DH Motoring

Regular price $645.00

Shipping calculated at checkout.

Customer Reviews

Based on 45 reviews
Surprising how big of an improvement.

It is still under break in stage, so far you can definitely feel a huge improvement.
I don't like the oil cooler bracket though, but not a big deal I just built my own bracket to relocate to a higher location and used a longer inlet hose.
David is an awesome customer support.


Steven Lorey
DHM kills it again

I can’t properly articulate how much I appreciate the quality of DHM parts. Everything arrived as advertised, it was well packaged and arrived quickly. The install wasn’t too difficult despite my limited mechanical capabilities. There are YouTube videos to make it pretty simple. Also, I am confident if I had an issue, I could email David and receive a timely response. DHM will remain to be my go-to place for Grom performance parts. Thanks David!

Adam Robinson

Man everything bolted up great absolutly no issues. This kit is well put together . The low end tourqe is prety nasty. Still in break in but even at 5-6k shifts. It pulls. Hard! If your thinking about buying this kit. Just buy money will be well spent. 2.5 hours total with the right tools. Easy power up wheelies and hills are absolutly not a bother with power to spare. 225lbs rider plus gear. 1/4 throttle 16t sprocket GPS confirmed only at 6500 rpm 66 mph . PLENTY OF POWER TO SPARED breaking in currently though! And davids service and response 5🌟

1k mile overview

I have 1,600 miles on the kit and still running strong! On a good day I can get the bike to 79mph and 77mph on average tucked(GPS confirmed) with 16/34 gearing, 210lbs with gear. The bike is honestly just a beast for what it is! As stated below in other reviews the oil cooler mounting bracket pictured is different then what was received. Just some filing is needed to get it to sit flush on the head. I actually just broke the mount recently from the cooler hitting the fender, Just be careful if you bottom out the suspension, I have an ohlins fork kit and this happens when riding silly. Overall totally worth the money, you’ll never wanna ride a 125cc grom ever again lol!

Johnathan Strickland
186 big bore blast! Wow wow

What a ripper! Absolutely love the big bore kit. No issues at all. (Professional install is key to problem free install).

**Kits are now shipping within 2 weeks from order date!**

We are proud to introduce the first "all inclusive" big bore package for the Honda Grom!

The DHM 186 kit includes everything you need to convert your Honda Grom in to a big bore machine. You no longer have to piece parts together with expensive components to get more power out of your bike. Our kit utilizes hardware from TBParts along with an OEM Honda injector and our software tuning to create a reliable and powerful kit that makes the Grom easier and more fun to ride. 


  • 186cc Cylinder and Piston
  • Complete Oil Cooler Kit
  • OEM Honda PCX150 Fuel Injector
  • DHM Reflash
  • TB Camshaft (you can select an option to reduce the price if you already own one)

Performance Data:
  • 16.5 HP and 14.5 Ft/lbs of torque! A gain of 74% in HP and 67% in torque over a stock bike. (Dyno chart available in photos above)
  • 20-50mph in 5 seconds compared to a stock bike at 9 seconds


TB Camshaft. This kit is designed to be run with the TB camshaft. If you already have one, you do not need to select it. If you do not already own one, please add it by using the drop down menu when ordering.

TB Clutch Kit. If your stock clutch is worn, or you'd just like a harder bite, we offer the TB clutch kit as a $40 upgrade.

How the Reflash works: 

You will need to send us your ECU to be reflashed for proper fueling to run with the kit. If you do not wish to send us your ECU, we can provide a core ECU for an additional fee. Please select your ECU option from the drop down menu when ordering. You can receive a refund of $75 when returning your ecu if you order a core when checking out.

Since our primary business is building race engines, we take a lot of pride and care in our engine assembly. Before shipping, we will check all ring gaps, and we will also align the rings in the piston and install the piston in to the cylinder for you with the alignment we recommend! 

**An upgraded intake and exhaust are not required but are highly recommended!

Completely upgradeable:

Since we are flashing the OEM ecu, you can still use piggybacks on top of this kit should you decide to make any changes. If you would like to add a big valve head or change the cam, etc., you can do so and use your piggyback fuel controller (pcv, etc) to tune it as you normally would. The fact that we've reflashed the stock ecu for fuel and timing will actually provide a better base than the stock, unmodified ecu! 

***Requirements and General Information:

The ECU reflash requires your stock narrowband O2 sensor to be installed and plugged in. If you have removed this to use another fuel controller, you will need to reinstall it. The ecu will learn and adapt over the first 15-20 miles, so acceleration and overall ride ability will increase up to that point. 

Installation instructions are not included. If you are not capable of working on these engines, having a shop perform the install is recommended.

This kit is designed to be bolt-on, reliable power. However, at the end of the day the Grom engine is a very simple air-cooled engine, and heat is the enemy. If you plan on taking regular highway trips, an oil temperature gauge is highly recommended to make sure the engine does not overheat. OPMID and Koso both offer great options.