How to tell if a GY6 125 / 150 engine needs a 743-20-30, 835-20-30 or 842-20-30 drive belt?
Dimensional breakdown of GY6 50/125/150 drive belts, variator boss lengths, and SYM Attila FI belt cross-references.
Count the CVT cover bolts or measure crankshaft-to-clutch shaft distance: Short-case GY6 125/150 (10-inch rear wheel, 7 or 8 bolts) uses 743-20-30; Long-case GY6 125/150 (12/13-inch rear wheel, 8+1 center bolt) uses 835-20-30 or 842-20-30.
Technical Specification & Interchange Matrix
| Belt Code | Length × Width × Angle | Engine / Case Type | Typical Wheel Size |
|---|---|---|---|
| 669-18-30 | 669 mm × 18 mm × 30° | 139QMB (GY6 50/80cc Short Case) | 10-inch rear wheel 50cc scooter |
| 743-20-30 | 743 mm × 20 mm × 30° | 152QMI / 157QMJ Short Case | 10-inch rear wheel 125/150cc |
| 835-20-30 | 835 mm × 20 mm × 30° | 152QMI / 157QMJ Long Case (Standard) | 12-inch rear wheel 125/150cc |
| 842-20-30 | 842 mm × 20 mm × 30° | 157QMJ Long Case (Large Pulley) | 12/13-inch wheel with oversized rear pulley |
Step-by-Step Verification & Diagnostic Procedure
- 1Decode the 3 numbers printed on the back of any CVT belt
In 743-20-30: 743 = outer circumference in mm, 20 = top width in mm, 30 = V-angle in degrees. Replace the belt when width wears down by more than 1.0 mm.
- 2Verify Aramid vs standard chloroprene material for FI models
Fuel-injected scooters with higher low-end torque require Aramid-cord belts (such as PD-A-7 ADB Belt for Attila FI) to prevent stretching.
Referenced OE Part Numbers & Dossier Links
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