If you have a carbureted CRF250 you are out of luck for an effective big-bore. The 2010 and 2011 CRF250s have thicker cylinder liners that offer more than enough room to go bigger.

As a rule, the MXA wrecking crew keeps our test bikes in stock trim. Even though we have access to every hop-up part in the world, we need to keep our bikes as stock as possible to ensure a fair platform for testing individual aftermarket products and different settings throughout the year. When we test major modifications, which would require us to totally remake our bike, we simply get another bike?keeping our test bike OEM perfect.

We consider our heavily modified bikes to be “project bikes.” We always have some work in progress where we are combining modifications and testing the effectiveness of individual parts as part of a race package. Some project bikes appear in the pages of MXA, and some are sent to the ignominy of the dust bin as abject failures. A project bike has to work well to make the cut, and last year’s Honda CRF264 project bike was one such winner.

The reason that our 2010 CRF264 was a success had as much to do with a decision in the Honda engineering department as any choices that MXA made. We could never get a big-bore CRF250 to work before 2010, because the cylinder liners weren’t thick enough to accommodate a larger bore?all that changed in 2010 when Honda went thicker, which led us to Millennium Technologies. They had been chomping at the bit to show what they could do to a CRF250 with a little more room in the cylinder.

Last year’s MXA CRF264 project bike didn’t have any tricks or mods done to it apart from the 264cc big-bore kit. The MXA gang didn’t want to be distracted by too many flashy gizmos, anodized parts or conflicting components. We wanted to focus on the modified cylinder and the big piston?that is what we brought to the party, and that was what we were going home with.

Now, a year later, we decided to dust off our venerable CRF264 and go for broke?in the classic “throw money at it” tradition. So, for this year’s CRF250 project, we set our bone-stock rule aside and decked out the bike with every treat that caught our eye. In no way was this an economy ticket?it was first class and big bucks. The all-or-nothing approach enabled us to put a lot more thought and effort into a bike that was already a success with just the big-bore 264cc kit.


The individual components of this year’s 264cc package come from many different companies, but the heart of the project is still the $500 264cc kit by Millennium Technologies. It includes the cylinder work, Vertex piston and Cometic gaskets. When a customer ships his stock cylinder to Millennium, it is bored out from 76.8mm to 79mm and replated with a Nickel Silicon Carbide coating. Vertex’s 79mm forged racing piston gives the engine a modest bump in compression (from a 13.2:1 rating to 13.5:1). There was no head porting on this project, although Millennium does perform this service with their five-axis CNC machine.

Cams: From our previous 264cc starting point, we added Hot Cams‘ Stage 2 camshaft. Since the Honda CRF250 only has one cam, the price for the cam upgrade is only $270. We minimized the cost even more by sticking with the stock valve springs.

Exhaust: FMF’s new RCT Factory 4.1 muffler, along with their titanium MegaBomb header and mid pipe, were bolted onto the cylinder. The spark arrestor/insert was unclipped, removed and set aside.

Throttle body: On the fuel delivery side, Injectioneering was recruited for the throttle body modification. A complete, isolated test of Injectioneering’s throttle body on our stock CRF250 can be found on page 134.

Mapping: Fuel mapping was tuned with a Bazzaz Z-FI MX Offroad mapping tool. It uses an air/fuel sensor bung in the head pipe to read the gas mixture. FMF doesn’t offer this service, so you’re on your own. The Bazzaz system averages the air/fuel mixture ratio throughout the rpm range and makes recommendations to get to the desired ratio.

The MB1-tuned forks were progressive and plush…a little too plush when spiked by the extra power of the QTM brake.

Suspension: MB1 Suspension was assigned to work on the CRF250 fork and shock. They addressed the CRF’s stinkbug stance with firmer 0.48 kg/mm fork springs and a softer 5.2 kg/mm shock spring. Additionally, a longer Ride Engineering shock link was used to lower the rear of the chassis for more setup options.

With the fork valving, MB1 tried to get a more progressive feel by softening the initial part of the stroke and ramping up high-speed damping to prevent bottoming in the last part of the stroke. On the shock, MB1 utilized their “High-Speed Adjuster Spring” (to help distribute the load at high shock-shaft speeds) and their “Works Bladder Cap” for more volume. We also added 21mm offset Ride Engineering triple clamps.

Ride Engineering’s 21mm offset triple clamps don’t cure what ails the CRF chassis, but they reduce the symptom’s severity.

Clutch: The CRF264 featured Hinson‘s SS (single spring) system. It uses a cone-shaped Belleville washer instead of six separate clutch springs. Although only the inner hub and pressure plate are needed for the SS setup, MXA’s project bike got Hinson’s billet clutch basket, outer clutch cover, clutch plates and clutch springs. The clutch was actuated by a Works Connection Elite clutch perch and lever.

Hinson’s SS (single spring) clutch offered a quick engagement, which complemented the CRF264’s bottom and mid power.

