Project KX250F: The Heart of an Engine 13

Article and pictures by Michael Lindsay

Walking the pits at the Supercross season-opener at Anaheim, I caught myself (as usual) drooling over the bikes used by the top 250 teams, and wondering what it would be like to ride one of their machines. Once I returned from my time-wasting daydream, I was now a little bummed, knowing I would never have the opportunity to ride one... but, I could build something myself. So with the racing over and a budding concept in mind, I went home, pulled the engine out of a KX250F, and got to work.

There is no shortage of aftermarket companies in MX to choose from. You can choose to go for flash or function...or sometimes even both at the same time. You can truly customize and personalize your bikes like never before. After a little brainstorming while tearing down the engine, I decided I was going to go all out. I didn’t want to leave anything to chance, so I took on a higher price tag in trade for reliability and peace of mind down the road.  After some spinning some T-handles and wrenches here is what I ended up with.

Starting off with the backbone of the engine, I needed a piston and rod, which led to a trip down to CP-Carrillo to snag one of their "Platinum" piston and rod kits ($595.00). What makes this kit so special? Outside of a higher compression piston (14.2:1), the Carrillo rod in this kit is 1.5mm longer than the stocker. Now I’m sure you’re thinking, "Wait, doesn’t that make it stroker?" Not in this case, because the piston pin is also moved up 1.5mm into the piston.  This means the piston is still traveling the same length as before (retaining the stock bore and stroke). The advantage with the rod being longer is that it's at less of an angle every time the crank pushes the rod and piston up and pulls it back down, meaning the rod is actually traveling straighter, allowing for less side load on the piston against the cylinder wall. On top of that, it also means the piston spends a little more time at top dead center, for a more complete burn during each ignition period. In the past this was something that was normally only available to race teams, who would present the specs of what they wanted to have made to piston and rod manufacturers, and pay to have a small batch made. Thanks to CP-Carrillo’s massive in-house manufacturing capabilities, knowledge, and experience in both the piston and rod market, they were able to bring this level of performance to the general consumer. Their Platinum rod & piston kits are available for most current four-stroke motocross bikes and performance ATVs.

From there, I jumped into the heart of the engine, the crank. Since the Carrillo Rod needed to be placed between the crank halves, I took the opportunity to take it a step farther and sent the rod and stock crank off to Crank Works in Arizona. Having built cranks for Star Racing, Kawasaki Team Green, and many other teams (that choose to keep it on the down low), they're known as one of the best, not just for rebuilding cranks, but modifying and building custom cranks to whatever your needs may be. Once it arrived, they disassembled the stock crank and went to work. They lightened ($285) the crank halves (removed mass) and put the halves through their friction reduction process ($195). All of this was done in mind to not only improve power, but to get a lighter and snappier feel from the engine. From there, they assembled the crank with the supplied rod and balanced ($200) it based on the weight of not only the rod, but the piston, clips, rings, and wrist pin to help eliminate as much vibration and unwanted wear as possible. Then they trued it and welded ($65) the halves to the center pin to keep everything in place.

After cleaning the cases and changing any oil seals, (it's always better to be safe than sorry, once you're in this deep) it was time to re-assemble. First up, I slid in a freshly undercut and polished transmission done up by RCR transmission ($300). The undercutting improves the ability of the gears to engage properly, and I had this done because the stock KX250Fs can be a little “notchy” and harder to shift than some bikes under a heavy load. It’s always nice to have a bike that shifts like butter, so there are no worries the next time I’m trying to upshift coming into a jump face.

Next up was instillation of the crank and torquing the center case bolts (always have the torque specs in something this critical). With the cases back together, I installed an absolute must...if you guessed Hinson clutch, then you’re right on the money. When adding more power, one of the last things I wanted to skip out on was the clutch. With better oil flow, wear resistance, and a more consistent feel under heavy abuse; there is a reason why almost every major team in the pits runs Hinson’s clutch components.  After installing their basket, inner hub, clutch plates, pressure plate, springs, and clutch cover ($1094.99 complete kit) the bottom end was buttoned up and ready to go up from there.

