The engine case of the KDX is wide than the KX so the swing arm mount or rear motor mount as it is called needs to be modified. 1 things need to be done. The rear motor mount needs to be reamed 2mm wider as the KX swing arm pin is 2mm thicker than the KDX. The good news is the rear motor mount seems solid, but it is not so drilling is pretty straight forward. Do not cut the cases as the motor mount is a pressed in sleeve about 1 inch in depth on each end. In-between is actually hollow space that bears no load. See the Purple color. The sleeves are shown in yellow. The KX125 swing arm will hit the cases about where the red lines show.
The motor mount sleeve is shown below, looking into it. You can see inside where it stops. If you cut the case you will remove most of the motor mount! If I recall it is 15mm and needs to be drilled out to 17mm. Measure first.
So how does the swing arm it. Well the swing arm has a lot of meat on it and between the bearing and where he swing arm meets the engine case is mostly air and non load bearing. Cut it but measure first. See graphic below. It is big and high resolution so watch out. You can se where the needle bearing stops until the dust seal at the end is just air. Download it and study it close and look where the space should be and cut appropriately. I used Hall’s Precision racing in Phoenix. They have a full machine shop and CNC facility. Actually Jerry used to be a team Green mechanic. You can send it all there and they will do it. Send the frame, motor and swing arm.
Hall's Precision Racing Products
5614 N 52nd Ave Glendale, AZ 85301
623 - 931 - 8132
I got the original author of the drawing to reproduce it.
