I've lost my mind (Hybrid project)

I know this isn't a dirtbike but its got a dirtbike heart :devil: I wanted to share it with you guys. I took a stock Aprilia RS50 and shoehorned a YZ426 engine in it. I just did a writeup on it and have a video also. I know this is long but very informative if you want to know what it takes to build something like this :thumbsup:




I started out by finding just the right RS50 to do the conversion. I looked for a few months and finally came across a 2000 model RS50 that was in perfect shape. I then started stripping it all down and selling off the various parts I did not need including the stock motor, exhaust, oil tank, battery tray etc. Then I began to search for the perfect MonoGP transplant motor which happened to be a YZ426F motor because of their availability and parts supply. After receiving the motor and mocking it up into the RS frame, it was evident that serious fabrication of the bottom cradle was going to be necessary to make this project work. Mike, a friend of mine who works for Brumos racing in Jacksonville, Florida

(a top contender in the Rolex Grand American sports car series with its Porsche fabcar) did all of the welding and fabricating on the bike. It slowly started coming together piece by piece. Once the motor was situated in the cradle, the center of the swingarm had to be cut out to make room for the rear engine mount. This is the main mounting point of the motor that takes most of the stress so it had to be perfect. After this was complete, the motor was sitting inside the cradle by itself. Now it was starting to look like a bike. Next up was many small things we had to contend with including the aluminum oil tank, kickstart extension, radiator mount to accept a cooling fan, rebend the header to clear the frame cradle and build a completely new tail piece with undertail exhaust and built in oil reservoir. Once these things were complete, I was able to completely disassemble the whole bike so I could have it all powdercoated gloss black with silver rear sets. Now that I had the motor back out, I wanted to get everything installed in it before being put back into the frame. Since I am building this bike to ride on the street, I had to buy a new stator and flywheel to power lights. Since the motor is from a dirtbike it does not have the capability to power any lights or accessories. I also installed an automatic decompression exhaust camshaft from a 2003 YZ450F in the motor to eliminate the decompression lever on the handlebars and ease in starting the bike. Ok, now that everything is mechanically together I can fire the engine for the first time. I anxiously await while the gas is being poured into the tank and then it is time. I sit on the bike, give it a few twists of the throttle to get some gas into the carb, engage the kickstart once and it immediately fires, I was relieved to know that everything was functioning properly. Now that I know the bike runs, I can get on to the other things. The wiring was next on the list. Everything on the bike had to be redone to accomodate the new dual 50w xenon halogen bulbs that I installed as well as the LED tag lights, running lights, blinkers and custom made brake light. The stock battery was way too small to run all of the accessories I had on the bike so I installed a bigger sealed battery to handle the job. Now that I had a functioning charging system, it was time to start working on the dash cluster. My goal was to make every light and gauge work like a factory stock bike. After countless hours of cutting and testing wires, I had them all functioning properly. I was even able to tap into the neutral postion sensor on the 426 motor so the neutral light would work on the dash. Now on to the gauges. The stock RS has speedo, tach and water temp gauges . The speedo was ok since it works off the front wheel but the tach and temp gauge had to be modified. I installed a temperature sensor on the head for the temp gauge which worked fine. The tach was the tricky part. Since the stock tach runs off a cable-driven gear in the motor, it couldn't be used but I wanted to retain the look of the stock dash with stock gauges so I sourced out a specially made ATV tach made by Autometer for use in single cylinder engines. Most tachs used on singles are digital because they are easier to find, but I wanted the analog style tach to work so I took the Autometer unit and my stock tach to a local speedometer calibration shop and had him take the internal components out of the Autometer unit and install them into the stock gauge. It then had to be recalibrated because the Autometer unit was smaller in diameter than the stock gauge so the sweep of the needle was incorrect. Once he finished the tach and calibrated it properly I installed it back into the dash. Now back on to the rest of the wiring. Since the headlights consume a lot of power to run, I mounted a switch on the dash to turn the headlight on and off manually. I used an LED cluster in the center of the headlight for day time riding and only use the headlights for night time use when they are needed. Next to the headlight switch I mounted an adjustable knob for the radiator fan. It is pre-set to come on at 190 degrees but just in case I want to manually turn it on or off I can. That about raps up the electrical install for the bike. The exterior of the bike was also changed a little. The stock RS runs a 90/80/17 front tire and a 110/90/17 rear tire. These tires are obviously too skinny for the bike now so I upgraded them to race DOT tires in a 110/70/17 front, 130/70/17 rear. You also have the option of running 125GP slicks. The driveline also had to be upgraded. The stock motor used a 420 chain, again too small for this motor so it was swapped for a 520 DID ERV2 x-ring chain and a custom made 38 tooth rear aluminum sprocket for the 520 chain. The last thing to be completed on the bike was the shift linkage. It is a combination of the stock linkage and the 426 linkage and they are welded together in the middle and adjusted to function perfectly. The time had finally come, it was now ready for the open road. The first time out was kind of a test and tune ride to make sure everything worked properly, no leaks, loose bolts etc. Everything checked out ok so now it was time to see what this beast really had to offer. I took it to a rural area so I could open it up and see how it would run with the experimental gearing I had chosen for street duty. I was very pleased with how it ran. I had 14/38 gearing on it and it ran a touch over 105mph on the rev limiter. This bike is amazingly quick and very lightweight. It's a sportbike killer in disguise. It easily walks all over SV650's and gixxer 600's until it tops out. It's the ultimate sleeper! Overall I am very pleased with how the bike turned out, everything worked out well and I have not experienced any problems with it yet. The bike took a total of 10 months to build but for a one-off bike it was worth every minute. Chris

Here is the video: RS-426 video

What the???

Sweet! You did an excellent job on that bike. :thumbsup:

Nice! Now wheres the bike in action video!?!?

Awesome! :thumbsup:

Thaks guys, I'll get an action video soon :thumbsup:

That is killer, I like how the exhaust is centerede in the back. How does it ride?

Hey Chris, nice to see you over here on TT. That write up will work great with my site. I'll use it when I complete your page and photo album. Bummer is that I have to move my site to a new host, so it may be a few weeks before I can get things together.

I picked up my CRF450R a few weeks ago and I'm loving it to death.

You didn't mention if you had to do anything to make that 426 motor more street worthy. Can the more handle running constant rpms like it would be exposed to on the street?


Cool Ben, let me know when you get a chance to put it on the website? The only things I did to make it more street worthy is increase the oil capacity to almost 2 quarts and add the heavier WR flywheel to smooth out the powerband :thumbsup:

Nice work!!! Did you make any changes to the suspension for weight differences? Looks like a blast to ride :thumbsup:

Yes, I put a Race-tech spring kit in the front and Bitubo adjustable shock on the rear. Handles much better now :thumbsup:

You must not be married. Mine would scalp if I threw that much time and money into a project. But that thing is sweeeeeeet. :devil::thumbsup:


Great job in detail :thumbsup:

That's beautiful Chris!!! Nice job. Reminds me of this.



Thanks for all the replies guys, I really appreciate other people taking an interest in my project :thumbsup:

beef up that swingarm and you have a serious contender in WERA Clubman etc..... !!

Yes, the swingarm has been braced in the middle.....it has been beefed up for all that power :thumbsup:

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