YZ conversion

Ratranger

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So last year I built a cr80 with an xr100 motor, it's a blast but a little on the small side for long trails. So starts a new build, 1998 yz125, with an xr200 motor.
The motor is going to be a bit mix n match, xr200 bottom end with the crank stroked 8mm, atc200x cylinder for the big fins, crf230 piston because it has a 4mm smaller deck height, crf230 head because bigger valves and straighter ports, crf cam modified for XR ignition.
Frame will eventually get an 18" rear wheel, and sprung/valved for my weight.
20211106_115634_HDR.jpg
20220115_124526~01~01.jpg
 
So last year I built a cr80 with an xr100 motor, it's a blast but a little on the small side for long trails. So starts a new build, 1998 yz125, with an xr200 motor.
The motor is going to be a bit mix n match, xr200 bottom end with the crank stroked 8mm, atc200x cylinder for the big fins, crf230 piston because it has a 4mm smaller deck height, crf230 head because bigger valves and straighter ports, crf cam modified for XR ignition.
Frame will eventually get an 18" rear wheel, and sprung/valved for my weight.View attachment 205437View attachment 205438
'Splain to me like I'm a five year old what you're doing in the lathe pic?
 
I plugged the hole in the crankshaft for the pin, tacked it in place, then using a fixture i made drilled/bored a new hole offset 4mm. This increases the stroke once the crank is reassembled by 8mm. Combine that with the 1mm over piston and it'll go from a 195cc motor to a 223cc.

The CRF230 is basically an updated xr200. Same bore, longer stroke. The crf piston has the wrist pin 4mm closer to the top than the xr does. So instead of doing like powroll used to and squishing the rod to make everything clear, you can just drop a CRF230 piston in.
 

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I plugged the hole in the crankshaft for the pin, tacked it in place, then using a fixture i made drilled/bored a new hole offset 4mm. This increases the stroke once the crank is reassembled by 8mm. Combine that with the 1mm over piston and it'll go from a 195cc motor to a 223cc.

The CRF230 is basically an updated xr200. Same bore, longer stroke. The crf piston has the wrist pin 4mm closer to the top than the xr does. So instead of doing like powroll used to and squishing the rod to make everything clear, you can just drop a CRF230 piston in.
That's some cool stuff right there.
Your skills and builds... wow
 
Some progress to note. The rear mounts are done, lower is tabs on the frame, upper uses the swingarm bolt. It's a tight fit, but it'll be fine. Next up is front mounts, and the new cradle.
20220122_120139.jpg
20220122_120146_HDR.jpg
20220122_151846_HDR~01~01.jpg
 
This is slowly coming along, doing some work on the crank and cleaning up the ports. Haven't decided on a cam yet, I kinda want something with more than stock, but I'm not sure I want to spend $3-400 on a cam.
 
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