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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 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 25 reviews
Loved it!

easy installation… woke up my monkey in an angry way..🤣🤣 more power low end and definitely top end.

Ben Smith
She's torqueeeee

Massive difference in power and torque, very happy with it. Thanks DHM 🙏

149 BBK

I have the first gen on my 22 grom I liked it so much I purchased the second gen for my 23 grom. Get one it will really wake your bike up !!!!

daniel k
Not really sure yet

I plan to change my review in the future. The owner always writes back and does his best to answer any and all questions. That said, I’m on my 2nd 149bbk. The first one had the piston blow up. I bought a 2nd kit to try again. I plan to have the bike dump tuned after install to be sure nothing goes wrong.

Dhm 149cc big bore

This kit really wakes up the 22+, more power and torque paired with the s1 cam and more top end paired with the s2 cam. Don't forget to add a ported head, intake and throttle body! That will definitely maximize this package. Customer service is top notch!