SKU: UR3401-MXRX Categories: ,


Availability: 2 in stock (can be backordered)

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This is the MXR “Team Edition” engine from Ultimate Racing. The “Team Edition” concept uses the same Ultimate engines that are for sale, but the engines are assembled directly in our factory, completely broken-in and then thoroughly checked by our competition team to extract 100% of the performance of Ultimate engines.

There are many manufacturers that offer pre-break-in services performed on benches with propellers. The problem is that this method only works as a pre-break-in. It is still necessary to continue breaking in the engine on the track.

At Ultimate Racing we wanted to go a step further and offer our customers the possibility of purchasing fully broken-in and ready-to-race engines without the need for any additional break-in on track, i.e. engines that can be used in the most demanding competitions as you receive them, directly from the box.

To ensure quality and that all “Team Edition” engines are broken-in exactly the same, we have evolved a fully monitored break-in bench where temperature, RPM and break-in time are controlled at all times. The whole break-in process is performed with the engine perfectly lubricated to minimize the loads and wear of all the parts, significantly increasing its useful life.

During each of the break-in stages the engine’s resistance torque is also controlled. Using this parameter, we know when the engine break-in is finished, ensuring that all of our “Team Edition” engines are perfectly broken-in and ready to compete.

Once the break-in process has completed, the engines are dismantled, checked piece by piece and assembled again by our competition team, thus ensuring the highest quality of product reaches our customers.

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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