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

$349.00
by DH Motoring

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 39 reviews
85%
(33)
3%
(1)
8%
(3)
3%
(1)
3%
(1)
D
Danny S.
When in doubt, talk it out.

I have had the best experience possible with this company. I bought a kit and experienced issues. It happens; it’s how the world spins. However; I can full heartedly state that every step of the way, David at DHM was on the phone helping me figure it out. It took all my fear of not getting the bike running away. I cannot express enough how much I recommend a quality kit from David. You’re the best man. Thanks for all your time and help; it really was all appreciated!

A
Alexander

Loved. You can feel the difference

G
Gabe N.
2025 Dax big bore

Got the ported DHM head along with the s1 cam and the ecu reflash. Bike ran great but lacked bottom end. Now with this I can gain speed up hills where I was losing speed. Well worth it. Mpg is out the window but I didn’t buy this bike for mpg. Buy it and never be afraid to pass a car again.

P
Paul Kem
Just what the Grom needed

This with the supporting mods is just perfect!
Well done DMH. We appreciate what you do for the community.
Continue to be great!

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!

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