HOW TO BUILD AN INEXPENSIVE TWO-STROKE THAT JUNIOR CAN RACE IN THREE CLASSES

December 25, 2013
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At first glance, this machine might make you think that Honda is bringing back two-strokes for 2014, but that isn’t the case. Just looking at this bike, you would never guess that it is a seven-year-old Honda CR85 Expert. Or, that it was bought off of Craigslist for $700.

MXA was interested in a cheap, well-used CR85 Big Wheel because we thought that it would be the perfect transition bike for a young rider making the step up from the minicycle class to the big bikes. Imagine Junior getting another year or two on his downsized minicycle while racing the Supermini class, 125cc two-stroke class or 250 Novice class. Best of all, he could stay on an engine that he is familiar with and a chassis that is only a couple inches longer and a few pounds heavier than his minicycle. Best of all, this project can be applied to a YZ85, KX85, RM85 or KTM 85SX.


Tom Morgan Racing was in charge of the powerplant and went to town. He bored and stroked the engine from an 85cc stocker to a 112cc rocket ship.

The first step was to find someone who could take our Craigslist find and build an engine that would be competitive against bigger engines, so we took the bike to Tom Morgan Racing (TMR). Tom is well-known for his two-stroke expertise. TMR bored and stroked the CR85 out to 112cc. This 27cc increase in displacement added 9 horsepower and dented our bank account by $2500. With the TMR engine, we knew that we had the power to run with 250cc four-strokes, thanks to a favorable power-to-weight ratio.


With engine modifications came increased engine temperatures. ICW took the stock radiator and welded an additional tank to it to keep the engine cooler, along with adding a bracing stem for durability.

As for the almost-a-decade-old suspension, we sent our CR85 parts to Race Tech for their bolt-in Gold Valve assembly ($189.99). Sparkle Silver is the OEM Honda frame color, so we had the frame powdercoated a sparkle silver by San Diego Powder Coating for $225 (plus our sweat equity in stripping it down). When the frame and engine came back, the rest of the bike was inspected, lubed and put back together.

THE TRANSITIONAL FRAME

With MXA’s CR85 being seven years old, the suspension components were out of date and needed to be modernized. Race Tech offers a complete Fork Gold Valve assembly that makes updating this bike a breeze.

Not every kid is ready to make the giant step up to a 224-pound, 250cc four-stroke. And, not every parent is willing to bite the $8000 bullet for a bike that he can’t fix. MXA’s goal was to build a bike that would be legal in three different classes, didn’t cost an arm and a leg to own, and could be maintained in the garage by an attentive dad. Plus, Junior’s trusty old 85cc minicycle could be used as the foundation. We think that this project could be a win-win for many kids coming out of the 85cc class.


Fast mini kids require supreme braking power. Applied offers a dual-piston front-brake carrier and a 260mm oversize rotor kit that allows this mini bike to stop on a dime.

We chose a Honda CR85 Expert because it comes with bigger, 19-inch front and 16-inch rear wheels that allow the bike to accommodate a growing kid. We must confess to spending unnecessary money on cool stuff like RAD wheels, Works Connection components, DeCal Works graphics and Applied Racing triple clamps, but our goal wasn’t to limit our expenses on this project. We just wanted to create a road map that parents could follow.


The triple clamps are made by Applied Racing as a set, which has reversible bar mounts, allowing for a more aggressive bar position, either 8mm or 14mm farther forward than stock.


Of course, the stock OEM pipe was replaced to uncork this mini beast. FMF is still going strong on not only producing but also developing the two-stroke technology.


With a significant increase in horsepower, the drivetrain needed to be buffed up to support the load of the engine. A heavy-duty Renthal chain with matching sprockets was installed to ensure reliability.

Jumping from an 85cc minicycle to a bored-and-stroked, 112cc fire-breather was an eye-opener. The MXA wrecking crew was surprised by the abundance of low-to-mid power. Not that we didn’t expect it; it’s just that we didn’t think it would mimic the power curve of a four-stroke so closely.

For more information, go to Tom Morgan Racing at www.morganracingengines.com and Race Tech atwww.racetech.com.

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