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
Features
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Big bore upgrade: 50cc to 89.3cc (stock stroke) or 94.9cc (44mm stroker crank).
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Performance head: Non-emissions design with larger 23mm intake and 20mm exhaust valves.
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Improved flow: 20.8mm intake port and 18.7mm exhaust port.
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Complete kit: Everything needed for the top-end swap in one box.
Specifications
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Part number: 137-4
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Cylinder bore: 52.4mm
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Cylinder sleeve OD: 54.4mm
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Wrist pin diameter: 13mm
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Total piston height: 36.3mm
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Compression height: 20mm
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Intake valve: 23mm
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Exhaust valve: 20mm
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Intake port: 20.8mm
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Exhaust port: 18.7mm
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Valve stem length: 64mm (short style)
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Displacement: 89.3cc (41.4mm stock stroke) / 94.9cc (44mm stroker)
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Brand: Naraku
Model Fitment
| 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.
