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CRF250-300 Big Bore Kit

CRF250-300 Big Bore Kit
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Price: $2,500.00

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Honda redesigned the CRF250 in 2010 after 6 years in the old configuration. The motor for 2010 was redesigned and completely different. Fuel injection was added and several other modifications were made to suspension and chassis. The power output on the newly designed motor seemed kind of lazy and lackluster at best. Bottom end power decent through the mid-range we felt the peak power and over rev was ok but the motor felt like it was missing the pull we have come to expect in a 250. We didn’t want to lose that 250 rev-ability we have come to love in our 250's.

When we first looked at this project we started out using the same formula that works so well on the old motor when we built the 270 kit: forged boxed style piston and ductile iron sleeve for strength. Since there was more room to work with on the 2010 bore we started out with a ductile sleeve and 83mm piston. We found after testing and several short lived cylinder and piston combinations copying the design of the old kit just wouldn't work. After months of design and testing, different, sleeve styles, pistons, compression ratios, bore sizes, cams, porting, weeks of dyno and 100’s of hours of track tuning, fuel injection mapping and pushing the motor to its limits and beyond.

We built the ultimate 250 big bore kit. For 2010 the stock cast aluminum cylinder is good for a 2mm overbore (264cc) any larger and you risk cracking a cylinder. For the 300 kit Thumper went back to the drawing board using our Cool Flow sleeve design which increases the water to cylinder contact area by 25% in this kit. Thumper used a billet aluminum sleeve with 2x the tensile strength of the stock cylinder which allowed us to take the piston out to 81mm and still maintain cylinder wall thickness, integrity and strength. In order to make the 300 it took a little case modification and we found we could add another 4.2mm of stroke which gives a full 58mm stroke verses the stock 53.8mm stroke. This netted a 300cc stroker motor and transformed the CRF250 into one of the best 250 big bore motors Thumper has ever built. Most big bore 250’s don’t rev out anything like a stock 250. One of the main goals was to keep the light fast reving 250 feeling without making it overpowering or unridable.

The stock cams left the big bores feeling very strong on the bottom and mid but in typical big bore fashion they left the motor a little flat and needing to be ridden more like a large bore bike and was happiest being short shifted. The stock cam gives the motor incredible bottom and mid power which is great for woods and off road riding but for MX and faster desert riding we needed more topend and over rev. We tried 10 different cams before we came up with a cam option that would ensure the larger motor pulled down low and wouldn't feel flat on top like the typical 250 big bore. Because of the installation and case machining involved in this engine, the 300 kit is only sold as complete kit installed in your motor. The motor must be sent into Thumper Racing for installation. Fits 2010+ CRF250r Kit includes: Billet Aluminum Cool Flow sleeve installed in your stock cylinder and Nikasil plated, custom 81mm 12.5:1 compression box style piston, crank stroking and balancing, case modification, head porting, gasket set, installation labor -engine blue printing, cam, valve shimming and fuel injection map. Please note the Fuel injection mapping must be changed any time a change is made from the stock motor settings to ensure peak performance. Thumper Racing will supply a FI map that your dealer or you can download to the bikes CDI provided you have the FI mapping tool OR we offer an piggy back fuel controller premapped with our custom mapping for this kit.

The is a closed loop fuel injection option as well that will make all other controllers obsolete. The close loop system maps for any altitude, temperature, humidity you ride in as well as will automatically make mapping recommendations for any modification done to the motor you can add cams, exhaust, port the head or anything else you want to do and the close loop system will monitor and recommend changes. No dyno tuning will ever be needed.
Call us for more details on the closed loop system.

CRF 300 big bore kit $2500

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