WPB Welsh Pit Bikes 125 4-Speed Gearbox GN125 BD125
New unbranded replacement 4-speed manual gearbox shaft assembly for selected 125cc pit-bike engines commonly identified as GN125 / BD125. The assembly is supplied as the two complete transmission shafts with the gears already installed, making it suitable for replacing worn, chipped or damaged internal gearbox components where the dimensions and tooth counts match.
Verified Application
- Welsh Pit Bikes / WPB 125 fitted with the GN125 / BD125-type 4-speed manual engine
- 125cc air-cooled pit-bike engine
- 4-speed manual clutch transmission
Important identification note: “GN125” in this listing refers to the GN125-type pit-bike engine designation. This gearbox is not advertised for the genuine Suzuki GN125 motorcycle, which uses a different 5-speed transmission.
Gearbox Specifications
Output Shaft Assembly
- Overall shaft length: 119.5 mm
- Output / sprocket-side section diameter: 17 mm
- Bearing-end diameter: 12 mm
- Gear tooth counts: 23T / 20T / 16T / 11T
Input Shaft Assembly
- Overall shaft length: 116.5 mm
- Spline diameter: 17 mm
- Bearing section diameter: 12 mm
- Gear tooth counts: 36T / 31T / 27T / 24T
Components Identified From the Product Photos
- Complete input / main-shaft gear assembly
- Complete output / countershaft gear assembly
- Four transmission gears on each shaft
- Splined shaft ends for transmission / drive engagement
- Machined bearing journals at the shaft ends
- Sliding and fixed transmission gears with engagement dogs / selector features
- Factory-assembled spacers / retaining hardware visible on the shafts
OE & Manufacturer Part Number Information
No genuine motorcycle OE number or verified manufacturer MPN could be established for this gearbox. Matching aftermarket examples are sold as Unbranded with manufacturer part number listed as Does Not Apply. Your Moto Mania UK stock SKU is 02289-ENG; this is a shop SKU rather than an OE or manufacturer part number.
Compatibility Check — Please Measure Before Ordering
Several visually similar 4-speed 125cc pit-bike gearboxes exist and they are not automatically interchangeable. Before ordering, compare your original gearbox against all of the following: shaft lengths, 17 mm splined/output sections, 12 mm bearing journals, gear tooth counts, selector-dog positions, gear widths and the photographs in this listing.
Do not order by engine capacity, bike colour or model name alone. Engine suppliers changed internal gearbox designs during production, so the physical measurements and tooth counts are the most reliable identification method.
Installation Notes
Gearbox replacement requires complete engine disassembly and crankcase splitting. Inspect the selector drum, selector forks, bearings, bushes, thrust washers, circlips and crankcases before reassembly. Any worn selector or bearing components should be replaced at the same time. Reassemble using the correct workshop procedure, gasket/seal components, lubrication and fastener torque settings.
What's Included
- 1 x complete input-shaft gear assembly
- 1 x complete output-shaft gear assembly
Selector drum, selector forks, bearings, crankcases, gaskets and other engine components are not included unless clearly shown in the photographs.
Delivery & Returns
- UK-stock replacement engine component
- 30-day returns, subject to the item being returned in its original condition