Big Bore Kit Buyer Guide — Honda Grom, Honda Monkey & Kawasaki Z125

Overview

Big Bore Kits are one of the most transformative engine upgrades for Honda Grom, Honda Monkey, and Kawasaki Z125 models. These kits increase the engine’s displacement beyond stock 125cc, giving your bike significantly more power and torque without replacing the entire engine. They’re a popular choice if you want a stronger throttle response, better top-end speed, and a more exciting ride.

HardRacing.com carries a range of options to suit different build goals — from easy bolt-on increases to larger performance-oriented setups.

This guide will help you decide:

• If a Big Bore Kit is right for you
• The core benefits you'll get
• What supporting mods or considerations may help

What is a Big Bore Kit ?

A Big Bore Kit replaces your bike’s stock cylinder and piston with larger components that increase total displacement (measured in cc). More displacement means the engine can draw in more air and fuel, leading to a bigger combustion event and more horsepower and torque.

Unlike just swapping an intake or exhaust, a big bore kit fundamentally changes how much work your engine can do — so the gains are substantial compared to other bolt-on mods.

Key Benefits

• More Power & Torque: Increased displacement directly adds power, helping with acceleration and top speed. 
• Better Midrange: Big bore engines have stronger pull in the mid-range where stock engines are often weaker.
• Great With Tunes: When paired with proper management modules and supporting mods, power delivery becomes smooth and optimized. 
• Build Flexibility: Depending on kit size, you can go from mild upgrades to serious performance builds.
  
  

Who Should Consider a Big Bore Kit ?

Good Pick If You Want:

• Noticeable power gains over stock 125cc motor
• Better throttle feel and midrange torque
• A foundation for more serious engine mods (ECU upgrade + camshaft + oil cooler)
• To make highway cruising more practical
  
  

Installation & Supporting Mod Tips

Big bore kits are not always simple bolt-ons -- some require case work, tuning & fuel upgrades

• Fueling: Larger displacement often needs a bigger injector or tuning (like aRacer ECU or DynoJet PC6)
• Tuning:  After installation, a fuel controller is strongly recommended to get safe, optimized performance from your newly installed big bore kit.  To protect your investment by allowing it to be tuned properly
• Supporting Parts:  Camshaft, high-flow exhaust, intake upgrades, and stronger clutch components help to get the most from your new Big bore kit.
• Installation:  If you're unfamiliar with engine work, a trained mechanic can help avoid mistakes during the installation process
  
  

Common Questions

Do big bore kits make a huge difference ?

Yes — since they increase engine displacement, the overall power and torque gains are generally much more noticeable than simple add-ons like air filters or exhausts.

Will my stock fuel system keep up?

Often not. As displacement goes up, proper fueling (injectors + fuel controller module) becomes more critical to avoid running incorrect fuel parameters.

Ready to Upgrade?

Big Bore Kits are a transformational performance upgrade for Honda Grom, Honda Monkey, and Kawasaki Z125 riders who want improved power, torque, and rideability. They’re especially effective when paired with proper fueling and tuning — and HardRacing carries a range of kits to fit different build goals.

Shop Big Bore Kits