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DHM High Compression Stock Bore Piston (Gen 2) 2014-2020 Grom / Monkey

Include NGK CR9E Spark Plug

DH Motoring Stock Bore Racing Piston (Gen 2)

The DHM Race-spec stock bore piston has produced more HP than any other stock bore kit and is the highest compression stock bore piston on the market. It has a high compression dome that is made exclusively for us and isn't available anywhere else. It's gone through extensive testing on the racetrack and has proven to provide the most power possible from the stock bore. 

  • ~13.2:1 compression 52.4mm stock bore piston
  • Larger ring and skirt oiling holes than other pistons on the market
  • Larger oil spring to increase oil volume between the oiling rings
  • Oil ports under the pin holes to keep the wrist pin lubricated
  • 360 Degree skirt design to increase overall strength
  • Racing piston ring kit with pins and circlips
  • Valve reliefs that clear valves up to 27.5/23.5 cylinder head valves
  • Includes a new base gasket and head gasket
  • Requires NGK CR9E (6263) spark plug 
  • Requires at least 91 octane fuel

Over the past 6 years we've raced with and tested over a dozen different stock bore pistons. Using our experience on the racetrack we've now designed and produced the most competitive and reliable stock bore piston on the market. We targeted increased lubrication while also pushing the compression limits for pump fuel and ended up with what we feel is the most refined 125cc piston available. By enlarging the oiling holes that feed the oil spring channel and skirt, the piston stays well lubricated at all RPM which allows for extended high RPM use. Valve reliefs have been sized to allow the largest valves possible with the stock bore and they have been cut deep enough to ensure compatibility with even the highest lift cams on the market. 

Install video:

**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-.009" on the top ring and .009-.011" on the bottom ring. The silver ring goes on top and the black ring below. Unknown Overland does a great job of explaining this in the video below.

Customer Reviews

Based on 42 reviews
Mike Domer

The customer service was top notch. She made sure i got the right tune for my bike.

Dhm high compression 125cc

Totally worth the money. More power everywhere without sacrificing reliability. Highly recommended.


Dave’s a great dude very helpful pleased with my purchases

Andrew O.
DHM High Comp Piston - rocks!

Very happy with my DHM high comp piston. I previously had intake, exhaust, TB cam and CJR ECU flash. That was a huge improvement over stock. Now, adding the high comp piston puts me at 'Stage 3' and has restored some of the bottom end I lost with the TB cam but also added more power everywhere else. It was my first time doing a modification like this but it was very straightforward. Very pleased with the results and DHM customer service is top notch!

Stage 3

Parts came in a timely manner will install after Christmas.