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2016 kx250sr af

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    2016 kx250sr af

    Some pics on the development and completion of my alloy frame KX250 bored out to 270cc in a 2016 KX450 frame. Through this thread I'll put up all the photos and explanations of how I managed to build this bike, some of the problems I ran into and how I fixed them. ENJOY.
    Last edited by Danny; 05-15-2020, 11:30 AM.

    --How it all Started--

    This is how the bike started, on a 60+ hour 2016 KX450f I bought of a racer in South Australia. I luckily managed to sell the engine very shortly after buying this bike for a good sum of money since this design of 450 was still very recent at the time. During this time I also managed to buy the engine and electrics off someone parting out a 1999 KX250, now I had key ingredients needed to make it happen.
    Last edited by Danny; 05-15-2020, 12:02 PM.


      --The Taredown and Initial Fitout--

      First thing to do was strip the KX450f down to the frame and put the shell KX250 engine in through the swing arm bolt. This gave me a quick idea of where I'd needed to cut the front of the frame, what angle the engine needed to be and also gave me an idea of how big the engine mounts/brackets needed to be.

      Fortunately, the KX450 swing-arm bolt is the exact thickness dimension as the 1999 KX250, so the bolt goes straight through no problems. Only downside is that the area the bolt goes through the engine is slightly thinner than the 450, so I had to machine some spacers to take up the empty space between the swing-arm and the engine on each side. You'll see these with all the CNC'd parts in a later post.
      Last edited by Danny; 04-19-2020, 06:18 AM.


        --Machining the Y-Piece and Brackets--

        From here, I was able to cut out the original Y-piece and then measure the exact dimensions required to machine the new Y-piece and engine brackets.
        I develop these parts in a cloud based CAD platform called OnShape. From here I simply sent the XLT file and dimension pages to a heap of CNC machining businesses to see who could machine these pieces up for me. After a few quotes I picked a small business and we got to work. I think Y-piece and all brackets set me back about $400AUD, but was well worth it in the end.

        When fitting the stock pipe I quickly realised that we were going to have troubles getting the exhaust to clear the frame. More on this issue later on.
        Last edited by Danny; 04-19-2020, 06:18 AM.


          --Machined Parts Before the Welding--

          Here are some more pics of the machined up parts to enable the fit of the 1999 KX250 engine. As you can see , instead to machining the y-piece to exact fit with the 45 degree cut-out, I actually got it machined longer than needed, so we could cut it back to exact fit on install. I got two copies of engine mounts/brackets made just incase we made any mistakes on install with the welding.
          Last edited by Danny; 04-19-2020, 06:19 AM.



            Next step was to get the Y-Piece and engine mounts welded to the frame. I didn't take any chances here, I got all the welding done from a highly regarded aviation welder who welds aluminium and titanium for a living. I got him to spot weld everything in draft position first to check everything fitted when bolted on (e.g. fitted with radiators on, engine mounted, all hosing systems fitted etc.). After some very minor adjustments I gave him the go ahead to weld everything up.

            While I cut the edges of the radiators down to fit around the frame with the hoses on, I then handed them over to the welder to seal them off, you'll see this work in the next post.
            Last edited by Danny; 04-19-2020, 06:20 AM.


              --Fitting the Engine to the Modified Frame and Radiators--

              After picking up the frame I was then able to bring it into the shed and see how everything fitted, and everything seemed to work like a dream.

              To combat the exhaust clearance, I invested in a Pro Circuit works pipe and while it was much better clearance wise than the stock, it still ONLY JUST cleared the frame rails. Despite the clearance, this still wasn't enough and caused problems later on bing a clean fit into the engine's exhaust manifold... More on this problem in a later post.
              Last edited by Danny; 04-19-2020, 06:29 AM.


                --Fitting Electrics...Part 1--

                Fitting the electrics was like musical chairs with this bike, where do you put it all?!

                Starting with the biggest component, the KX250 ignition.. This thing usually sits in the air-box on the 1999 steel frame but there obviously wasn't the space for that using the 450 air-box, so I decided to fabricate myself a bracket which bolted onto the right side engine mount the 450 used to utilise.