Next up was the CP piston, which has a higher compression ratio than stock (14.2:1 - up from stock’s 13.8:1), and is also lighter than a stock piston. CP-Carrillo also includes a DLC-coated wrist pin, circlips, rings, and a Cometic gasket kit, for a nice all-in-one package. Once it was all in place and a new OEM cylinder slid snugly over it, I turned my attention to the cylinder head. The guys over at Tokyo Mods took the head and gave it some TLC with a port and polish job ($495.00) to improve flow and efficiency. While they were at it, they replaced the stock valve seats with copper beryllium seats ($139.95 each installed), which dissipate heat more efficiently, offer a better valve seal, and reduced valve wear. Valve wear was particularly important, since I opted to add a set of Del West flat-faced titanium valves ($99.95 per valve). The majority of stock bikes have domed valves. The “flat-faced” valves add more compression since they take up more area inside the combustion area.

Before closing up the engine, I added an Xceldyne valve spring kit ($389.95 includes springs, retainers, and spacers), to better support the new and slightly heavier valves (flat-faced has more mass), and to keep them opening and closing consistently and reliably. To top them off, I added a set of their DLC-coated buckets ($79.95 per bucket), which have reduced friction and wear from the cam lobe contacting them (very useful if you’re running an aggressive cam grind). Sliding into the cam journals was a set of Hotcams ($179.95 intake, $199.95 exhaust). For most applications, they produce quite a few specs to accomplish what you’re looking for (check their website for individual cam specs). In this case, I grabbed a set of Stage 1 cams, looking for more mid-range out of the motor and more rideability than a dyno king. With everything in place, I timed the engine. To ensure that it would stay in time, I grabbed a manual cam chain tensioner while I was over at Tokyo Mods ($99.95). The manual tensioner is more reliable than a stock automatic adjuster, but requires you to re-check it every so many hours as the cam chain stretches. With reliability in mind, quality oil is a must. Having put about 99% of my riding time since 2006 on four-strokes, I have some preferences when it comes to engine oil. For this I chose to run Motorex’s CrossPower 4T 5w40 to keep everything moving in a smooth, working order.

With everything torqued to spec and back in the frame, what’s left? On the outside of the engine, there were a few things to address. First up, I took the throttle body over to Wade Wilcox from Injectioneering ($295). Wade has taken care of many of the top team’s injection duties (Factory Connection, TLD, Kawasaki, etc.). Wade modifies the stock body with a different butterfly and some other internal magic. The goal is to improve power and throttle response, most notably in the first 25% of the throttle. Once the throttle body was reconnected to the cylinder head, I went to the other end of it to the airbox. I replaced the stock air filter and restrictive cage with one of DT1’s filter cages ($209.99) and air filter ($25.95). This not only makes the air filter easier to install, but it also eliminates the stock backfire screen to promote more airflow.

Now to the other side of the motor, where I capped it off with a longtime favorite, an FMF 4.1 RCT/mega bomb exhaust system ($899.99) to complement the other modifications to the motor.

Finishing it off, I took off the stock ECU box and replaced it with a Vortex ECU from Tokyo Mods ($750). The Vortex box allows for the use of ten different maps, in this case it was setup for the race motor with the rev-limiter raised by 350 rpm. Since the motor is no longer suited for pump gas, I grabbed a can of VP’s T4 fuel ($60.99 MSRP for five gallons). VP does produce more “potent” batches of fuel, which would produce more power, especially with the proper dyno tuning. For my case however, the T4 blend is more affordable to run constantly and still has additives to help the performance of the modified engine.

All said and done, it may look like a large laundry list of items, but this about covers the basics of a full engine build. Could I have done more? You bet, and it can be a bit of an endless road. I could have decked the cylinder to raise the compression even higher, had custom cam grinds built, and replaced all the internal bearings with ceramic bearings for less friction. Then there's the option of spending more dyno time to further customize the mapping for a higher end fuel, and even more exotic parts. All of this is where the top shelf race teams and experienced motor builders have their advantage. Time and testing are invaluable resources. With this in mind that’s why I went the direction I did. It was built with high quality parts to keep it reliable and fun, plus the grin on my face while building it will almost be as big as the grin when I kick it over to ride it.

Check out Vital MX for the additional installments on this build! Next up we'll take on a trip through FMF and put this bad boy on their dyno, before we finally take it out, put some laps on it, and find out if all the hard work was worth it.

Check out the companies involved in this build: - (949-567-9000) - (480-897-1746) - (888-457-9403) - (909-946-2942) - (800-990-2779) - (336-475-0201) - (515-402-8005) - (763-417-1377) - (310-935-2915) - (310-631-4363) - (210-635-7744)

RCR Race Transmissions - (407-880-6163)


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