GY6 Big Bore Kit 50cc / 139QMB, 1P39QMB

Also referred to as "GY6 50." The 139QMB, 1P39QMB are 49/50cc are air-cooled 4-strokes engines commonly found in Chinese scooters such as Tao Tao 50cc.

Its' engine code if we were to decipher each character of "139QMB" the "1" means its single-piston and the "39" would mean that it has a 39mm piston with a cylinder outer skirt of 44.78mm, which may vary by manufacturer slightly off by 0.2mm thickness, allow it slip into the 139QMB engine's crankcase bore opening 52.5mm max. So you can see their room for a bigger cylinder upgrade for the 139QMB and 1P39QMB engine, which will be covered here on GY6 Big Bore Kit 50cc.

NOTE: If you have a letter “P” after example “1P”, stands for (P)air valve horizontal engine. The P series engine has a taller cylinder head and a shallow recess cam seating that gives it slightly more performance than non-P GY6’s engine code. However, if both cylinder and cylinder head is swap out it will be identical to the non-P GY6 50 engine. So, both GY6 50cc 139QMB and 1P39QMB engine parts are interchangeable as long as the 1P39QMB top-end is replaced with a 139QMB cylinder, cylinder head, and rocker arm convert 1P to non-P. But why? Because they're so many more GY6 50cc performance parts available for non-P type engines includes GY6 performance parts 139QMB big bore cylinder139QMB big valve cylinder head, 139QMB 64mm rocker arm, and performance 139QMB camshaft, etc.

The fourth code letter: “Q”

Simply means it’s a forced-air-cooled engine. "Q" means is not a liquid-cooled but forced-air-cooled engine.

The fifth and sixth code letters: “MB”

The “M” means the engine type is a motorcycle(motor-cycle relates to scooters\atv\buggies as well). The sixth letter thereafter is code for engine displacement size measure in "cc" (cubic centimeter). See the table below. Notice as each alphabetical order B-to-Y so is the engine displacement CC increases incrementally 10cc, 15cc, 25cc, 50cc, and 100cc.

  • MB = 50cc
  • MC = 60cc
  • MD = 70cc
  • ME = 80cc
  • MF = 90cc
  • MG = 100cc
  • MH = 110cc
  • MI = 125cc
  • MJ = 150cc
  • MK = 175cc
  • ML = 200cc
  • MM = 250cc
  • MN = 300cc
  • MP = 350cc
  • MQ = 400cc
  • MR = 500cc
  • MS = 600cc
  • MT = 650cc
  • MU = 700cc
  • MV = 750cc
  • MW = 800cc
  • MX = 900cc
  • MY = 1000cc
  • YG = 1100cc
  • YL = 1200cc
  • Y? and so on...

GY6 engine code can help you determine what size cylinder the GY6 engines can handle 50cc scooter part or GY6 performance parts big bore kit upgrade available. As far as the GY6 49cc-50cc engine code 139QMB and 1P39QMB engine it will be able to handle the biggest GY6 Big Bore Kit slip-on available on the market...

Naraku Cylinders’ Combo 52.4mm 94.9cc 2V w/Cast Piston (139QMB)

Description: High-Quality Naraku Cast-Iron Cylinder 52.4mm 94.9cc with Cast Aluminum Piston offers better durability and reduced friction for 139QMB 49/50cc+ engines. The cylinder bore is 52.4mm. Naraku Cylinders' Combo 52.4mm 94.9cc 2V w/Cast Piston including the following parts Naraku 52.4mm (94.9cc) cylinder kit with cast piston and Naraku oversize 2V head 23/20. Naraku Cylinders' Combo 52.4mm 94.9cc 2V is suitable for all air-cooled 4-strokes, 49-50cc Kymco engines (scooters), and most Chinese imported vehicles 49/50cc (scooters, quads, and buggies).

When used in conjunction with a stock crankshaft stroke 41.4mm the engine displacement comes out to 89.3cc! When used with a 44mm stroker crank, engine displacement = 94.9cc!

NOTE: Please, measure with a caliper 139QMB crankcase bore (opening) for 54.5mm cylinder outer diameter sleeve need a minimum 54.6mm bore clearance or boring and machining are required. If, engine case must be bored, order an engine bore tool (Part# 1400-1001)Most stock 139QMB cylinder head's valve height is 69mm which comes standard with its 69mm rockerarm. This Naraku Cylinder Head's valve height is 64mm and will work only with a 64mm rockerarm. If you need a set of 64mm rockerarm see Part# 151-101 sold separately.


100% Naraku Products – NO SUBSTITUTE.

Includes the following Cylinder, Base gasket, Piston, Rings, Piston pin, Circlips, Cylinder head, Head gasket, Exhaust gasket.

