Motocross

KTM 125 SX 2027

Motocross, 125cc 2T, US version

Verified

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

From the owner’s manual: premix 40:1, p. 146; coolant 1.2 l, p. 145; front tire 80/100-21 TT, p. 146; rear tire 100/90-19 TT, p. 146. Checked 16 Oct 2026.

Model year on file: 2027 only. Other years are added as their manuals are read.

No CPSC recall found for the 2027 KTM 125 SX, checked 2026-10-15. Recalls

Spec card PDF

Gearing

4 of 5 filled

Stock front sprocket: 13T; Verified, read off the manual page: Manual p. 146 · checked 2026-09-27 source [1]
Stock rear sprocket: 50T; Verified, read off the manual page: Manual p. 146 · checked 2026-09-27 source [1]
Chain pitch: 520; Calculated, not printed: The trade name of the printed size 5/8 x 1/4" · Manual p. 146 · checked 2026-09-27 source [1]
Stock chain links: Not yet verified
Stock final drive ratio: 3.85 : 1; Calculated, not printed: Calculated from the stock sprockets, printed on manual p. 146

Wheels and tires

2 of 4 filled

Front rim: Not yet verified
Rear rim: Not yet verified
Front tire: 80/100-21 TT; Verified, read off the manual page: Manual p. 146 · checked 2026-09-27 source [1]
Rear tire: 100/90-19 TT; Verified, read off the manual page: Manual p. 146 · checked 2026-09-27 source [1]

Suspension

6 of 7 filled

Fork: WP XACT AER; Verified, read off the manual page: Manual p. 145 · checked 2026-09-27 source [1]
Fork travel: 310 mm (12.20 in); Verified, read off the manual page: Manual p. 145 · checked 2026-09-27 source [1]
Shock travel: 300 mm (11.81 in); Verified, read off the manual page: Manual p. 145 · checked 2026-09-27 source [1]
Stock fork spring rate: Not yet verified
Stock shock spring rate: 39 N/mm; Verified, read off the manual page: Manual p. 149 · checked 2026-09-27 source [1]
Static sag, bike only: 35 mm (1.38 in); Verified, read off the manual page: Manual p. 149 · checked 2026-09-27 source [1]
Race sag, rider on: 105 mm (4.13 in); Verified, read off the manual page: Manual p. 149 · checked 2026-09-27 source [1]

Fluids and capacities

3 of 3 filled

Coolant capacity: 1.2 l (≈ 40.6 fl oz, converted by MotoXSports); Verified, read off the manual page: Manual p. 145 · checked 2026-09-27 source [1]
Premix ratio: 40:1; Verified, read off the manual page: Manual p. 146 · checked 2026-09-27 source [1]
Fuel capacity: 7.2 l (≈ 1.90 gal, converted by MotoXSports); Verified, read off the manual page: Manual p. 146 · checked 2026-09-27 source [1]

Service specs

6 of 6 filled

Gearbox oil: Engine oil (15W/50), Partially synthetic: 0.80 l (0.211 liq. galUS); Verified, read off the manual page: Manual p. 145 · checked 2026-10-16 source [1]
Fork oil: Fork oil (48601166S1) (SAE 4): left outer assembly 240 ml (8.12 fl. ozUS); right outer assembly 240 ml (8.12 fl. ozUS); right cartridge 380 ml (12.85 fl. ozUS); Verified, read off the manual page: Manual p. 148 · checked 2026-10-16 source [1]
Brake fluid: DOT 5.1; Verified, read off the manual page: Manual p. 95 · checked 2026-10-16 source [1]
Coolant type: Coolant; antifreeze protection to at least: −25 °C (−13.0 °F); Verified, read off the manual page: Manual p. 145 · checked 2026-10-16 source [1]
Fuel octane: Super unleaded (98 octane); Verified, read off the manual page: Manual p. 146 · checked 2026-10-16 source [1]
Idle speed: 1,400 rpm … 1,500 rpm (23.33 Hz … 25.00 Hz); Verified, read off the manual page: Manual p. 144 · checked 2026-10-16 source [1]

Maintenance

3 of 3 filled

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

Dimensions

3 of 3 filled

Seat height: 958 mm (≈ 37.7 in, converted by MotoXSports); Verified, read off the manual page: Manual p. 146 · checked 2026-09-27 source [1]
Wheelbase: 1493 mm (≈ 58.8 in, converted by MotoXSports); Verified, read off the manual page: Manual p. 146 · checked 2026-09-27 source [1]
Weight (weight without fuel approx.): 92 kg (≈ 203 lb, converted by MotoXSports); Verified, read off the manual page: 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 2026-10-16. Rows marked not yet verified are blank on purpose.

Buying parts

The stock sizes and specs this bike left the factory with, each with its source. Take them to any shop.

Retailer links below are affiliate links: we may earn a commission, at no cost to you, and it never changes the specs. Affiliate disclosure.

Chain: Chain pitch 520 Owner's manual p. 146520 chains at MotoSport 520 master links at MotoSport
Sprockets: Stock front sprocket 13T Owner's manual p. 146Stock rear sprocket 50T Owner's manual p. 14613T front sprockets at MotoSport 50T rear sprockets at MotoSport
Tires: Front tire 80/100-21 TT Owner's manual p. 146Rear tire 100/90-19 TT Owner's manual p. 146Front tires 80/100-21 at MotoSport Tubes 80/100-21 at MotoSport Rear tires 100/90-19 at MotoSport Tubes 100/90-19 at MotoSport
Coolant: Coolant capacity 1200 ml (40.6 fl oz) Owner's manual p. 145Coolant at MotoSport
Suspension springs: Stock shock spring rate 39 N/mm Owner's manual p. 149Suspension at MotoSport

In stock at MotoSport

Prices from MotoSport’s catalog of Oct 26, 2026; check today’s price before you buy.

Front tire, stock size 80/100-21

Rear tire, stock size 100/90-19

Prices and stock from MotoSport's product catalog on Oct 26, 2026; they change, so check the price at MotoSport before you buy. Products are chosen by exact stock size, code or oil viscosity from the specs above, known brands first, then price. We have not tested them.

2 part specs not yet verified; see data coverage.

Use this bike

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Service schedule

58 rows from the owner's manual schedule, pp. 38-40, counted in hours, at these intervals:

After one operating hour
After 15 operating hours / Every 15 operating hours
Every 45 operating hours
Every 90 operating hours
Every 24 months

Check what's due or track it in My Bike. Buying one used? The hours check lists what the schedule calls for by its meter reading.

Gearing calculator

Gearing for this bike

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 8–16 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—

Ratio change—

A —

B —

Taller · more speed Shorter · more pull

—

Changing the rear sprocket changes the chain length too. The chain length calculator works it out from the chain you have now, with where the axle ends up.

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

What's missing

26 of 30 fields read off a manual page.

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

See coverage for every bike

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, 95, 144, 145, 146, 148, 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 (weight without fuel approx.), weight, as labelled, gearbox oil, fork oil, brake fluid, coolant type, fuel octane, idle speed; checked 2026-10-16.

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.

Your manual prints something different? Report a value with the page it is on; every fix is listed in the corrections log.

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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