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Athena Race Kit YZ250F 19-.. Big B.85mm EXTREME 300CC


The standard Yamaha engine is considered one of the best in the first part of power delivery: the initial thrust is exceptionally powerful, even though it is within the medium and high steady states, and strong initial progression seems to stabilize a lot.

Athena's Big Bore Extreme increases the displacement up to 304 cc, bringing the bike to have a very strong, effective and consistent engine output over the entire arc of the power delivery curve.


Extensive experience in the Offroad and Racing sectors has allowed the Athena R&D Department to create something that was not yet present on the market: a Big Bore Cylinder Kit with Ø 85 mm steel barrel dedicated to 250cc offroad bikes for an increase of displacement never achieved before, without making any changes to the engine blocks!

The entire kit guarantees maximum performance, better delivery, greater torque at low and medium steady states, better reactivity and - at the same time - the same reliability as original parts, thanks to all the technical devices resulting from the challenge won by the Athena R&D team: all this is combined with the proverbial good handling of a 250cc Offroad for a simply exceptional driving experience.

The kit includes the aluminum alloy cylinder, the forged piston complete with pin and rings, and all the necessary gaskets. 

Athena's technical team manages the entire development and approval process of the cylinder kits. The team designs the kit elements together, and then their test facility technicians test them and validate the parts before they are used directly by drivers. The result is a combination of engine components that ensure maximum synergy with each other.

More power, more performance and more fun: are you ready to never get off your bike again?



The cylinder with extreme Ø 85mm oversized bore is made of aluminum alloy and high silicon content.

To allow such an increase in bore while maintaining the original block, the Athena technicians decided to use a steel cylinder liner.

This cylinder liner is not inserted inside the cylinder bore with pressure, but instead, it is positioned directly in the mold through an innovative co-fusion production process. This method, studied and tested internally, makes it possible for the aluminum casting to "embrace" the steel, making it integral with the aluminum. This allows excellent heat transmission and an efficient mechanical grip that prevents reciprocal sliding, even after many hours of use, guaranteeing perfect union and reliability over time.

The inside of the cylinder liner is finished with a galvanic nickel-silicon coating treatment developed by Athena and subsequent polishing, to obtain maximum smoothness and optimal resistance to wear.

The winning feature of this new Athena project is to have developed a cylinder resulting from the union of two different materials, with particular attention to the design of the cylinder liner. The superior geometry of the cylinder liner was designed to ensure maximum sealing in the combustion chamber and to guarantee the right contact pressure between the components, under all conditions, avoiding pressure losses and leakage of liquids and/or gases.



The forged aluminum alloy piston is a guarantee of resistance and lightness. Furthermore, machining with latest generation CNC machines ensures perfect geometries and reduced tolerances.

The undercrown design has a fully open boxed bridged design to combine greater resistance to fatigue, lightness and better cooling of the piston crown.

The piston crown was engineered with particular care to optimize it for greater resistance, better heat dispersion and an optimization of the compression ratio to ensure greater driving elasticity and reliability over time.

The shell incorporates a molybdenum disulfide coating, which is applied by screen printing to reduce friction and noise, and improve sliding. The segments included are made in Japan out of special steel, with a chromium nitride coating applied with the PVD process, guaranteeing low friction, better smoothness, and a longer lifespan.

The gudgeon pin, specifically designed for this type of extreme application, is biconical and made of aeronautical steel alloy with a nitriding treatment with DLC "Diamond-like carbon" surface coating, which guarantees high hardness and low friction.



The set of gaskets included in the kit, developed by Athena technicians, were specifically designed for our new aluminum cylinder with steel liner, to ensure maximum seal and durability. 

The head gasket is an MLS (Multi-layer Steel) type, designed specifically to withstand increasingly higher pressures and temperatures. It was made using the latest generation laser cutting technology.

The material used is stainless steel with high elastic return, for high adaptability to the surfaces, and to guarantee an excellent, uniform seal on the whole surface. To further improve the seal capacity, it is equipped with specific raised points around the cylinder liner and the oil and water passages.

To maintain the synergy ensured by our internal design process, the cylinder base gasket is also made by laser cutting in stainless steel with a specific rubber coating.

The rubberized inserts and the raised relief around the cylinder liner are the Athena team's design response to effectively and constantly support the cylinder body subject to the closing load of the cylinder head stud bolts.

More Information
SpecsCylinder Kit
Bike specific Bike specific
Fits - UseBig bore kit
EAN / Bar Code805330071651
Ride Bike Model Year
Enduro YAMAHA WR 250F 2020
Enduro YAMAHA WR 250F 2021
Enduro YAMAHA WR 250F 2022
Enduro YAMAHA WR 250F 2023
Enduro YAMAHA WR 250F 2024
MX YAMAHA YZ 250F 2019
MX YAMAHA YZ 250F 2020
MX YAMAHA YZ 250F 2021
MX YAMAHA YZ 250F 2022
MX YAMAHA YZ 250F 2023