1.    Includes Naraku Cylinder 52.4mm 2V, Piston + Rings, Gaskets (chain tensioner gasket, base gasket, steelhead gasket, exhaust ring), Wrist pin, and Circlips.
•    1x Cast-Alumin Piston: 51.86mm
•    5x Piston Rings (installed): 52.4mm
•    1x Wrist Pin w/G-clips: 41mm x 13mm x 8mm
•    1x Cast-Iron Cylinder: 52.33mm
•    Outer Sleeve Diameter 54.5mm
•    Overall Height (w/sleeve): 70.44mm
•    Cylinder Height (w/o sleeve): 53mm
2.    Includes Naraku Cylinder Head 52.4mm 2V NON-EGR, Valve Assembly.
•    1x Cylinder Head Stud Space: 50mm / H: 60mm / Intake Port: 20.8mm / Exhaust Port: 18.7mm
•    2x Valves: 23x64mm Intake / 20x64mm Exhaust / Stem: 5mm
•    Overall Height (w/valves) H: 66.1<67.3mm
•    Cylinder Height (w/o valves): 59.75mm
•    6mm Intake Threads
•    6mm Exhaust Threads
•    Made in Taiwan


•    Fit Intake Studs: M6 / Nuts: M6 Socket: 10mm
•    Fit Exhaust Studs: M6 / Nuts: M6 Socket: 10mm
•    Fit Rocker Arm Assembly: 64mm / Feeler Gauge: .004in / .005in / Socket: 9mm
•    Fit Cylinder Studs: 168mm 176mm / Nuts: M7 / Washers: M7 / Side Bolts: M6 / Socket: 8mm-10mm
•    Fit 50mm Stud Spacing /Diagonal 70.5mm
•    Fit 64mm Rocker Arm 2V Assembly
•    Fit Crankshaft 41.4mm to 44mm
•    Fit Dowel Pin Set (
•    Fit QMB 49/50cc+

Supported Models: 139QMB 49cc-50cc engines, Baja SC50, Boreem (BMS) 50, Filly 50, Kymco 50, Panterra 50, Qingqi 50, QLink 50, Roketa 50, SUNL 50, SYM 50, Tank 50, TaoTao 50, and virtually any other scooter engine with these measurements.

Item Weight & Product Dimensions:
•    1.456 Pounds | 4.567" (L) x 4.275" (W) x 2.776" (H)
•    2.193 Pounds | 5.227" (L) x 5.323" (W) x 2.598" (H)

After replacing your stock 50cc scooter parts with a GY6 Big Bore Kit 50cc upgraded 89.3cc up to 94.9cc you want to upgrade more GY6 performance parts to handle bigger engine displacement "CC" power like a GY6 performance CVK 24mm carburetor by Naraku. Perfect for GY6 Big Bore Kit 50cc upgraded!

Naraku Carburetor CVK 24mm (139QMB, GY6)

Description: Naraku 24mm pressure-controlled tuning carburetor. 24mm intake for 85cc+ cylinders with relevant sized heads. E-choke and 102 Main Jet, with 42mm air filter connection. No accelerator pump to prevent over oiling. Great for a performance increase on 85-180cc 4-stroke engines. Supplied jets are for 85cc engines. 24mm intake manifold set recommended. Carb can also be used as a replacement item on standard 125cc engines. Air filter connection: 42mm. Intake connection: 32mm. Jet Size is 5mm.

NOTE: Installation of Naraku Carburetor CVK 24mm on a 139QMB big bore cylinder head will require a bigger intake manifold like the NARAKU V.2 Intake Manifold (Part# NK200.32).


  • E-Choke
  • Non-Accelerator Pump
  • Cable/Rear Pull Linkages
  • Throttle Adjustment Knob
  • Open & Adjust Enricher Screw


  • Airbox Side ID: 38mm / OD: 42mm
  • Engine Side ID: 24mm / OD: 32mm
  • Venturi: 24mm


  • Main: 102 (5mm Thread) CVK Style
  • Pilot: 35s (6mm Thread x L: 28mm)


  • Fit Air-Filter Port ID: 3mm max. ± 41mm
  • Fit Intake Manifold ID: 2mm max. ± 30mm
  • Fit GY6 125/150cc+
  • Fit QMB 49/50cc+

Supported Models: GY6 139QMB 49cc-50cc 4-stroke, 50QMF 90cc-100cc, 152QMI 125cc, 157QMJ 150cc, 161QMK 170cc, Chinese Scooter, ATV, Genuine Buddy, Kymco, Polaris RZR 170, Ruckus, TaoTao, Vento, and virtually any other GY6-based engine with CVK carburetor.


Item Weight & Product Dimensions:

•    1.55 pounds | 5.375" (L) x 4.772" (W) x 4.680" (H)

When purchase BBK know what the cylinder size is you wanting and the current standard crankshaft is 139QMB it is 41.4mm calculated with cylinder piston size = true power in "cc."

Here how to determine your actual power displacement small engine is always measured in a cubic centimeter (cc) not horsepower like cars. 

Here what it would look like Naraku Cylinder 52.4mm w/Piston. Keep in mind the only thing that affects engine power increase top-end speed is just that the top-end changes the cylinder jug w/piston and its paired crankshaft stroker length. That it. The CVT/Transmission Kit can only help make engagement at the wheel faster but not add top-end miles per hour gain. With that said here is my calculation engine displacement power of what you could have gotten with Naraku 52.4mm BBK with standard crankshaft 41.4mm and with an extended crankshaft 44mm installed:

You can use the same "Engine Displacement Calculator" the links are below. And, there are many more high-quality aftermarket scooter parts available at the NCY STORE. So if your OEM 50cc scooter parts fail or get bogged down, let NCY Performance Scooter Parts be your source for finding better and upgraded replacement parts for your GY6 50 aka 139QMB 50cc scooter.

To check out the latest 139QMB CVT Transmission performance parts and accessories, click here.


Check out the related link Which Big Bore Kit?

Check out the related link Engine Displacement Calculator

Check out related video 139QMB vs 1P39QMB: