A big bore kit can increase the displacement of a compatible 50cc GY6 engine, but the right choice depends on its engine code, valve length, and desired upgrade. The Glixal 50mm kit with a large-valve head is best for buyers seeking a more complete 100cc-style rebuild; the KYMLAA kit adds ignition components; and the Glixal 47mm kit is a more modest 80cc-style option. Confirm fitment before ordering, since similar-looking GY6 engines can have different specifications.
| Kit | Stated upgrade | Best for |
|---|---|---|
| Glixal 50mm large-valve kit | 100cc-style rebuild | Buyers needing a larger-valve head |
| KYMLAA 50mm kit | 50cc/60cc/80cc to 100cc | Buyers wanting included ignition parts |
| hauertzx 50mm kit | 50mm cylinder and supporting parts | Buyers seeking a broad parts bundle |
| Glixal 47mm kit | 80cc-style upgrade | Buyers choosing a smaller bore |
| TRKIMAL 47mm kit | 80cc-style upgrade | Buyers whose engine uses 69mm valves |
Glixal 50mm Kit With Large-Valve Head
This Glixal rebuild kit is listed for GY6 139QMB, 139QMA, and 147QMD engines and includes a 50mm cylinder with a large-valve head assembly. Its listed valve sizes are 18.5/20.5mm, compared with the stated stock sizes of 16/18.5mm. It is best for a buyer who has confirmed the engine and head configuration and wants a kit that includes a cylinder head rather than a cylinder-only upgrade.
Check before buying: The product specifies a 69mm long-valve-style head. If the original head has 64mm valves, the listing says the rocker arms must be changed to the ones included in the kit. Avoid choosing it based only on “GY6” or “50cc” in the vehicle description.
KYMLAA 50mm 100cc Upgrade Kit
KYMLAA describes this kit as an upgrade for GY6 50cc, 60cc, and 80cc engines to 100cc. The bundle includes a 50mm bore set, a 6-pin racing CDI, an ignition coil, a performance spark plug, and a clutch spring. It is best for a buyer who wants several listed engine and ignition components together rather than sourcing each item separately.
Check before buying: The listing identifies the 69mm valve-length version and says the kit is without EGR. Confirm those details match the scooter’s existing setup. “Simple installation” is the seller’s description; it does not establish compatibility with every GY6 model.
hauertzx 50mm Kit With Supporting Parts
This bundle lists a 50mm cylinder and piston, 69mm valve specification, A9 crankshaft, CDI, ignition coil, spark plug, 1500RPM clutch spring, and 6.5g copper roller. It is stated to fit most 49cc and 50cc Chinese scooters, mopeds, and ATVs equipped with 139QMB or 139QMA four-stroke engines. It is best for buyers who want a broad set of components in one purchase.
Check before buying: The listing says “most,” not all, vehicles with those engine codes. Compare the engine code and included-part specifications with the original setup before ordering; the broad vehicle-brand list does not guarantee fit for every model year or configuration.
Glixal 47mm 80cc-Style Cylinder Kit
This Glixal option uses a 47mm cylinder bore and lists a 13mm piston-pin outer diameter. Its features also specify durable piston rings. It is best for a buyer considering a smaller-bore upgrade than the 50mm kits in this guide or replacing cylinder and piston components on a compatible setup.
Check before buying: The supplied details do not list a cylinder head or ignition components, so do not assume they are included. Verify the piston-pin measurement and the rest of the engine’s dimensions before selecting this kit.
TRKIMAL 47mm Kit For 69mm Valves
TRKIMAL lists a 47mm non-EGR cylinder and a 69mm valve length for 49cc, 50cc, 60cc, and 80cc GY6 four-stroke engines. It is described for many Chinese scooters, ATVs, and go-karts. This kit is best for buyers who have verified the 69mm valve specification and want a 47mm upgrade option.
Check before buying: The listing’s broad engine-size range does not mean every version shares the same head or valve arrangement. Avoid this choice if the existing setup requires a different valve length or EGR configuration.
Buying Guide: How To Choose A 50cc Big Bore Kit
Which engine code and measurements should you verify?
Check the engine code stamped on the engine or documented in the service information. These products name GY6 codes such as 139QMB and 139QMA, but that label alone may not settle fitment. Compare the existing bore, piston-pin diameter, valve length, head style, and EGR configuration with the kit listing. When a listing specifies 64mm or 69mm valves, treat that measurement as a key compatibility check.
Should you choose a 47mm or 50mm bore?
The listed 47mm kits are positioned as 80cc-style upgrades, while the 50mm kits are described as 100cc upgrades. Choose based on the kit’s stated application and the engine’s verified specifications, not simply on a larger-is-better assumption. The supplied product information does not establish that every engine can use either size without other changes.
Do you need a cylinder head or extra components?
Compare what is explicitly included. The Glixal large-valve kit specifies a cylinder head assembly, while the KYMLAA and hauertzx listings identify additional ignition or drivetrain-related parts. The Glixal 47mm details emphasize the cylinder, piston-pin size, and rings. Confirm the complete package contents before purchase, especially if you expect a head, gaskets, or ignition parts.
What should you plan for during installation?
A big bore conversion changes engine components and may require mechanical experience, careful measurement, and setup checks. Follow the manufacturer’s instructions and consult a qualified mechanic if you cannot verify clearances or compatibility. Do not assume an advertised power or speed change is guaranteed for a particular scooter.
Frequently Asked Questions
What does a big bore kit do on a 50cc scooter?
It replaces selected engine components with a larger-bore cylinder and matching parts. The listed kits describe upgrades from 50cc-class engines to 80cc- or 100cc-style configurations.
Are all GY6 50cc big bore kits interchangeable?
No. Engine code, valve length, piston-pin diameter, EGR configuration, and head style can differ. Match the kit specifications to the actual engine.
Does every kit include a cylinder head?
No. Check the included-parts description. The Glixal large-valve kit explicitly lists a head assembly; other listings may focus on different components.
Is a 50mm kit always better than a 47mm kit?
Not necessarily. The appropriate size depends on the engine’s compatibility and the buyer’s intended rebuild. A larger bore is not a substitute for checking specifications.
Can a novice install a big bore kit?
Installation difficulty depends on experience and the engine setup. Anyone unsure about measurements, clearances, or assembly should seek help from a qualified mechanic.