Naraku
Naraku 52.4mm Big Bore Kit 89cc - 139QMB 50cc GY6 Scooters
Description
This Naraku 52.4mm Big Bore Cylinder & Head Kit (SKU 137-4) is a complete top-end upgrade for 139QMB (QMB139) 50cc 4-stroke scooters, the Chinese GY6 50cc platform based on Honda's GY6 design. It bumps displacement to 89.3cc on the stock 41.4mm stroke, or 94.9cc with a 44mm stroker crank. The kit includes a ready-to-bolt-on head with larger valves, so you get a real jump in power and acceleration over a stock 50cc.
This product is intended for off-road use only. This product is not intended for use on public roads or land.
What's Included
- 52.4mm cylinder
- Naraku piston with rings, wrist pin, and circlips
- Cylinder head with valves and springs installed
- Gasket set, including head, base, and exhaust gaskets
Key Features
- Big bore upgrade: 50cc to 89.3cc (stock stroke) or 94.9cc (44mm stroker crank).
- Performance head: Non-emissions design with larger 23mm intake and 20mm exhaust valves.
- Improved flow: 20.8mm intake port and 18.7mm exhaust port.
- Complete kit: Everything needed for the top-end swap in one box.
Technical Specifications
- Part number: 137-4
- Cylinder bore: 52.4mm
- Cylinder sleeve OD: 54.4mm
- Wrist pin diameter: 13mm
- Total piston height: 36.3mm
- Compression height: 20mm
- Intake valve: 23mm
- Exhaust valve: 20mm
- Intake port: 20.8mm
- Exhaust port: 18.7mm
- Valve stem length: 64mm (short style)
- Displacement: 89.3cc (41.4mm stock stroke) / 94.9cc (44mm stroker)
- Brand: Naraku
Compatibility
| Engine | Engine Codes | Notes |
|---|---|---|
| GY6 50cc 4-stroke | 139QMB, QMB139 | Engines with the 64mm short-valve head |
Check the engine code stamped on your engine case before ordering. Engines marked 139QMB may use this kit, but confirm your original head uses 64mm short-style valves. The 52.4mm sleeve may require machining the engine case for clearance.
Installation Notes
A big bore kit adds fuel and airflow demand. Plan to rejet or upgrade your carburetor and retune your variator rollers and clutch springs for the best results.
Product Diagrams
Features
Specifications
Part Number: 137-4 | Bore: 52.4mm | Sleeve OD: 54.4mm | Wrist Pin: 13mm | Piston Height: 36.3mm | Compression Height: 20mm | Valves: 23mm intake / 20mm exhaust | Ports: 20.8mm intake / 18.7mm exhaust | Valve Stem: 64mm short | Displacement: 89.3cc stock stroke / 94.9cc 44mm stroker | Brand: Naraku
Model Fitments
QMB139 / 139QMB 49cc/50cc 4-stroke engines. May require engine case modification to accommodate larger cylinder. Replaces 64mm short-style valve stem heads only. Off-road use only.
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Same day shipping for orders placed before 2PM (CST) M-F
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30-day return period on eligible items
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Flat-rate shipping for orders shipped to the 48 Contiguous United States.
