GY6 50cc (139QMB) Scooter 52.4mm Big Bore Kit 89cc

52.4mm Big Bore Kit for GY6 50cc Scooters

This 52.4mm Big Bore Kit is built for GY6 50cc scooters that use the 139QMB engine. With the stock stroke, it raises displacement from 50cc to 89.3cc, and with a 44mm stroker crank it reaches up to 94.9cc. It is one of the GY6 50cc scooter parts available for riders who want more power for off-road use and better efficiency on the trail.

Better Airflow and Power

The kit includes a 52.4mm cylinder and a performance cylinder head with larger valves, 23mm intake and 20mm exhaust, to improve airflow. Port sizes are 20.8mm for the intake and 18.7mm for the exhaust. Together these changes give noticeably quicker throttle response and higher top speed, which suits off-road riders looking for a clear step up in performance.

Installation and Compatibility

Fitting this kit takes some mechanical skill, since the larger engine needs more fuel and air. For the best results, plan to adjust or replace the carburetor and retune the variator rollers and clutch parts. The kit works with GY6 50cc engines fitted with the 64mm short-valve head, so confirm your engine code before you order.

Specifications

• 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)
• Fits: GY6 50cc 4-stroke 139QMB (QMB139) engines with the 64mm short-valve head

Fitment

Fits:

• GY6 50cc 4-stroke engine (139QMB, QMB139 engine code, 64mm short-valve head)

How to choose: Look for the engine code 139QMB or QMB139 stamped on the engine case, and confirm that your original head uses 64mm short-style valves.

Shop more: GY6 50cc Scooter Parts (139QMB) · GY6 Scooter Parts · Scooter Big Bore & Cylinder Kits · Scooter Parts