All orders over $150 ship for free to the United States!

DHM 149cc Big Bore Kit for the 2022+ Honda Grom/Monkey (Gen 2)


Our second generation 149cc kit is now ready to go! We've made a few updates to make the install process easier and more simplified. We've also eliminated any potential clearance issues and streamlined the manufacturing process even further to help pass on savings to the customer. 

Generation 2 updates:

  • Wider skirt bridges for more squirter clearance
  • One additional oil control ring lubrication hole compared to the previous piston
  • Two additional piston skirt lubrication holes compared to the previous piston
  • Compression on pump gas is now 12:1 to reduce detonation when hot

Kit includes:

  • 55mm Forged Piston with ring kit
  • 55mm Sleeved cylinder 
  • Base and head gasket set

This kit provides a significant horsepower and torque increase across the entire rev range. Expect gains of 30% in the midrange over a bike with intake, exhaust and cam, and double the horsepower of a stock bike at 8500rpm.

Our ECU flash or an aRacer MiniX and AF2 combo will be required for proper installation.

ECU Flash located HERE

aRacer MiniX HERE and aRacer AF2 HERE

Required supporting mods other than the ECU tuning include an DHM S1 or DHM S1R camshaft, oil cooler kit, crankcase vent and fuel injector. We have options listed below:

Injector: PCX150 Injector

Starter (optional but recommended): CJR Performance HD Starter

Spark plug: NGK CR9E

Install Notes:

  • Two base gaskets included. Use both the OEM Honda base gasket and the grey paper gasket for pump fuel setups, use only the OEM Honda base gasket for race fuel.
  • Oil coolers are required. Keep an eye on oil temps.
  • An NGK CR9E spark plug must be used
  • Ring gaps must be set during install, please see the setup sheet for further info.
  • 91 octane is the bare minimum, and 93 is preferred.
  • When using aRacer, download the quickburn for 149cc + S1 cam
  • Honda injector 16450-KZY-701 is the required injector for this kit.

Download setup sheet HERE


Customer Reviews

Based on 30 reviews
brandon Haas
Great power gains from the 149kit !

Great products , great power along with great people and customer service is top notch

Matthew Mattox
Great but not Awesome

Okay, we purchased the big bore kit, S1R Cam, Spark Plug, Tune, Valve Covers, Camshaft cover, and injector from DHM for my wife's 2023 Grom and it did wake up the bike at 5,600 RPM and up. Install wasn't bad because I have rebuild engines before and Youtube is readily available, but there is ZERO instructions, maybe DHM can start adding that in the future. I did a 300 mile breakin and let the wife loose on it, she likes how it will do a regular 55mph now in Northern Arkansas and you can actually pass someone on a 2 lane road if they're going slow. Lower gears are very torquey now, 5,000-5,600 RPM the bike doesn't have any power or pull (weird), 5,700 RPM and up the bike wakes up and runs great! MPG went from 115-90 but that's alright. Other things I did at the time of install was an upgraded oil pump, bigger oil cooler, intake and exhaust. I do recommend but I wish that this kit would be more straight forward; 2 pages of instructions would be perfect. Thank you for reading!

Ricardo Serrano
Fast shiping best

happy 👍👍👍👍👍👍👍

Andrew Torres
What do you think about DHM 149cc Big Bore Kit for the 2022+ Honda Grom/Monkey (Gen 2)

Super happy with the performance for being 220lbs. David did an amazing job with installation. I have no issues or complaints. I highly recommend for all the big boys out there with a grom.

Jim H
Great products and fantastic customer support!

I ordered the 149cc kit, S1 Cam and flashed ECU for my 2023 Honda Grom. I sent my ECU to them and it was received on Wednesday. They flashed the ECU and sent all my parts that same day. I received the order the next Monday. I had some questions during g the install and David responded quickly to my emails, allowing me to manage through the install within one day. First tap of the start button and it started right up. Runs great. I really appreciate a company that is responsive from beginning to end. I will use DHM for future upgrades as well.