KLX250/300 to KX Frame Conversion
Article and photographs by Dave
Jigs available from Francisco

All the plastics I am using for this are from a 2000 KX 250. My bike is a 98 KLX 300. To enable all of the KX plastics to be fitted up rather than buying a KX subframe  I decided to build one. A standard KX rear guard can't be fitted to a KLX  as it comes in contact with the airbox, among other things. To determine the length of the horizontal part of the frame. I placed the new radiator shrouds and sideplates in their positions. This will give a good indication of the distance to come back. Two little lugs were welded on to allow it to bolt onto the rear fender. I went for square tube as on the KX as this helps in lining up the angles of the support struts. You just follow the angles on the rear fender plastic. Once these are in place I cut the support struts - used the angle of the plastic on the fender and just followed it down.
I mounted the new right spar of the subframe just below where the other one was cut off. The left side I mounted lower down so as to increase the stength.
To fit the seat I mounted a piece of metal to the right shape to match the curve of the guard and welded some threaded supports which I cut from the original KLX subframe. It doesn't have to be overly strong as the plastic piece on the rear fender supports the weight of the rider not the brace the seat bolts onto. It all fits together pretty well and I can use the grad handles which is cool.
With the subframe made and plastics fitted up a new tank had to be made to enable the KX seat to fit. The standard KX tank won't fit by miles as it touches the head and the frame angle is all wrong. Firstly I had to make an exact copy of the shape you require as the molds will taken off this. Expanding foam is the best way to go because  it is very easy to shape. I put a plastic bag in the cradle part of the frame and filled it up with liquid foam. It expands quite a bit as you can see. Then you shape the foam into a tank... Using the plastics makes it easier. You can see here I have the radiator shroud in place to assist in getting the shape right. This way I can mold the shape I require exactly.
Once I had the shape about right, the foam is the covered in fiberglass epoxy to give me a hard surface and then 5 coats of automotive spray putty. After that I used body filler to fill any imperfections and get the shape exactly how it will be. To make the filler thread I bought a used gas cap and took a rubber mold of the inside. The cap I got is actually off a YZ I think...(PlanetKLX does recommend using any Yamaha parts as this will cause hard starting)
Showing the new subframe and the mold for the tank.


Fitting up the plastics to check the shape of the tank. It's actually starting to look a KX!!


The lower subframe supports need to be bent in about 10 deg to follow the shape of the new plastic and allow them to join up with the existing frame. The tank shape is pretty much finished and ready for the molds to be made. Punch marks are made where the threads have to go as these will be added later. I allowed 2 holes for the fuel tap so I will still be able to use reserve. The tank will be made up of 4 sheets of kevlar then a sheet of carbon fiber over the top for looks.
When making the molds you don't want any under cuts so the mold wont separate from the shape. Therefore the more complex the shape, the more molds you will need. This tank will be made in four parts. You can see the dividers here where the first mold will go. The threaded inserts are placed in position and then will be held in place by the molds.
Making the top mold from fiberglass.


You can see the threaded inserts in place.


Making the two part mold for the bottom.


Completed fiberglass molds inside.


and  out side. Carbon/kevlar sheets are layed in place and ready to separate from the mold.
This tank is very strong and very light. It's about half the weight of the original and holds just under 10 litres(2.6 gals) - more than the stock tank as I used up as much space as I could underneath. It clears the head by about 10mm and the top of my FCR carb by about 1mm! I used a rubber strap to hold it down at the back as on the KX.
I later needed much longer fuel lines to avoid kinks when I bolted the fuel tap back onto the bike as the fuel outlets hang much lower than before. All Done!
The exhaust angle had to be changed... so it would fit better with the new plastics.