SKU: UR3401-MXR Categories: ,


Availability: 1 in stock (can be backordered)

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This is the Ultimate MXR Ceramic 3 Transfer .21 Competition Engine. Like its predecessor the MXS, this engine is manufactured by the prestigious Japanese brand O.S. Engines, in this case based on their square stroke Speed B21 engine. Apart from the modifications that the MXS already had, such as the crankshaft and back plate, the sleeve of the MXR has been modified, modifying the ports to be able to vary the inlet and exhaust timing. The inner head has also been modified and includes the innovative FlowMax venturi to improve performance, more specifically, to achieve greater power without compromising fuel consumption.

With this engine you will have a really good power band with a ton of usable power. It has a smooth bottom end which worked well on the bumpy track and then has a lot of mid-range pop and feel to it. It is one of the best idling engines you can find on the market.


The MXR is equipped with the same cooling head as its predecessors (MXS, MTS). This 8-fin cooling head has a very low center of gravity and has been designed to focus most of the airflow directly to the glow plug. By cooling the glow plug we get a more stable carburetion and idle.


Once again, the MXR comes with the 21 J3 carburetor. Unlike the original OS carburetor, its high-speed needle holder was redesigned for a better refrigeration of the fuel flow into the carburetor, lowering the temp of the carb, and consequently it is more stable with a great idle stability, improves fuel mileage and overall engine performance.

Besides this, the uniball joint of the carb slide was also modified with two stoppers instead of one. With this new modified design, you can easier adjust the angle of the uniball joint without having to disassemble the engine.

This carb offers a higher accuracy tuning, a very interesting advantage to experienced drivers.


Same as in the MXS, the MXR backplate has been modified. The backplate is CNC machined and made of 7075-T6 Swiss Aluminum. It has been milled to improve the gas flow going to the piston and improving engine efficiency and performance.

Thanks to its design, you can remove the backplate without damaging the conrod even when it is on its lower position (PMI). The lateral grooves allow to easily remove the backplate without having to use any tool that can damage it.


The crankshaft of the MXR is an evolution of the DLC B2102 but with two balanced tungsten in order to increase the inertia to rotation.


The MXR comes with the original OS ceramic bearing in the rear and Ultimate Racing steel double sealed in the front.


The MXR comes with the 6.5mm Ultimate Racing FLOWMAX venturi. With an innovative design, longer than the standard O.S. venturis and wider at the base, unlike all others, makes the inferior part of the venturi and the carburetor have exactly the same diameter. This new geometry improves fluid dynamics, providing extra power that allows the use of smaller venturis without decreasing power, and as a consequence improves fuel mileage.


Starting from the great base of the MXS, the sleeve of the MXR has been modified, modifying the ports to be able to vary the inlet and exhaust timing to achieve more power without compromising fuel consumption.

The sleeve with 3 inlet port and an exhaust port guarantees a low fuel consumption with a very high but smooth and linear power.

Both, the sleeve and piston are made from a high strength aluminum bar to improve its durability.


Displacement: 3,49 cc

Power Output: 2.65 PS/2.61 hp @ 34,000 rpm

Practical Range RPM: 4,000 – 42,000 rpm

Bore x Stroke: 16,40×16,40mm

Sleeve: 3 + 1 ports

Rear Ball Bearing: ceramic

Front Ball Bearing: steel

Crankshaft: 14 mm

Carburetor: 21 J3

Glow plug: P3 Turbo

Exhaust Position: rear

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