44mm Stroker Crankshaft Assembly for 50cc GY6 / QMB139 4-Stroke Engines

Increase the displacement and performance of your 50cc GY6 QMB139 engine with this 44mm Performance Stroker Crankshaft Assembly. Designed for serious engine builders, this crankshaft increases stroke over the factory crank, allowing your engine to produce significantly more torque and horsepower when paired with the proper big bore cylinder kit.

Manufactured from high-strength steel with a precision-balanced crank assembly, this stroker crank is built to withstand higher RPMs while delivering smooth, reliable performance for both street and racing applications. It is an excellent upgrade for 72cc, 80cc, 90cc, and 100cc performance engine builds utilizing the QMB139 platform. The 44mm stroke is approximately 2.6mm longer than the stock crankshaft, increasing engine displacement and improving low-end torque and acceleration. 

Specifications

• Engine Type: GY6 50cc / QMB139 4-Stroke
• Stroke: 44mm Performance Stroker
• Overall Length: 247mm
• Thread Size (Tapered Side): M10 x 1.25
• Thread Size (Spline Side): M12 x 1.25
• Bearing Outside Diameter (Magneto Side): 42mm
• Bearing Outside Diameter (CVT Side): 47mm
• Number of Splines: 26
• Connecting Rod Small End (Wrist Pin): 13mm
• Oil Pump Drive Gear: 16 Teeth
• Required Oil Pump: 33-Tooth Oil Pump
• Precision Balanced Assembly
• High-Strength Forged Steel Connecting Rod
• Heavy-Duty Crank Pin
• High-RPM Performance Design

Performance Benefits

• Increases engine displacement
• Produces more low-end torque
• Stronger acceleration
• Better hill-climbing performance
• Improves throttle response
• Higher horsepower potential
• Smooth operation at elevated RPM
• Ideal foundation for high-performance engine builds
• Precision-balanced to reduce vibration
• Durable construction for long service life

Recommended Engine Builds

This crankshaft is commonly used with:

• 47mm (72cc) Big Bore Kits
• 50mm (90cc) Big Bore Kits
• 52.4mm (100cc) Big Bore Kits
• Performance camshafts
• High-compression cylinder heads
• Performance CDI units
• 19mm–24mm carburetors
• Racing variators
• Lightweight roller or slider weights
• Performance clutch assemblies
• Free-flow performance exhaust systems

Compatible Engines

Fits most Chinese-built 50cc 4-stroke engines including:

• GY6 50cc
• QMB139
• 139QMB
• 139QMA

Common Vehicle Fitment

Compatible with many Chinese scooters using the QMB139 engine, including numerous models from:

• TaoTao
• Roketa
• Jonway
• Peace Sports
• BMS
• Ice Bear
• Wolf Brand Scooters
• Tank Sports
• SUNL
• Lance
• Znen
• JMStar
• Baja
• Eagle
• Dongfang
• Vento
• Coolster (QMB139-equipped scooters)
• Yerf-Dog scooters equipped with QMB139 engines

Always verify engine code, crankshaft dimensions, bearing sizes, and oil pump gear compatibility before purchasing.

Installation Notes

• Requires a 33-tooth oil pump due to the integrated 16-tooth crankshaft drive gear.
• New crankshaft bearings and seals are highly recommended during installation.
• Crankcase machining may be required for certain large-displacement engine combinations.
• Proper crankshaft alignment and professional installation are recommended for maximum reliability.
• Always check piston-to-valve and piston-to-head clearance when using stroker crankshafts with aftermarket cylinder kits.
• Pairing this crank with a quality big bore kit and properly tuned CVT will provide the best performance gains.

Why Upgrade to a 44mm Stroker Crank?

Unlike a standard replacement crankshaft, a 44mm stroker crank increases piston travel, creating a larger engine displacement without sacrificing reliability. The longer stroke improves cylinder filling, resulting in stronger torque throughout the RPM range, quicker acceleration, and increased horsepower when combined with supporting performance components. It is one of the most popular upgrades for riders building dependable high-performance QMB139 engines.