there's a few answers to this. and if you look through he pictures you'll see the XR4 motor sitting on the work bench completely put back together. So I'll answer a few questions as to why I'm doing this. the XR was going to be my new DS bike. I got it cheep in boxes. and missing a oil cooler. I now have every part here. I built fixed the problem with the motor but in rebuilding it I found 5 bolts striped. one head stud and 4 valve cover bolts. I had robed the headlight and needle from it for the Old XL at this point. Also I don't have a tittle on the XR and we don't know what state it first came from. I got it from a guy in FL. and they couldn't find any info on it with that state. Now I could ge a bonded tittle but I don't feel like it. so I have a dirtbike that's in pieces with a motor that i need to pull completely back down to fix 5 striped bolt holes and i can't get a tag for it regardless. Nope not going to happen. I'll keep it for parts and I'll find a XR4 I can DS and have a lot of good parts for it. So i have my Old XL. I ride it. I love it, but it's out dated. I can't find front brake shoes and it needs them. Humm i wonder if the XR front will fit. so i look the bearings up and they are the same. so I do a little looking at the two bikes. Yes putting the 500 motor in teh 400 frame crosed my mind, but the 500 is a lot bigger. see the 500 is also the bottom part of the frame. and it's not a dry sump motor. the 400 is. won't work. So now I'm on the making one cool XL500R. the fate of the 400 is probably going to be cut up to make my one wheeled trailer later on. I think that covers the whys. At this point I could sale the bottom end of the XR and some of the other parts i don't need and I would only have $1150 in my Old XL. As it sits if you include what i've spent on the XR I'm up to $1500. Like I said. I'm broke. i have the two bikes sitting here. I've spent $26 on this conversion so far and after i buy a new tire a mount for my mirror and a brake light switch I'll have about $120 in it. i could cut that down if I swapped the tire from the XL rim to the XR rim but it's at 50% ware so I might as well replace it. Also i have a KTM200MXC so i don't need the XR for a dirtbike. It took me 6 months to decide to do this. I've looked at the XR for a year, and I've tinkered with it and bought things for it all along the way. in the end I wanted to do something cool. this is what I came up with.