Harley-Davidson SX125: 2-Stroke Dual-Sport Legend

I still remember the first time I kicked one over. It was a drizzly Saturday in 1994, behind my uncle's barn in rural Ohio, and the thing looked like a half-eaten banana someone had forgotten in a hedge. Kick, sputter, silence. Second kick, a pop like a wet firecracker, then a cloud of blue smoke that smelled suspiciously of lawn-mower oil. Third kick—brap-brap-BRAAAP—and suddenly I was twelve years old again, convinced I'd just discovered the secret to flight. That was my introduction to the Harley-Davidson SX125.

Background

Nobody ever talks about it in the same breath as Knuckleheads or Fat Boys, and that's probably fair. The SX125 is the family secret Milwaukee parked in the corner and coughed about in marketing meetings. Between 1975 and 1978 Harley's Italian outpost—still called Aermacchi on the pay slips—built roughly 4,800 of these yellow two-stroke singles, painted the tank black, slapped a bar-and-shield decal on each side, and shipped them to every dealer who'd once muttered, "We need something for the kids."

Frame Details

The frame is nothing fancy: mild-steel cradle so narrow you could hide it behind a cereal box. The backbone doubles as the oil tank, which sounds clever until you realize the filler cap is directly under the seat hinge. Drop the seat too hard and you've just invented an instant oil shower for the next rider. Rear shocks are Girling units that feel like repurposed pogo sticks—three preload clicks, all of them optimistic. Up front, 31 mm Ceriani forks slide through triple clamps that look suspiciously like bicycle parts. Total dry weight: 86 kilograms. That's less than my high-school backpack.

Engine Characteristics

Engine first, because it's the only part that doesn't apologize. 124 cc, air-cooled, piston-port two-stroke, fed through a 28 mm Dell'Orto that loves to drip when you forget to close the petcock. Factory claimed 14 horsepower at 8,500 rpm. On a cool morning with fresh plug and 32:1 premix, it feels closer to twenty. The expansion chamber is a hand-welded steel pretzel that snakes around the right side and ends under the swing-arm, spraying a fine mist of castor on the rear tire—free traction compound, or so I told myself every time the back end stepped out.

Transmission

Four-speed gearbox, right-foot shift (because Aermacchi never got the memo about the other foot), ratios spaced like teenage growth spurts: first is stump-pull short, second and third are almost identical, fourth is a polite suggestion. Neutral lives somewhere between second and third, but only if the bike is hot, the rider is patient, and Mercury is in retrograde.

Cycle Components

Cycle parts? Charming until you need them. Front brake is a 160 mm single-leading-shoe drum—pull the lever and count to three before anything happens. Rear is identical but smaller, which means it locks instantly and then laughs at you. Wheels are 19-inch front, 16-inch rear, both wrapped in knobbies that look aggressive until you hit mud, at which point they behave like slicks on butter.

Riding Experience

Ride the thing and you spend the first five minutes wondering why the bars are so wide—then you hit a fire road and realize they're perfect for counter-steering your way out of trouble you definitely created yourself. Power arrives in one lazy, two-stroke swoop. No hit, no flat spot, just a gradual gathering of noise and momentum that feels faster than it actually is. Forty-five mph on the SX125 is pure cinema: engine screaming, suspension pogoing, every rock in the trail trying to knock your fillings loose.

Personal Anecdote

I once tried to jump ours over a drainage ditch. I cleared the ditch. I did not clear the landing. The Girling shocks bottomed so hard the seat smacked me in the tailbone, and for a moment I was weightless again—just not in the good way. The bike shrugged it off. I still have the scar.

Pros & Cons

Pros? Cheap to buy, cheaper to crash. Parts swap straight across half the Italian two-stroke catalogue: same piston as an early Husky 125, same crank seals as a Cagiva 175. Carb kits still show up on eBay for less than a pizza. And when it's running right, the smell—half burnt premix, half hot gearbox oil—is better than any candle my wife ever bought.

Cons? You'll become intimate with the phrase "two-stroke jet." The Dell'Orto has six jets, none marked in English, and the slide needle has more steps than a courthouse. Also, the side-stand is a piece of flat bar that bends if you breathe on it. Mine snapped on the third ride; after that I leaned the bike against anything vertical—trees, barn doors, the occasional cow.

Fuel Range

Fuel range is optimistic. The steel tank holds six litres if you rock the bike and ignore the slosh. At full tilt that's maybe 45 minutes of single-track, followed by a hike back to the truck with a two-stroke strapped to your shoulder.

Conclusion

Still, the SX125 has a kind of stubborn charm. It never tried to be a KTM or a Maico; it just wanted to be small, loud, and Italian enough to make Midwestern parents nervous. Every time I kick ours now—yes, it still lives in the same barn—it fires on the second try, barks once, and settles into a lumpy idle that sounds like it's clearing its throat.

Harley never made another real off-road two-stroke after 1978. The SX125 disappeared into swap meets and muddy memories, replaced by XR750s on oval tracks and Softails on boulevards. But if you listen on a quiet evening, somewhere in the backwoods of Ohio or the scrub outside Sacramento, you might still hear that little yellow bike rattling through the gears, refusing to admit the party ended decades ago.

Harley-Davidson SX125 – the bare-bones cheat-sheet

(no brochure polish, just the raw numbers)

1. Core Specifications (1974-1976)

Item	Figure
Engine	123 cc air-cooled 2-stroke single, piston-port
Bore × Stroke	56 mm × 50 mm
Compression	10.8 : 1
Rear-wheel power	13 hp @ 7 000 rpm
Rear-wheel torque	~14 N·m (10.3 lb-ft) @ 6 500 rpm
Transmission	5-speed, right-foot shift, chain final-drive
Carburetion	Dell'Orto VHB 27 AD
Ignition	Flywheel magneto, points
Frame	Aermacchi mild-steel cradle
Front suspension	30 mm Ceriani telescopic forks
Rear suspension	Twin Girling shocks, 3-way preload
Front brake	160 mm single-leading-shoe drum
Rear brake	160 mm single-leading-shoe drum
Wheelbase	1 359 mm (53.5 in)
Dry weight	112 kg / 247 lb
Fuel capacity	10.5 L / 2.77 US gal
Top speed (stock gearing)	105 km/h / 65 mph

2. Achievements & Key Years

Year	Event / Rider	Class / Result
1974	AMA District 36 Enduro	Privateer SX125 1st place (R. Linn)
1975	AMA 125 GP support race	SX125 podium – 2nd place (J. Hensley)
1976	AHRMA Vintage Revival	SX125 exhibition win (A. Cortez)

Total production 1974-1976: ~4 800 units.

3. Performance Snapshot Table

(dyno pull, 1975 stock engine, 32:1 premix, 25 °C)

RPM	Rear-Wheel HP	Torque (N·m)
4 000	6.5 hp	11.5 N·m
5 500	9.8 hp	12.8 N·m
7 000 (peak)	13.0 hp	13.5 N·m
8 000	11.7 hp	10.9 N·m

4. Quick-Read Charts

Power Curve (graphical)

RPM  →  4k  5k  6k  7k  8k
HP   →   6  10  12  13  11

Weight-to-Power Comparison (1975 125 cc class)

Bike	Dry kg	Peak hp	kg/hp
Harley SX125	112	13.0	8.6
Yamaha DT125	110	12.5	8.8
Kawasaki KE125	115	11.8	9.7

Sources: factory service bulletins, Cycle World dyno test May 1975, AMA District 36 results.