Wheels: Other pieces of moto jewelry were TCR’s turned-down and polished stock hubs and anodized stock rims. The TCR wheels were equipped with a Dunlop MX31 on the front and a 110/80-19 MX51 on the rear. A red anodized Renthal Ultralight sprocket was used in the rear. Renthal also provided the front sprocket, R1 Works chain, Twinwall handlebars, dual-compound grips and Moto hand guards. Ride Engineering bar mounts accommodated the larger-diameter bars.

Graphics: Without the telltale graphics, this bike could be raced in the 250 class with no one knowing the difference. But that would be cheating. To make sure that everyone knew how big the engine was, we had DeCal Works draw up a custom CRF264 graphics kit and printed backgrounds (best of all, they installed them on new plastic for us).

Miscellaneous: Moto Seat‘s Custom Cool seat cover had some design details that matched the custom DeCal Works graphics. Works Connection supplied some shiny parts, including front and rear billet brake master cylinder covers, engine plugs and chain blocks. Works Connection also supplied a Factory II stand and was thoughtful enough to install radiator braces in case the bike fell off the stand.

The rear brake system was modified with Fasst’s rear brake clevis and rear brake pedal return spring kit. The spring creates preload and allows the rider to tune pedal resistance and the feel of the actuation. For actual stopping power, QTM got the nod with their 270mm oversize front rotor kit.

Other performance trickery on the bike included LightSpeed’s titanium footpegs, a Uni Filter air filter, Bel-Ray lubricants, Applied Racing’s vent kit, and a CV4 fuel line heat wrap and silicone coolant hoses.


Increasing the displacement of any bike is a sure way to gain power. The most obvious hazard of going big is negatively impacting the location of the power or how it’s delivered. In truth, we liked our original CRF264 kit. It was worth the money?even with the stock pipe. But, it really came to life on top as we started to add more pieces. After one lap, testers knew that the modest displacement increase, increased compression and cam change kept the bike feeling like a free-revving 250F.

MXA‘s dyno tests of the stock CRF250 engine hinted that the 2010-2011 CRF250 produced a lot of peak and top-end power. Not so. On the track, every MXA test rider felt like he had to ride the bike in the midrange of the powerband?and the midrange only?because both the low end and top end were weak. Our original Millennium CRF264 made most of its power gains in the middle. Consequently, even though it made a little more power on top than stock, it felt flat on top. Last year’s 264 kit pulled strong through the middle, but it had to be shifted before letting it rev too high.

So, how did our full-race version of the same big-bore engine run? We feel bad, but with the exception of the FMF exhaust, which helped with top-end power, our new bike felt very much like the CRF264 that we started with. No shame. It was still a very powerful CRF250 (a.k.a. CRF264), but we expected more. We had, in the parlance of hop-ups, hit the hop-up wall. Each part individually benefited, but paired together, we didn’t get the crescendo effect.

Through tighter sections of the track, our new CRF264 worked well, but faster tracks required more shifting. This aspect made gearing more important. There were situations where we had to shift right before a series of jumps?which left us low in the powerband. Our solution was to gear the CRF264 up one tooth (one tooth less on the Renthal rear sprocket).

Some of our issues were related to Honda’s decision to fuel inject the CRF250. It seemed to make the stock CRF very electric and bland. The Millennium 264’s powerband had more personality and was more fun. We would hazard a guess that if we had ever been able to make a big-bore 2009 CRF250, we would have been a lot happier.

As for the MB1 suspension, it was smooth and fluid. There were no harsh spots in the travel, but it used up all of its travel easily. That meant that it worked best over small-to-medium bumps and gave a plush ride for 160-pound testers?not so with faster or heavier riders. The softer shock spring seemed good in principle, but we don’t think that would ever be our choice again.

We put a lot of effort into handling fixes to lessen the CRF250’s tendency to oversteer into turns and understeer out. That said, our project CRF264 was still nervous from time to time, but gave us a bit more confidence. Having tested different shock linkages on our stocker, we had no doubt that Ride Engineering’s link played a big part in calming down the steering input. And maybe, just maybe, Ride’s 21mm triple clamps (stock is 20mm) were a baby step in the right direction, but we would have liked a more significant change.

Things that we really liked about our project were: (1) The rubbery and tacky Moto Seat seat cover. (2) Renthal’s Moto hand guards. (3) We love LightSpeed’s 57mm-wide titanium footpegs, because Honda specs their bikes with vintage footpegs. (4) Hinson’s SS clutch had a bit more of an accurate friction zone that felt nice and racy once the test rider acclimated to it.



With all our modifications, the Millennium Technologies CRF264 was a potent race bike?but we would have liked to get more power with fewer mods. And, sadly, we think that the big-bore engine and the exhaust pipe were probably all that we needed. Perhaps on a stock 250cc engine, the cams, piston, pipe and other mods would have meshed more fluidly. The bottom line?whether you’re a cheater, Vet or play rider?is that cubic centimeters make the most horsepower per dollar.

crf250four-strokehinsonhondahonda crf250millenium technologiesmxarenthal