                This mount then slipped onto the rubber ignition sleeve and sits comfortably under the seat and above the rear shock, while wiring still comfortably reached the carb and coil/solenoid thing.

                As for the remaining electrical components, I'll show you were those went toward the end.
                Last edited by Danny; 04-19-2020, 06:29 AM.


                  As I was pretty confident I knew the engine fitted in the brackets (with spacers), the 1999 radiators cleared the frame and I had found a home for most of the electrics, I stripped everything down and sent the frame and subframe off for anodising, which I had done in black.

                  Once returned, I started to fit everything back together again. While this was going on, I also had the engine cases hydro-blasted, and other engine covers Cerakote in burnt bronze. Cerakote is great as it is resistant to basically all oils and fluids, making it incredibly durable. You'll see a second set of cases as I was also prepping a 2001 Mike Brown build at the time, but I'll save that rebuild story for another time.

                  While I outsourced the Cerakote, I now do all these sort of coatings myself, they're a walk in the park if you have a quality spray gun, the right cerakote/catalyst and a half decent oven to cure the parts. Again, I'll save how thats done for another time.
                  Last edited by Danny; 04-19-2020, 05:34 AM.


                    Next step was to put the rebuilt bottom end in. When I bought the engine and electrics, it was honestly clapped. So I dismantled the entire engine, bought all new seals and bearings and then handed the split cases to a mate at our local motorcycle shop to rebuild from the ground up. I now try and tackle bottom ends myself, but being my first one and a lot of other things going on at the time I didn't want to risk it. While it was costly, I'm glad I outsourced it as the bike works like as dream and this obviously is a big part of that.

                    While that was going on I also had the top end ports polished and had the cylinder bored out to 270cc. This almost had to happen, as the cylinder was in such bad shape when I bought it that it had to be re-sleeved, so boring wasn't going to cost much more than re-sleeving to 250cc anyway. Although the bore caused jetting issues later on, it was still the best decision I made with this build. You can see it compared to a stock 250 bore (2001 KX250 Mike Brown build cylinder) how much more volume you get from the extra 20cc, and you definitely feel it on the track!


                      When fitting the swing-arm up to the fresh bottom end, I quickly noticed that the swing-arm had next to no room and would grind against the engine when you simulated the swing-arm compressing (e.g. landing from a jump). I used a dremmel to shave the swing-arm bit by bit until I was happy with the clearance between the swing-arm and engine through the entire range of suspension motion the swing-arm would experience.

                      From here I bolted the shock straight on. On the second and third photo in this post, you'll see the swing-arm spacers I got machined up being put to good use!
                      Last edited by Danny; 04-19-2020, 12:47 PM.


                        The day the wheels arrived was cool.

                        I would love to have gone black excel rims with billet magnesium/bronze coloured hubs, but they were just way out of my price range at the time. After keeping my eye out for wheel sets for a few months, MXStore popped up with a sale on the black/green SM Pro wheel set for a few days, so I bit the bullet and went for those.
                        Last edited by Danny; 04-19-2020, 06:30 AM.



                          Coming up with a solution for the air box was easily the hardest part of the whole AF conversion, harder than designing any parts, getting the electrics to fit etc. This part was not fun and at times made me want to quit, or made me wonder why I was even doing this... But then I remembered building this bike would be so sick so I kept going anyway.

