Motocross

KTM 125 SX

Motocross, 125cc 2T, 2027, US version

Verified

Illustration: a KTM motocross bike, not this exact model.

Gearing

4 of 5 filled

Stock front sprocket: 13T; Owner's manual p. 146, checked 2026-09-27 (source 1)
Stock rear sprocket: 50T; Owner's manual p. 146, checked 2026-09-27 (source 1)
Chain pitch: 520; Owner's manual p. 146, checked 2026-09-27 (source 1)
Stock chain links: Not yet verified
Stock final drive ratio: 3.85 : 1; Calculated from the stock sprockets

Wheels and tires

2 of 4 filled

Front rim: Not yet verified
Rear rim: Not yet verified
Front tire: 80/100-21 TT; Owner's manual p. 146, checked 2026-09-27 (source 1)
Rear tire: 100/90-19 TT; Owner's manual p. 146, checked 2026-09-27 (source 1)

Suspension

6 of 7 filled

Fork: WP XACT AER; Owner's manual p. 145, checked 2026-09-27 (source 1)
Fork travel: 310 mm (12.2 in); Owner's manual p. 145, checked 2026-09-27 (source 1)
Shock travel: 300 mm (11.8 in); Owner's manual p. 145, checked 2026-09-27 (source 1)
Stock fork spring rate: Not yet verified
Stock shock spring rate: 39 N/mm; Owner's manual p. 149, checked 2026-09-27 (source 1)
Static sag, bike only: 35 mm (1.4 in); Owner's manual p. 149, checked 2026-09-27 (source 1)
Race sag, rider on: 105 mm (4.1 in); Owner's manual p. 149, checked 2026-09-27 (source 1)

Fluids and capacities

3 of 5 filled

Engine oil spec: Not yet verified
Engine oil capacity: Not yet verified
Coolant capacity: 1200 ml (40.6 fl oz); Owner's manual p. 145, checked 2026-09-27 (source 1)
Premix ratio: 40:1; Owner's manual p. 146, checked 2026-09-27 (source 1)
Fuel capacity: 7.2 L (1.90 gal); Owner's manual p. 146, checked 2026-09-27 (source 1)

Maintenance

3 of 3 filled

Tire pressure (off-road): 1.0 bar (14.5 psi) front and rear; Owner's manual p. 145, checked 2026-09-27 (source 1)
Chain slack: 58-61 mm (2.28-2.40 in); Owner's manual p. 83, checked 2026-09-27 (source 1)
Spark plug and gap: NGK BR10 ECMVX, gap 0.80 mm; Owner's manual p. 144, checked 2026-09-27 (source 1)

Dimensions

3 of 3 filled

Seat height: 958 mm (37.7 in); Owner's manual p. 146, checked 2026-09-27 (source 1)
Wheelbase: 1493 mm (58.8 in); Owner's manual p. 146, checked 2026-09-27 (source 1)
Weight: 92 kg (203 lb); Owner's manual p. 146, checked 2026-09-27 (source 1)

Solid marks are owner's-manual pages; hollow marks are manufacturer web or spec-sheet figures still waiting on a manual page. Last reviewed 2026-09-27. Rows marked not yet verified are blank on purpose.

What's missing

20 of 24 fields read off a manual page.

Not yet verified (4): chain links, front rim, rear rim, fork spring rate.

See coverage for every bike

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Tools for this bike

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Buying parts

The stock sizes and specs this bike left the factory with, each with its source. Take them to any shop. Retailer links will appear here once a store approves MotoXSports.

Chain: Chain pitch 520 Owner's manual p. 146
Sprockets: Stock front sprocket 13T Owner's manual p. 146Stock rear sprocket 50T Owner's manual p. 146
Tires: Front tire 80/100-21 TT Owner's manual p. 146Rear tire 100/90-19 TT Owner's manual p. 146
Coolant: Coolant capacity 1200 ml (40.6 fl oz) Owner's manual p. 145
Suspension springs: Stock shock spring rate 39 N/mm Owner's manual p. 149

4 part specs not yet verified; see data coverage.

Garage tool 01

Sprocket calculator

Put your current gearing in Setup A and the setup you are considering in Setup B. Everything updates as you type.

Start from a bike's stock gearing

Pick a bike to fill both setups with its stock sprockets.

Front sprockets 10–16 teeth, rear 34–60. Values outside that range are brought back to the nearest limit.

Setup A — what you run now

Front teeth

Rear teeth

Setup B — what you are thinking about

Front teeth

Rear teeth

Final drive ratio

— — : 1

—

Pull at the wheel—

Top speed—

Chain (est.)—

A —

B —

Taller · more speed Shorter · more pull

—

Ratios are exact. The chain figure is an estimate of roughly one link per two teeth of change — confirm against your manual before ordering.

Sources

1
KTM Owner's Manual 2027 125 SX / 125 XC, Item no. 3240379en (07/07/2026); owner-supplied copy from the KTM Group print-on-demand portalpp. 83, 144, 145, 146, 149Manual page, verified. Covers stock front sprocket, stock rear sprocket, chain pitch, front tire, rear tire, fork, fork travel, shock travel, stock shock spring rate, static sag, bike only, race sag, rider on, coolant capacity, fuel capacity, premix ratio, tire pressure (off-road), chain slack, spark plug and gap, seat height, wheelbase, weight; checked 2026-09-27.

Always confirm against the manual for your exact year before ordering parts or setting up suspension. Torque specs are deliberately not published here. How we verify a value.

My settings

Date / track
Sprockets run
Race sag / free sag
Fork air pressure or spring
Fork clickers (compression / rebound)
Shock clickers (low / high / rebound)
Tire pressures (front / rear)
Notes

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