Motocross

Yamaha YZ125

Motocross, 125cc 2T, 2027, EU/AU identical

Verified

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

Gearing

5 of 5 filled

Stock front sprocket: 13T; Owner's manual p. 2-3, checked 2026-09-28 (source 1)
Stock rear sprocket: 49T; Owner's manual p. 2-3, checked 2026-09-28 (source 1)
Chain pitch: 520; Owner's manual p. 2-7, checked 2026-09-28 (source 1)
Stock chain links: 112; Owner's manual p. 2-7, checked 2026-09-28 (source 1)
Stock final drive ratio: 3.769 : 1; Owner's manual p. 2-3, checked 2026-09-28 (source 1)

Wheels and tires

4 of 4 filled

Front rim: 21x1.6; Owner's manual p. 2-5, checked 2026-09-28 (source 1)
Rear rim: 19x1.85; Owner's manual p. 2-5, checked 2026-09-28 (source 1)
Front tire: 80/100-21 51M; Owner's manual p. 2-5, checked 2026-09-28 (source 1)
Rear tire: 100/90-19 57M; Owner's manual p. 2-5, checked 2026-09-28 (source 1)

Suspension

5 of 7 filled

Fork: Telescopic fork, coil spring; Owner's manual p. 2-5, checked 2026-09-28 (source 1)
Fork travel: 300 mm (11.8 in); Owner's manual p. 2-6, checked 2026-09-28 (source 1)
Shock travel: 315 mm (12.4 in); Owner's manual p. 2-6, checked 2026-09-28 (source 1)
Stock fork spring rate: 4.2 N/mm; Owner's manual p. 10-12, checked 2026-09-28 (source 1)
Stock shock spring rate: 48 N/mm; Owner's manual p. 10-13, checked 2026-09-28 (source 1)
Static sag, bike only: Not yet verified
Race sag, rider on: Not yet verified

Fluids and capacities

3 of 3 filled

Coolant capacity: 930 ml (31.4 fl oz); Owner's manual p. 2-2, checked 2026-09-28 (source 1)
Premix ratio: 30:1; Owner's manual p. 2-2, checked 2026-09-28 (source 1)
Fuel capacity: 7 L (1.85 gal); Owner's manual p. 2-2, checked 2026-09-28 (source 1)

Maintenance

3 of 3 filled

Tire pressure (off-road): 100 kPa (15 psi) front and rear; Owner's manual p. 2-5, checked 2026-09-28 (source 1)
Chain slack: 48-58 mm (1.89-2.28 in), on a maintenance stand; Owner's manual p. 2-7, checked 2026-09-28 (source 1)
Spark plug and gap: NGK BR9EVX, gap 0.6-0.7 mm; Owner's manual p. 2-2, checked 2026-09-28 (source 1)

Dimensions

3 of 3 filled

Seat height: 980 mm (38.6 in); Owner's manual p. 2-1, checked 2026-09-28 (source 1)
Wheelbase: 1445 mm (56.9 in); Owner's manual p. 2-1, checked 2026-09-28 (source 1)
Weight: 96 kg (212 lb); Owner's manual p. 2-1, checked 2026-09-28 (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 2026-09-28. Rows marked not yet verified are blank on purpose.

What's missing

22 of 24 fields read off a manual page.

Not yet verified (2): static sag, race sag.

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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. 2-7Stock chain links 112 Owner's manual p. 2-7
Sprockets: Stock front sprocket 13T Owner's manual p. 2-3Stock rear sprocket 49T Owner's manual p. 2-3
Tires: Front tire 80/100-21 51M Owner's manual p. 2-5Rear tire 100/90-19 57M Owner's manual p. 2-5
Coolant: Coolant capacity 930 ml (31.4 fl oz) Owner's manual p. 2-2
Suspension springs: Stock fork spring rate 4.2 N/mm Owner's manual p. 10-12Stock shock spring rate 48 N/mm Owner's manual p. 10-13

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
Yamaha 2027 YZ125 Owner's Service Manual, D68-2819U-52; owner-supplied copy from Yamaha's Owner's Manual Librarypp. 2-1, 2-2, 2-3, 2-5, 2-6, 2-7, 10-12, 10-13Manual page, verified. Covers stock front sprocket, stock rear sprocket, chain pitch, stock chain links, stock final drive ratio, front rim, rear rim, front tire, rear tire, fork, fork travel, shock travel, stock fork spring rate, stock shock spring rate, coolant capacity, fuel capacity, premix ratio, tire pressure (off-road), chain slack, spark plug and gap, seat height, wheelbase, weight; checked 2026-09-28.

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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