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

$349.00

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


Installation:

Customer Reviews

Based on 35 reviews
83%
(29)
3%
(1)
9%
(3)
3%
(1)
3%
(1)
K
Kevin Heng
Great

I finally installed it and started my bike for the first time with the kit and it sounds amazing. Can’t wait to go out and ride and get the tune dialed in!

A
Anthony Mastro
Big bore kit.

The big bore kit performance is great. It really gives the bike a much improved ride ability experience. Your fifth gear will come alive.

A
Alberto Soto
Don’t not regret buying ! 5 stars!

Easy install, called DHM for info and they were very nice and respectful and knowledgeable, these guys know what they’re doing ! Made my 2025 grom 149cc with their cam and A Racer and chimera intake and Brock’s exhaust and this thing rips!! Thank you DHM will continue to support you guys and keep shopping here!

R
Robert Weeston
Holy Moly

I got the BBK installed. Wasn’t that bad and I have never worked on a motorcycle EVER. I got all the items that DHM said you so needed. Just finished, maybe 10 minutes ago, installing the oil cooler. Add 93 octane fuel 10w-40 and started her up and holy moly. Sounded mean. No hiccups. Started right up and just purred. Gave it one rev and my exhaust even sounded better after the BBK and all the recommendations by DHM. Plus the installs were easy to medium. If you watch the videos over and over again like I did, makes the install a breeze. Sorry for the long review. But this bike sounds like the fastest slowest bike in the world. lol.

E
Eduardo Ramirez

Wow what a difference In power! Small bike small gains matter. If you're looking for something do give a little more top speed or better pick up off the , or even to maintain 55mph without the throttle full open this is it everythingwent in as it should looks greatand sounds great!!