                          My original plan was to just buy an adaptor kit (similar to what you might see with the KX125 restyle kits), but I just couldn't find any at the time to fit a 1999 KX250 air boot to a 2016 KX450 airbox, at the time I could only find the 03-08 KX125 to a 250F airbox adaptor kit and from what I could tell, that spec just wouldn't work on the bigger/older engine and my patience was running out. So I decided to execute my home job plan which followed the below steps:
                          1. Use a dremmel to cut out a shape in the original 450 air box plate which replicated the shape of the 1999 air boot entry. This would allow the end of the rubber boot to slip inside the 450 air box plate.
                          2. Once I had a shape which the 1999 air boot could fit through, I cut out a 1.5mm think alloy plate to fill in the remaining gaps left from the original 2016 KX450f air boot entry.
                          3. Now what I needed to do was to find a way to seal this new shaped hole I had made myself. For this I used a 5mm thick rubber sheet, cutting it to the shape of the 1999 air boot again.
                          4. Before pulling the air boot through the new plate hole I made, I lined everything with silicon sealant. This made sure the were no gaps around the air boot and the air box entry
                          5. I then drilled new holes where the 1999 air boot needed to be held down to the new plate in accordance the the 1999 KX250 cage screws (I filled in the old holes which the 450 used).
                          6. I then pulled the old 1999 rubber air boot through, put the cage over it to seal it and screwed the cage onto the plate through the new holes I just drilled.

                          Below are some photos of my DIY efforts. Does it look bad on the inside? Yeah, probably... Is it sealed? Yes it is definitely sealed 100%...Does it look bad from the outside? No, you'd never tell and in fact, I think it looks way cooler than having these thick white plates sticking out like your standard restyle kit would give you. While this method looks heaps better from the outside, there were some other issues that arose, which I'l explain in the next post.
                          Last edited by Danny; 05-15-2020, 10:57 PM.


                            --Airbox Issues--

                            So I now had a 1999 KX250 boot which successfully met with 2016 KX450f airbox, home and hosed! Well, almost...

                            Key issue I still had is that the air boot was about 30-40mm short of the carb. I tried to literally just stretch the boot to the carb and screw the air boot clamp on but it got to the point where it was just deforming the boot and rubbing the rear shock, so I knew this wasn't going to work.

                            To combat this issue, I decided to drop the entire 2016 KX450f air box further down the sub-frame. I did this by cutting out an alloy plate with new bolt holes by the rear fender, allowing the airbox to sit lower (see plates under rear guard in photos).

                            While this made ends meet (literally) and the boot could now reach the carb, new issues were introduced. The mud-flap would no longer smoothly slide down the swing-arm while riding and would get chewed up by the tire. To combat this I literally heated the mud flap and bent it to fit the new dropped down air box (see photo). While this worked for the mean time, I think it looked poor and even still, the air boot was still slipping off the carb while test riding, something had to change. I'll get to how I fixed this up later on. For now, let's get the electrics fitted and take some photos!
                            Last edited by Danny; 04-19-2020, 12:41 PM.


                              --Electrics - Part 2--

                              Ok, so I started off with fabricating a bracket to hold the CDI (Capacitor Discharge Ignition) using the right side engine mount the old KX450 engine used to bolt to. Despite this, there were still 3 key electrical components I needed to bolt up.

                              1. Condensor
                              2. Voltage Regulator
                              3. Ignition Coil

                              1. Condensor - I found this one best routed back into the air box as this is the way I have seen it done with many steel frames in the past. To do this I simply drilled a M5 hole through left hand brace of the sub-frame and bolted it straight on (see first picture below).

                              2. Voltage Regulator - This on I ended up bolting on using an existing bracket from the 450 frame. I ensured the Regulator was pointing upward before tightening so it remained out the way (see second photo below).

                              3. Ignition Coil - This part was difficult as it literally needs to float somewheres since it has holes on each end, but is quite thick. To combat this, I fabricated a bracket which was bolted to the right shroud/radiator brace, which then twisted down to meet with the top hole of the coil (see third photo below). The bottom hole of the coil then met with the exhaust bracket to complete the earth.

                              In addition to this, I had the top exhaust brace bolt from here across to the same bracket which is holding the Voltage Regulator (see fourth photo below). While this set-up doesn't sound so flash, it has held up really well so far, and shows no sign of giving up at the moment (fingers crossed). I'd like to take the Ignition Coil away from the exhaust bracket at some point as this is probably the highest vibration frequency point of the bike, but everything has seemed to work really well so far using the above mounting methods.

                              From here, I just used the existing KX450f wire guides to hold the wiring around the inner frame rails to avoid them from touching the fuel tank once fitted.