Chimera Engineering CBR 250/300 Engine Bolt-on Swap Kit - Honda Monkey (2022+)
Chimera Engineering CBR 250/300 Engine Bolt-on Swap Kit - Honda Monkey (2022+)
  • SKU: CH-1374

Chimera Engineering CBR 250/300 Engine Bolt-on Swap Kit - Honda Monkey (2022+)

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    Chimera Engineering offers engine swap kits for 250/300cc engines, which allow you to install either a Honda NC51 (300cc) or a Honda MC41 (250cc) engine onto your 2022+ Honda Monkey 125. With this upgrade, you can enjoy the benefits of a water-cooled Honda engine with six speeds, providing your Honda Monkey with increased power and reliability. What's more, there is no need for welding as this is a true bolt-on engine mount.

    • Honda Monkey (2022+):
      • The engine mount kit includes everything needed to bolt-on a NC51 or MC41 engine onto a 2022+ Monkey chassis. The kit does not require welding, but you will need to cut the right side engine tab for clearance. Although you could bend it for clearance, it is at the installer's discretion to modify this area. Additionally, you will need to cut a retaining tab on the lower left side of the frame near the kickstand.

        The Chimera CBR Engine Swap Mount Kit was designed specifically for the 2022+ 5 Speed Honda Monkey! The Chimera engineers found that the same dual stacked plates design was still the best solution to alleviate harmonic vibrations over the solid thicker 1pc design that others have made. Overtime, the single thick plates the other brand uses would transfer the vibration frequency more severely, in turn, would cause stress cracks on the frame over time.

        Please review and read everything below carefully. There are many options listed in the drop-down menu so that you could customize the kit to work with your preference and set up.

        Engine Mount Kit Content ($265.00):

        • 4x Chimera bolt-on engine mounting brackets NC51/MC41 engines
        • 7x Chimera Spacers
        • 4x M10 Engine nuts and bolts
        • 6x M6 Frame to Mount plate bolts and nuts
        • 2x Chain Guide Bearings


        • Chimera CNC Billet Aluminum Intake Manifold Elbow (SKU: CH-1239) OR Chimera Silicone Coupler Elbow (SKU: CH-1144)
        • Chimera Front Mount Radiator for Honda Grom 250/300 engine swaps (SKU: HND-1095)
        • Chimera Radiator Hose Kit for Front Mount Radiator (SKU: CH-1147)
        • Chimera Race Header for Honda Grom w/Chimera CBR Engine Swap Mounts (SKU: CH-1123)
        • Aftermarket ECU (SKU: ASG-1042X)
        • Standard Length Chain (+$53.95-$100)
        • Front Sprocket (+$22.95)
        • Rear Sprocket (+24.99+$30.99)
        • EGR Valve Cover Block Off Plate (SKU: CH-1221)
        • Coolant Over Flow Tank (SKU: MPRO-1041)
        • Breather Oil Catch Can (TBA)
        • H4 to SF Headlight adapter (55.00)

        Donor parts that are needed (USER SOURCED, NOT INCLUDED WITH KIT):

        • -Honda NC51 or MC41 engines, commonly found on CBR250, CRF250F, CBR300, CB300, and Rebel 300.
          • NOTE: For all NC51 300cc engines before 2016, there is a CRANKSHAFT recall on them by Honda. If you’re purchasing a complete donor bike, you would want to take it to the dealer to perform the crankshaft recall before installing it onto the Grom. Aftermarket performance crankshaft is also available to stroke the engine (details would be provided in the near future). Special Thanks to Dominic in TN for pointing this out!
        • Honda CB300F OR CBR250/300 wire harness.
          • We recommend the CB300F harness because it requires the least amount of work to get it to function on the Grom chassis (due to the speedometer). Modification is required for the harness, so that it would cleaner and some wires will need to be extended/lengthen.
          • The CB300F harness allows you to reuse the original Grom speedometer OR a CB300F Speedometer. The CB300F speedometer is the same as the 1st/2nd gen. Grom unit, so it’s a direct-replacement, however it will display higher RPM reading (12k RPM) and it has a coolant temp light. If you decide to keep the Grom Speedometer, max RPM reading is 10k RPM and it doesn't have a coolant temp. light.
          • If you have the CBR250/CBR300 harness, then you're require to run a CBR250 or CBR300 speedometer (requires custom mounting bracket) or a jumper harness to use the CB300F or Grom Speedometer. Typical harness modification is needed as well.
          • Regardless of what harness you use, it would require all the supporting relays, sensors, and modification to the harness.
        • Radiator and/or Radiator Fan:
          • If you go with Chimera Front Mount optional radiator, then you would need a radiator fan from one of these bikes: CBR250, CBR300, CB300, or Rebel 300.
          • If you decide to run your own radiator, then you would need to source something that could handle the cooling system of a 250cc or 300cc engine. If the radiator is too small, you will run the risk of overheating.
        • Throttle Body:
          • Any throttle body would work between the two models of engine.


        1. The chain guide is made with bearings, therefore it's a direct bearing-to-chain contact. This would produce a chain whirling noise as the chain rolls over the bearings. Very little noise would occur during acceleration or cruising. However, during deceleration in gear (engine braking), the noise would be much louder. Chimera thought of many ways to correct this, however all of the method would result in a wearable item, which they didn’t want.
        2. If you decide to run the Chimera Front Mount Radiator, trimming to the inner plastic shrouds are required to clear the width of the radiator.
        3. The OEM ECU from the NC51 or MC41 engines will not work alone. You would need to add a piggyback fuel controller OR run a full standalone such as ARacer. We highly recommend the following ECU
            1. ARacer RC Super 2 with AF2 option (RACE OPTION: Advance tuning features, full control of everything)
            2. ARacer RC Mini 5/Mini-X ECU with AF2 option (BEST OPTION: for price, tuning features, and user friendliness)
        4. 2022+ Model Monkey owners who wish to retain the stock led headlight will require to purchase Tripage headlight conversion sku TRIP-1001.
        5. If you ever decide to put the original 125cc engine back on, you’re able to unbolt everything CBR related and reverse it, but you would need to reweld or purchase the Chimera Switch Back bolt on bracket to put the mounting tabs back on for the 125cc engine.

        WARNING: Due to the short standard wheelbase of the Monkey, the new swap will have the tendency to wheelie during initial acceleration, rapid low speed accelerations, or while changing gears. These kits were designed for off road use only. 

        ⚠ WARNING: California Proposition 65


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