(1) Replace the stock Nikasil liner with a steel sleeve (something that most tuners were opposed to).
(2) Try to squeeze a few extra cc out of a miniscule 1mm bore job (which at best would give you a 256cc engine). No matter how you cut it (or bored it), a big-bore CRF250 faced a major disadvantage against the KX250F, RM-Z250 or YZ250F, all of which could be bored out 2mm or 3mm.
As a result, Vet racers, play riders and offroad guys on Suzukis, Yamahas or Kawasakis could tool around on powerful big-bore engines, while CRF250 riders were not allowed to join the club.
THE RESULT? 263.71CC. ON THE DYNO, THAT EXTRA 15cc WAS GOOD FOR OVER 1-1/2 HORSES AT PEAK, BUT, BEST OF ALL, IT FILLED IN THE LOW-END THAT IS SO SERIOUSLY LACKING ON THE 2010-2012 CRF250.
Suddenly, with a snap of the Honda engineers’ fingers, the CRF250 could be turned into a big-bore.
How big? The stock bore and stroke on the 2010-2012 CRF250 is 76.80 x 53.80 (for 249.23cc). The first step was to consulted with the technicians at Millennium Techonolgies (www.mt-llc.com), and they felt that we could safely increase the bore by 2.2mm (pushing the bore from 76.80mm to 79.00mm). The result? 263.71cc. On the dyno, that extra 15cc was good for over 1-1/2 horses at peak, but, best of all, it filled in the low-end that is so seriously lacking on the 2010 CRF250.
Of course, when you are building a project bike, you need to dot all the i’s and cross all the t’s, so we started thinking about our other complaints about the 2010 CRF250 and how to address them while we had the cylinder off.
SHOP TALK: WHAT IT’S ALL ABOUT?
While we were having the cylinder bored and replated at Millennium, we decided to go after the rest of the CRF250 with a six-pronged attack:
(1) MB1 Suspension: The 2010-2012 CRF250 comes with Showa suspension. The 48mm forks have Kayaba-style internals and new fork pistons. The Showa shock layout is a clone of the CRF450 shock, and it has also been moved lower and farther forward in the chassis.
Forks: The first thing MB1 did on the Showa forks was to change to stiffer springs (going from the stock 0.44 kg/mm to 0.46 kg/mm springs). This vastly improves the CRF250′s fore/aft balance and cuts down on the CRF250′s death-defying head shake. Additionally, MB1 reworked the Showa’s compression valving to give the forks a plush action, but with a firmer feel. We ran 335cc of oil with the rebound on eight and the compression on nine clicks out.
THE STOCK 2010-2012 CRF250 ENGINE IS A PURE MIDRANGE POWERPLANT. IT HAS A SURPRISING LACK OF LOW-END (GIVEN THAT EVERY FUEL-INJECTED 450 PUMPS OUT ABUNDANT LOW-TO-MID POWER) AND IS FAIRLY FLAT ON TOP.
On many big-bore 250s, you can just focus on the power plant and leave the rest of the bike alone. Not so with the 2010-2012 Honda CRF250. Making more power puts even more stress on the chassis and suspension.
(2) Shock linkage: The MXA wrecking crew heartily recommends a revised shock linkage on the CRF250. Honda gave the 2010-2012 CRF250 a 27.15 head angle (it was 27.90 in 2009). This radical head angle was part of Honda’s new chassis theory, but, in our opinion, they took it too far. The longer link lowers the rear of the bike, kicks the head angle out, offers more fork height options, and works well with MB1′s revalved shock. Aftermarket rising rate links are available from Pro Circuit, Factory Connection, Ride Engineering and MB1.
(3) Hoses: Although we have had good luck with Honda’s stock radiator hoses, CV4′s silicone coolant hoses are more reliable, stronger and color-coded. We also used a higher-pressure CV4 radiator cap from www.cv4.net or (800) 874-1223.
(4) Graphics: MXA has had a very good relationship with Face Lift Unlimited (FLU) graphics, and the bold red, white and blue design of MXA’s CRF264 graphics is a testament to their creativity. For more information, go to www.fludesigns.com or call (661) 256-2313.
(5) Controls: For the pilot’s compartment, we mounted oversize Renthal TwinWall 997 bars (www.renthal.com) to Applied big bar mounts (www.appliedrace.com) with a Works Connection Elite clutch perch (www.worksconnection.com).
(6) Fuel: We elected to run VP Racing Fuel’s U4.2 race gas (mixed 50/50 with pump gas). Our big-bore engine can be made capable of running on pump gas without pinging, but U4.2 adds a healthy dose of power (if you can afford it). For more info, call (210) 635-7744 or go to www.vpracingfuels.com
TEST RIDE: A BIKE WE WAITED FIVE YEARS FOR
The only CRF250s that you can use this big-bore CP piston in are the 2010-2012 models.
Before we tell you what our 264cc Honda CRF250 ran like, we need to discuss how much the engine work cost. Millennium bored and plated the cylinder to 79mm for $234.95. The matching forged CP piston (13.5:1) set us back $278.95 (www.cppistons.com), and the Cometic gasket set was $47.67. The total cost of going from 250cc to 264cc was $561.57 (if you take the engine apart and put it back together yourself).
The stock 2010-2012 CRF250 engine is a pure midrange powerplant. It has a surprising lack of low-end (given that every fuel-injected 450 pumps out abundant low-to-mid power) and is fairly flat on top. If the MXA test crew had our druthers, we’d want the stock Honda to produce a broader powerband?so that there were more options in tricky situations.
When we first started testing the CRF264 engine, we ran it with the stock exhaust system. It produced potent low-end power and was much stronger through the middle. Most of the Vet and Novice test riders loved the powerband, but faster test riders, as always, wanted more top-end. Our quickest and simplest solution was to install a $549.99 FMF Factory 4.1 Ti slip-on system and $299 PowerBomb head pipe. This added a ton of over-rev and, while it took away some of the chunky low-to-mid of the stock pipe, it broadened the powerband and made the CRF264 much easier to ride.
Strangely, the powerband was so broad that it lacked the kind of hit that we felt would make it a better race bike. The quick fix was to gear it down one tooth on the rear. What’s strange is that we rarely gear down big-bores, because the extra power makes up for any gearing flaws (which the 2010 CRF250 doesn’t have in 250cc mode).
As for the MB1 suspension fixes, we wouldn’t want to race a 2010-2012 Honda CRF250 without stiffer forks. In stock trim, it is a busy bike. It dives under a load and head shakes like a wet dog with the stock spring rates. The MB1 suspension ironed out the balance issues and made the 2010-2012 Honda CRF250 calmer at speed.
CONCLUSION: WHAT DO WE REALLY THINK?
This is a bike that we couldn’t build with the old Honda engone (because the previous Honda CRF250′s didn’t have enough meat in the cylinder liner to make it a safe choice). We think that Honda CRF250 riders will find the ability to punch up the horsepower and broaden the powerband to be more than worth the $600 that this engine kit costs.