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DHM High Compression Stock Bore Piston for the 2022+ Honda Grom / Monkey (Gen 2)

Include NGK CR9E Spark Plug

**Professional install is highly recommended. Ring gaps must be set, and base gaskets must be installed properly. Setup sheet is included and can be viewed HERE

DH Motoring Stock Bore Racing Piston (2022+ Grom / Monkey)

We've now produced the first high compression stock bore piston for the new 2022 Grom/Monkey. We took our development experience from the previous generation Grom and CRF110 and applied it to the all new Grom engine and have come up with a lightweight, high compression piston that still utilizes the OEM 12mm pin size. 

  • ~12.4:1 compression 50mm stock bore piston
  • 100% Forged construction
  • Larger ring and skirt oiling holes than other pistons on the market
  • Larger oil spring to increase oil volume between the oiling rings
  • OEM sized skirt area to maintain reliability
  • Racing piston ring kit with pins and circlips
  • Includes a new base and head gasket.
  • NGK CR9E spark plug required 
  • Requires at least 91 octane fuel
  • Requires ECU Flash or other Engine Management (aRacer, etc)

Increasing the compression ratio on the stock bore significantly increases the throttle response and overall power of the engine. In comparison to the stock piston, you can expect more low, mid and top end power while maintaining the use of your stock cylinder. We've designed this piston to be as close to the stock weight as possible without sacrificing any structural integrity. It can clear big valve head with 27/22 valve diameters and it will drop right in to your OEM cylinder. 

You will need either an ECU flash or some sort of fuel controller for this to run properly.


aRacer Standalone ECU

**Installing this piston requires setting the ring gaps. If you are not comfortable with this, please have your local shop perform the install. Failure to set the proper piston ring end gap can result in premature failure or seizure of the engine. We recommend an end gap of .007-.008" on the top ring and .009-010" on the bottom ring. The silver ring goes on top and the black ring below. 

Customer Reviews

Based on 17 reviews

I was wondering if I gap the oil rings to do all 4 rings need gaped

Andrew H
Great High Comp Piston!

The DHM High Comp stock bore piston seems to pair well with the Chimera intake, Yoshi exhaust and S1 cam. The aRacer Mini X with AF2 live tuning handled the adjustment well. Very happy!

Kevin Russell
Works as it should

I have the 2019 monkey and installed the high compression piston from DHM and it ran so well I decided I would install one in my Grom. So the 2022 Grom has a DHM high compression stock bore piston, DHM S1 cam, chimera intake, Kemimoto exhaust and the DHM stage 3 ecu flash. Bike runs beautifully, more low end TQ and just pulls better and seems less labored doing it. I would recommend ALL of DHM products. Not a hard install just some basic tools and mechanical knowledge. If you are not going big bore than this is the next best option.

Kevin G
Mo’ power baby!

Installed on my buddies bike and woke it up. Easy install and worth it. Paired it with an aracer but have previously used dhm flashing services in the past for other bikes with great sucess

HC piston

Loving this upgrade! Makes more torque right off idle all the way up thru the power band. Was an afternoon in the garage and worth every hour. Cant wait to go big bore next summer!!