DHM 186cc Big Bore Kit – All-Inclusive Honda Grom Engine Upgrade

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DHM 186cc All-Inclusive Big Bore Kit – Honda Grom

Stop piecing together expensive parts. The DHM 186 Big Bore Kit is the first truly all-inclusive big bore package for the Honda Grom — everything you need to transform your stock 125cc into a 186cc powerhouse, assembled and prepped by the same team that builds race engines for a living.

Using TBParts hardware, an OEM Honda PCX150 fuel injector, and DHM's proprietary ECU reflash, this kit delivers a reliable, dyno-proven power gain that makes the Grom faster, more fun, and completely streetable.

Performance Numbers

• 16.5 HP & 14.5 ft-lbs of torque — a 74% HP gain and 67% torque gain over stock
• 20–50 mph in 5 seconds vs. 9 seconds stock

What's Included

• 186cc Cylinder and Piston (rings pre-aligned and installed)
• Complete Oil Cooler Kit
• OEM Honda PCX150 Fuel Injector
• DHM ECU Reflash (send-in or core ECU options available)
• TB Camshaft (optional — deselect if you already own one)

Build Options

• TB Camshaft: Required for this kit. Deselect if you already own a TB or DCR R2 cam to save on cost.
• ECU Reflash: Send in your stock ECU or order a pre-flashed core ECU. Core ECU customers receive a $75 refund upon returning their original. Available for SF Grom (2017+), OG Grom (2014–2015), and Monkey (2019–2021).
• TB Clutch Kit: Optional $40 upgrade for harder clutch engagement or worn stock clutches.

How the ECU Reflash Works

Your stock narrowband O2 sensor must be installed and plugged in. The ECU will self-learn and adapt over the first 15–20 miles. Since we reflash the OEM ECU, you can still run piggyback fuel controllers (PCV, etc.) on top for further tuning — giving you a better base than an unmodified stock ECU.

Fitment & Compatibility

• Honda Grom SF (2017+)
• Honda Grom OG (2014–2015)
• Honda Monkey (2019–2021)

Recommended (not required): Upgraded intake and exhaust. For regular highway use, an oil temperature gauge (OPMID or Koso) is highly recommended to monitor engine heat.

Installation instructions are not included. If you are not comfortable working on these engines, professional installation is recommended.