kx 85 105cc big bore kit owners

4 replies to this topic
  • Dad Knaus

Posted August 06, 2009 - 08:33 AM


Looking for someone to confirm that a 105cc big bore kit will work on a kx 85. Do I need extra parts to make it work?

  • JBFL

Posted August 06, 2009 - 08:56 AM


52.50 bore 2mm stroke.

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Posted August 06, 2009 - 10:59 AM


52.50 bore 2mm stroke.

This is actually a 103cc. The 52.5 is a stock 100 bore. The stock strokes of the 85 and 100 are the same (45.8mm). A 105 is stock stroke, 54mm bore. 54.5 is a 107. You can get a +2mm stroker crank to get the following:
1. +2mm on 48.50 bore (stock 85) = 88.3cc
2. +2mm on 52.50mm (stock 100) = 103.47cc
3. 54mm bore on stock (45.8) stroke = 104.89cc
4. +2mm stroke on 54mm bore = 109.47cc
5. 54.50mm bore on stock stroke = 106.84cc
6. +2mm stroke on 54.50mm bore = 111.50cc

FYI...The max for any AMA supermini bike, if you plan to race an AMA sanctioned event, is 112cc.

When boring out cylinders, the intake and exhaust bridges will weaken under extremely hard riding. I prefer sleeving them. They tend be be stronger. If you assemble the motor yourself, you can have a Sick 112 for under $800. BUt be prepared, with gain of power comes loss of longevity. I build all my motors to exact tolerances. But when something as simple as a main bearing starts to go, it usually takes a few other things with it.

If you're looking for more power, a stock 100 setup or even a stroked 100 setup will be great. My son loved the 103 build. Gobs of lowend power and topped out just right. Believe it or not, this ran best on the stock 100 jetting, stock needle setting, Bills pipe and PC 304 shorty. The air screw was 1 1/2 turns out (stock 1 3/8).

  • Dad Knaus

Posted August 06, 2009 - 02:11 PM


The bore is 54.5mm. I have documented the problems on another thread posted a couple of threads below this one titled kx85 big bore overheating. If someone could read that thread it would be a great help. Thank you, Scott

  • JBFL

Posted August 06, 2009 - 02:48 PM


Hah can ya tell I don't have the kid on a super mini yet DOH!!:thumbsup:

I buy rings once a month and a piston every other month. Those Wiseco 52.5 and 54's are way more money!! Kid better start mowing lawns or something!

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