[new_rider]

I know it's an old thread, but this is pertinent ... I am just finishing up pretty much the same swap but using an older 250 frame. I was deciding between a 650 & 500 when I found this thread (and a $300 parts bike) and figured what the heck. The actual path to the swap was pretty long. After getting the parts bike, I first swapped the gauge clusters because I liked the one from the 500 better. Then, because I liked certain aspects of the 500's swingarm over the 250's (namely the chain adjusters), I swapped those parts. That really wasn't a big deal except for machining down the width of the 500's swingarm. Ironically, Kawasaki must have agreed with me because the latest revision to the 250 go the swingarm off the 500.
Then, I finally moved on to the engine. Since the 250 was my almost daily driver, I didn't want to dismantle and cut up the engine mounts before I had as many of the mounting brackets for the 500 motor ready as possible. So in a weekend I pulled my motor and slid the 500 into place. Since the 500's upper mounts are wider than the 250's, I cut slots in the engine's mounts so I could get the engine into place (big mistake). I managed to use the stock exhaust header and mufflers. Doing so required moving the mounting tabs on the mufflers and spacing the sidestand about 5/8" off the frame. That might be a problem for a bike in better condition than mine, but I've taken off quite a few times with my sidestand down (never fallen because of it ... not even close), so it's a little warn (and yes, I disable all that safety crap). For the carbs to fit between the motor and frame, I shorted the stock mounts so that the carb mounted about .1" from engine. That was it for that moment, so I put the 250 engine back in while I made the parts for the swap.
When winter finally came (we don't have much winter down in the South), I finally pulled the 250 for the last time. I cut the lower mounts off the frame and welded in new ones for the 500. Because of their odd placement, I had to do something really funky with that mount. From what I can tell in the other guy's pictures, I didn't tilt the engine as far forward as he did and ended up with the lower engine mount roughly in line with the lower frame (equivalent of a B pillar in a car), so a bolt could not be inserted from outside the frame. I also modified all the 500's engine mounts to be solid mounts since the 250's frame is designed for the engine to be a stressed member. The 500's urethane mounts probably would have been rigid enough, but I didn't want to impose the extra flex on the chassis. Moving to the upper engine mount, that slot I made caused the mount to crack when the bolt was tightened. I knew it would but decided to ignore the basic engineering calculations and give it a try anyway. I welded the cracked in the engine case and also welded up the two slots I had made, which in turn meant I had to move the upper mounts on the frame. No biggie ... a little grinding, a little welding.
Because of where my carbs ended up landing, I had to use the fuel petcock off the 500 because the 250's wanted to live inside the carb. To put the 500's petcock on the 250's tank, I had to cut off the lever off the selector, but I can weld a little finger on for that later. The 500's main pickup is so much lower than the 250's that I wouldn't have much reserve anyway, and with a vacuum operated petcock there isn't much use of the "off" position.
As for the electrical, it's almost not worth mentioning I used the 500's harness since I was using its gauge cluster. Though the plugs might not always be in the exact right position relative to the 250's, almost all of the connectors are identical to the 250s and the ones that aren't are easy to figure out. If I had used the 250's harness, I would have had to have run a wire for wire to the ignition box since there are a handful of extra wires there. Oh, and I forgot to mention that I snagged the coil-on-plug ignition from a 650r, so I don't have to worry about ignitions coils. The coil resistance is so close to that of the stock 250/500 that it's a drop-in replacement except for the connector, which has to be spliced.
There are only two things that I don't like, the radiator filler and thermostat housing and my intake. Due to space constraints, I had to use the filler and thermostat housing off the 250. I liked the 500's more, but there just wasn't anyplace good to put it, not without spending money on tubing. (Oh, I forgot to mention that I had spent no money on this project other than the $300 I paid for the parts bike and $10 for the ignition coils. In all fairness I do have access to a fairly large bin of scrap steel.) On the intake side of things, the edge of the #1 carburetor sits inline with the frame rail. I molded a piece of PVC to go around it, but I really just don't like it, and the mounting is crap. This just means that sometime in the future I will probably modify the fuel injection off a 650 to fit this engine, but since that will require money, that's a very long way off. The engines' outputs are close enough that they'll probably run just fine. Nothing a microcontroller and some code can't fix.
I know people love pictures. I'll have to get a camera from work because that's one piece of equipment I don't have. Well, technically, I have one, but it uses 2 AA batteries that are always dead because I've never replaced them.
In the beginning of this thread, some people asked about a kit. One could be made, but it would still require cutting and welding to the 250's frame. And no, a subsequent heat treatment is not required. Is the material weaker than before? Probably. Does that matter? No. The load applied to that frame in normal use is inconsequential relative to its strength. It may develop fatigue cracks in the distant future. My frame had 2 cracks when I bought it. I never would have known had I not stripped it down to repaint it (it was a flood victim of Katrina). I welded them up and have had no problems since. Anyway, the rest of the kit would probably be a piggyback wiring harness to complement the stock 250's, front engine & radiator mounting brackets, carb boots, air filter arrangement, some hoses and whatnot for the radiator. That's about all I can think of at the moment. It could be done, but it would be exceptionally expensive as a low quantity production.