So this particular build has been done many times over, but I figured I would throw it in here for posterity's sake.
By combining the amazing torque, powerband and easy maintenance of the CR500 engine with the modern handling of the aluminum frames you get an incredible bike... or at least that's the story. I'm going to find out for myself.
This isn't my first rodeo with the CR500. I had one years ago and hated/loved it. I started by feeling I bought a bike over my head but by the time I sold it I absolutely loved it. It was one of the best trail bikes I've ever owned. Since then I've gone through more than a few different dirt rides but after spending ridiculous amounts of money trying to keep my Husaberg 470 going I finally decided to swear off valves and get back to basics. I want the ease and power of the 500 but not in that buckboard of a frame - hence the conversion.
CR500 motors have been getting shoehorned into Honda aluminum CR and CRF frames for over 10 years now. Service Honda started it and while you can still buy one from them in the neighborhood of $10k, it's been done enough times at home now that it's a well-documented process. In fact, you can even buy the necessary billet y-piece and all motormounts for less than 400 bucks. Cut off your old Y-piece, clean off the old mounts and drop the whole thing by your TIG expert of choice - it doesn't get much easier than that.
I've found it's cheaper to buy a whole CR500 that someone has left rotting in a corner than just the motor. I bought this one for $1500 and it's even got an E-Line 200W stator. I was able to finagle a street title for it so I should be able to make back most of what I spent on parts. It's got some other aftermarket stuff as well to sell.
The donor frame comes from an '07 CRF250R. Service actually builds theirs using the 250 frames instead of the 450 frames too. I haven't researched why but I should. The 250 motors seem to be time bombs, so it's not hard to find a blown up one. They take a fair bit of maintenance that needs to be kept up. I bought this one actually almost back together after losing the big end crank bearing. The PO had the bottom end rebuilt and it just needed the top end put back together. I'm in the process of finishing it so I'll be able to sell the engine. I bought this one for $1800.
The plan is to finish the 250 engine, ride it briefly just to make sure it's good and then yank it and start cutting the frame. I'm done with riding the 500 so I can pull that one now along with the pipe and electrics. So far the engine seems to be in great shape. I took the 500 for a 35-mile ride last weekend and the engine ran flawlessly. I can't say the same for the suspension and handling, but I'm very happy with what I bought as a plant.
I'll keep tabs on the cost just to see how far under the Service Honda price point I come. Stick around if you're interested...