If you’re chasing more power, higher RPM, and improved top speed for motocross- or street-ready builds, a big bore kit is a practical upgrade. This guide highlights five well-suited options for 50cc–100cc class engines commonly used in GY6 and similar platforms. The selections focus on kits that increase displacement, expand valve sizes, and improve cooling, helping riders who want better acceleration and sustained performance. The buyers most likely to benefit are scooter or mini-bike enthusiasts, ATV and go-kart builders, and hobbyists tuning for sharper throttle response. Below is a quick table comparing the featured products to help you decide at a glance.
| Product | Engine Type | Displacement | Best For |
|---|---|---|---|
| Glixal GY6 100cc Big Bore Kit | GY6 139QMB/139QMA | 100cc | Max power with large valve head; best for scooters and ATVs |
| TRKIMAL 100cc Big Bore Kit | GY6 50cc–139QMB family | 100cc | Budget-friendly upgrade; easy install |
| Glixal ATMT1-008 80cc Big Bore | GY6 139QMB | 80cc | Compact boost; solid for small scooters |
| TRKIMAL Upgrade 100cc Big Bore Kit | GY6 49cc–139QMB | 100cc | Comprehensive kit with multiple components |
| Glixal ATMT1-011 139QMB 100cc Kit | GY6 139QMB/139QMA | 100cc | GZ2-ready upgrade with 50mm bore |
Glixal GY6 100cc Big Bore Kit With Large Valve Head
The Glixal 100cc big bore kit for 139QMB/139QMA/147QMD engines includes a performance big valve cylinder head assembly with 18.5/20.5mm valves (larger than stock 16/18.5mm). This design targets higher intake flow and better high-RPM breathing. Best for riders seeking noticeable power gains and a valve-driven performance boost in scooters or go-karts.
Who this is best for: riders prioritizing more torque and top-end power on GY6 platforms and who are comfortable with valve-head swaps. Why selected: the large-valve head can improve air intake efficiency and overall combustion efficiency, contributing to stronger mid-to-high RPM performance.
Caution: If your original head uses 64mm valves, rocker arms in this kit must be changed accordingly to match the kit components.
TRKIMAL 100cc Big Bore Kit For 64mm Valve
This kit upgrades gy6 engines from 50cc/60cc/80cc to 100cc without EGR, designed to deliver more RPM, higher power, and faster top speed. The 64mm valve length version is included in a complete kit that covers cylinder body, head kit, camshaft, gaskets, oil pump, gear, spring sets, and a racing AC CDI with ignition coil and spark plug. Best for: riders seeking a single comprehensive upgrade package that adds speed and responsiveness.
Best for: value-conscious builders who want a well-rounded upgrade with supporting components. Limitation: installation complexity may be higher due to the included multiple parts—ensure compatibility with your current engine setup.
Glixal ATMT1-008 80cc Big Bore Kit
Glixal’s ATMT1-008 80cc kit for the 139QMB engine family features a 47mm cylinder with a 13mm piston pin diameter, plus high-quality piston rings. Best for riders who want a compact, reliable power boost without going to a full 100cc upgrade. Choose this if you want a simpler, easier install with noticeable performance gains.
Best for: riders upgrading from stock 50–80cc setups who want a smooth power increase. Limitation: total displacement remains under 100cc, so gains may be more modest compared to full 100cc kits.
TRKIMAL Upgrade 100cc Big Bore Kit With Head Assembly
This Trkimal 100cc kit includes a head assembly with 69mm valve length and a complete upgrade package that covers cylinder head, camshaft, gaskets, oil pump, gear, ignition components, and spark plug. Best for: builders who want a robust, race-ready upgrade with broad component coverage. Why: the included high-RPM capable parts support stronger performance if matched to the rest of the engine design.
Caution: verify valve length compatibility with your existing head and rocker arms to avoid clearance issues.
Glixal ATMT1-011 100cc Big Bore Kit For 139QMB
This kit is designed for 139QMB/139QMA scooters with a 50mm bore and 13mm piston pin. It aims to offer a straightforward upgrade path for 4-stroke GY6 engines. Best for: riders who want a direct 100cc upgrade with clear compatibility notes and a focused fit for 139QMB platforms. Limitation: verify that your engine’s block and head match the 50mm bore specification.
Buying Guide: Key Considerations For Motocross Big Bore Kits
- Engine compatibility: Confirm your engine family (GY6, 139QMB/139QMA, etc.) and whether the kit’s bore and valve dimensions fit your cylinder head and crankcase, to avoid clearance or timing issues.
- Displacement vs. reliability: A larger displacement often improves power but can stress cooling and fueling systems. Match kit choice to your cooling system and exhaust setup.
- Valve size and head design: Larger valves improve intake flow, especially at higher RPM. Check valve-to-seat geometry and rocker-arm compatibility when swapping heads.
- Supporting components: Some kits include camshafts, CDI units, gaskets, and ignition parts. Ensure your electrical and fueling system can support these additions.
- Installation complexity: Comprehensive kits may require advanced mechanical skills. Consider professional installation if uncertain about tolerances and torques.
- Cooling and maintenance: Upgrades can raise heat output. Plan for improved cooling (coolant flow, radiator capacity) and regular maintenance intervals.
FAQ
- Q: Will a big bore kit improve top speed? A: Yes, by increasing displacement and improving airflow, which can raise peak horsepower and RPM capability.
- Q: Do I need to change the rocker arms with larger valve heads? A: In some kits, yes. If your kit’s head uses larger valves than stock, you may need to swap rocker arms to maintain proper valve timing and clearance.
- Q: Can I install a big bore kit myself? A: It depends on your mechanical skill. Some kits are straightforward, while others require precision machining, timing checks, and gasket sealing. Consider professional help if unsure.
- Q: Will bigger bore affect fuel economy? A: Larger displacement generally increases fuel consumption under heavy throttle, but improved efficiency can come with better air-fuel management at higher RPMs with proper tuning.
- Q: Are all big bore kits legal for street use? A: Local laws vary. Check regulations in your area regarding engine displacement and emissions compliance before installing upgrades on street-registered vehicles.
- Q: Should I upgrade the exhaust and intake when using a big bore kit? A: Yes, optimizing the intake and exhaust helps the engine breathe better at higher RPMs, maximizing the kit’s potential.