81cc / 100cc REPLACEMENT PISTON KIT

50mm BIG BORE

FOR QMB139 / 139QMB 4-STROKE ENGINES

Restore your engine's compression and performance with this premium 50mm replacement piston kit for QMB139 / 139QMB 4-stroke engines equipped with a 50mm big bore cylinder. Manufactured from high-strength aluminum alloy and precision machined for an accurate fit, this piston assembly is an excellent replacement for worn or damaged pistons during a top-end rebuild.

Whether you're refreshing an existing engine or rebuilding a performance setup, this kit provides everything needed to replace your piston assembly.

⭐ FEATURES

• Direct replacement for 50mm Big Bore Cylinder Kits
• Fits QMB139 / 139QMB 4-Stroke Engines
• Precision-Machined Lightweight Aluminum Piston
• High-Strength Aluminum Alloy Construction
• Restores Engine Compression
• Improves Throttle Response
• Reduces Blow-By
• Excellent Heat Dissipation
• Precision Ring Grooves
• OEM-Style Replacement
• Ideal for Complete Engine Rebuilds
• Ready for Installation

PACKAGE INCLUDES

✔ 50mm Piston

✔ Complete Piston Ring Set

✔ 13mm Wrist Pin

✔ (2) Wrist Pin Circlips

Everything needed to replace your piston assembly.

SPECIFICATIONS

Engine Type

• QMB139 / 139QMB
• Air-Cooled
• Single Cylinder
• 4-Stroke

Piston Diameter

• 50mm

Wrist Pin Diameter

• 13mm

Material

• High-Strength Aluminum Alloy

Ring Material

• Cast Iron / Steel

Fits

• 50mm Big Bore Cylinder Kits

Approximate Displacement

• 81cc (Commonly Marketed as 100cc)

PERFORMANCE BENEFITS

✔ Restores Lost Compression

✔ Improves Starting

✔ Better Acceleration

✔ Increased Low-End Torque

✔ Improved Throttle Response

✔ Reduced Oil Consumption

✔ Smoother Engine Operation

✔ Precision Fit

✔ Excellent Replacement for Worn Pistons

✔ Great for Top-End Rebuilds

✔ Long Service Life

FITMENT

Designed for 50mm Big Bore QMB139 / 139QMB Cylinder Kits

Fits many Chinese scooters including:

• TaoTao
• Wolf
• Ice Bear
• Jonway
• JMStar
• Roketa
• SUNL
• Tank
• Peace Sports
• Gator
• BMS
• Baja
• Dongfang
• Znen
• Redcat
• Kazuma
• NST
• Puma
• Eagle
• Lifan (QMB139 Models)
• Many other Chinese scooters equipped with QMB139 / 139QMB engines.

IMPORTANT FITMENT INFORMATION

⚠ This piston is designed ONLY for 50mm Big Bore Cylinder Kits.

It will NOT fit:

• Stock 39mm 50cc Cylinders
• 44mm Big Bore Kits
• 47mm (72cc / 80cc) Kits
• 52mm Big Bore Kits
• 54mm Big Bore Kits
• GY6 125cc Engines
• GY6 150cc Engines
• Minarelli 2-Stroke Engines
• Honda Ruckus
• Honda Metropolitan

Always compare your original piston before purchasing.

Please verify:

• Piston Diameter
• Wrist Pin Diameter
• Compression Height
• Ring Groove Configuration

Review all photos and measurements before ordering.

INSTALLATION NOTES

• Verify piston ring end gap before installation.
• Install the piston with the arrow facing the exhaust port.
• Lubricate the piston, rings, wrist pin, and cylinder during assembly.
• Install circlips securely into their grooves.
• Stagger piston ring end gaps according to manufacturer recommendations.
• Always install with new gaskets.
• Proper engine break-in is recommended.

Professional installation is recommended.

BREAK-IN RECOMMENDATIONS

For maximum engine life:

• Vary engine RPM during the first 100 miles.
• Avoid prolonged wide-open throttle.
• Change engine oil after the first 100–150 miles.
• Recheck valve adjustment after break-in.

? INDUSTRY NOTE

Actual Engine Displacement: Approximately 81cc

Although this piston is commonly advertised as a "100cc" replacement, a 50mm piston installed with the factory 41.4mm QMB139 crankshaft produces approximately 81cc of actual engine displacement.

The "100cc" designation has become the industry-standard name used by manufacturers, distributors, and online marketplaces because it is the term most customers search for when shopping for this upgrade.

81cc = Actual Engine Displacement

100cc = Industry / Marketplace Name

? MAXIMUM DISPLACEMENT OPTIONS

Looking for even more power? Pair your piston with a stroker crankshaft.

? BUILD A TRUE 100cc+ ENGINE

For riders looking to maximize the performance of a QMB139 engine, we recommend combining:

✔ 54mm Big Bore Cylinder

✔ 44mm Stroker Crankshaft

✔ Performance Cylinder Head

✔ 19mm–21mm Performance Carburetor

✔ Performance Camshaft

✔ High-Flow Exhaust

✔ Performance CDI & Ignition Coil

✔ Proper CVT Tuning

This combination produces approximately 101cc of actual engine displacement, delivering substantially more torque, stronger acceleration, and higher overall performance than a stock 50cc engine.

WHY REPLACE YOUR PISTON?

Over time, pistons and piston rings wear due to heat, friction, and normal engine operation. Replacing the piston assembly can help:

• Restore compression
• Improve power and acceleration
• Reduce oil consumption
• Improve fuel efficiency
• Reduce engine noise
• Extend engine life
• Restore lost horsepower

QUALITY PARTS • FAST SHIPPING • EXPERT SUPPORT

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