DROWsports Honda Grom/Monkey 198cc 4 Valve Performance Cylinder Kit

DROWsports LS 198cc 4-Valve Big Bore Cylinder Complete Kit

Designed for the 2014–2020 Honda Grom and 2019–2020 Honda Monkey (4-Speed), the DROWsports LS 198cc Complete Kit delivers an affordable, high-performance big bore upgrade built around the proven KOSO 4-Valve Cylinder Head.

Drawing from extensive experience pushing the KOSO 170cc kit to its limits, we developed this larger-displacement option with improved attention to detail and several key upgrades for better performance and reliability.

Cylinder & Piston

The DROWsports LS 4-Valve 198cc Big Bore Cylinder & Piston is a direct upgrade over the 170cc kit. It features our proprietary 66mm forged 4-valve piston with a 15mm wrist pin. Despite being forged, the piston remains lightweight — only marginally heavier than the 170cc version.

All components are CNC machined to tight tolerances and treated with a special friction-reducing coating. The kit achieves an 11.5:1 compression ratio, delivering strong, reliable power. Additional refinements include machined radius edges and a precision accumulator groove.

Cylinder Head

The KOSO 4-Valve Cylinder Head comes fully pre-assembled with camshaft, valves, rockers, and seals installed — ready to bolt on. Equipped with 24mm intake valves and 20mm exhaust valves, it provides significantly improved airflow, perfectly complementing the 66mm bore of this 198cc build.

Professional Engine Building Services (Optional Add-On)

As fellow performance enthusiasts, we offer professional engine assembly as an optional upgrade. We build each engine to the same standards we use on our own bikes, including:

• WPC surface treatment (greatly reduces friction and improves piston longevity)
• Race porting and polishing
• Crankshaft balancing

Fitment

• 2014–2020 Honda Grom 125
• 2019–2020 Honda Monkey 125 (4-Speed)

Kit Includes

• DROWsports 198cc Ceramic Cylinder
• DROWsports 66mm Forged Piston (15mm Wrist Pin)
• KOSO 4-Valve Cylinder Head Assembly
• KOSO Camshaft
• KOSO Intake Manifold
• Iridium Spark Plug
• Performance Break-In Oil

Addition Parts Required:

• Crankcase machining is required
• 15mm connecting rod or Crank with 15mm Rod
• Throttle Body
• Fuel Injector
• Heavy Duty Head Studs
• Oil Cooler