shock selects rebound adjustments. Compression damping adjusts via a small knob on the top of the frame-mounted shock reservoir. A damage-resistant braided stainless steel line connects the reservoir to the shock body, and a threaded collar sets spring preload.

Tuning the widely adjustable shock makes the KTM plush for leisurely trail rides or firm for high-speed trips across desert whoops. You can even tune it for light duty as a playtime half-miler. In fact, the 504 is happy playing in the left-turn-only mode. The front end sticks well when you jam the bike into a hard left at high speed—something almost unheard of with a knobby-equipped, long-travel-suspension motorcycle. Just snap it into the turn and bring in some throttle.

KTM uses Marzocchi forks on all its big-bore bikes. The 42mm tubes and four-bolt triple clamps form a rigid fork assembly. The triple clamps are supported by tapered caged roller bearings in the steering head. The leading-axle fork offers air caps on each fork leg for adjusting front-end air pressure. In especially rough terrain, the fork works best with four psi in each leg. With zero pressure, the fork bottoms over medium jumps. The rear shock also requires additional compression damping in these conditions.

All bodywork, fuel tank included, is made of durable plastic. The tank holds 3.4 gallons. Both right and left sidecovers shroud a large area, helping improve airbox waterproofing. Additionally, they prevent the bike from collecting as much mud as it might when conditions get really messy. Both front and rear fenders protect against tire-deflected objects, and they proved to be durable (having survived our typical crash program).

With the exception of the front brake assembly, all hand controls are the top-quality European Standard—Magura. The handlebar, made from chrome-moly tubing, is finished in non-glare matte-black. Although the bend is good, the bar is excessively wide. If you plan to ride the 504 in tight woods, and hope to have fingers that work at the end of the day, trim at least an inch off each end of the bar.

Front disc brakes are rare on off-road bikes. Though the total disc-brake system (disc, caliper, brake line and master cylinder) weighs more than a drum brake setup, the extra few pounds are worth the gain in brake feel, response and consistency in poor weather conditions. The KTM's Brembo system does indeed provide excellent feel and power at all times. However, the KTM's disc setup has one possible drawback—the aluminum master cylinder/plastic reservoir stands a fair-to-middlin' chance of being damaged in the first good right-side crash. We'd prefer to see an all-metal master cylinder/reservoir with a braided stainless steel hydraulic brake line, a la Kawasaki's.

Without a doubt, the 504 has one of the firmest seats ever found on a modern off-road motorcycle. If you want to train standing up, just hop on the KTM. If it's at all rough, you won't be sitting for long.

You won't be sitting on the 504 for long when you first start out for the day either. Starting the KTM is most easily accomplished when standing on the motorcycle's left and kicking the left-mounted lever with the right leg. Forget all those horror stories you've heard about starting big four-strokes. The KTM has a small window in the cam-drive cover which aids you in finding the point just past TDC. Merely pull the handlebar-mounted compression-release lever and kick the engine through until the yellow dot appears in the window. If the engine is cold, full choke, no throttle and roughly two kicks should have the MXC running. When warm, quarter choke, eighth throttle, one kick, and you're in business. The 504 halfheartedly kicked back exactly twice—with too little strength to do harm.

Throttle response from closed to quarter-open is excellent, but on our test unit response from half- or full-open throttle was poor. Whacking the throttle wide-open from small throttle openings always caused a great big cough followed by decent acceleration. It seemed as though someone hit the kill switch for an instant. Different pilot jets, main jets and needle positions all failed to cure the malady. Only opening the throttle smoothly brought clean response.

The MXC is happiest on fireroads and in wide-open riding areas. Its steep front-end geometry and long wheelbase work well together to make the 504 surprisingly agile and at the same time stable at high speeds. No handling peculiarities make the KTM unsuitable for closed-circuit motocross racing, but it does have handicaps—specifically too much weight and not enough power. With a maximum of 31 horsepower and weighing 299 pounds with one gallon of fuel, the KTM must surrender to the two-strokes. You can't spot the competition 70 pounds and 15 horsepower (at the peak) and expect to rumble into the winner's circle. It's a matter of physics.

If you're compelled to race and can find an all-four-stroke class, then stock for stock, the KTM is more than a match for any other four-stroke thumper you can buy.

At $3080, the KTM 504 MXC seems reserved for those with either a decent income or a rich uncle. Although considerably more expensive than the most popular of the lot, Honda's XR, the KTM is priced right in line with the other Rotax-powered thumper, the Can-Am Sonic. Price aside, the KTM is the most versatile and best overall performer among the current production four-stroke dirt bikes. Welcome to the new leader.■