DHM 2022+ Grom / Monkey 149cc Big Bore Kit

The 2022+ Grom and Monkey run great from the factory — until you start chasing more. The DHM 149cc Big Bore Kit is the cleanest way to add real, usable power to your Gen 2 without splitting the cases. Ditch the stock 123.9cc setup, drop in a bigger bore, and feel the difference everywhere the tach lives — stronger roll-on, harder pull out of corners, and more grunt to carry that extra weight when you've got a passenger or a loaded frame slider setup.

This kit is built specifically for the Gen 2 platform (2022 and up), so you're not fighting head-clearance or timing quirks that come from adapting a Gen 1 part. It's a straightforward install for anyone comfortable pulling a top end, and it plays nicely with the mods you've probably already got your eye on — intake, exhaust, and tuning.

Runs the stock 5-speed and factory EFI. Pair it with a tune to unlock the full package.

Bullet highlights:

• Bumps displacement from 123.9cc to 149cc for a noticeable mid-and-top-end gain
• Engineered for the 2022+ Gen 2 Grom & Monkey — correct fitment, no Gen 1 workarounds
• Bolt-on top-end install — no case splitting required
• Bore: 55 mm / Stroke: 63.1 (stock stroke retained) 
• Compression ratio: 12:1
• Works with stock EFI; tune recommended to maximize gains
• Kit includes: cylinder, piston, rings, wrist pin, clips, and gaskets 

Specs block: