Another update: I might just go with the current xr600r bottom-end with the XL top-end. If I go over by 1 or 2mm, I get 628-653cc of displacement out of only an 80mm crank. The reason I've been thinking of going this route is that it's the simplest solution and the cheapest, involving the least amount of case splitting, etc. The reason for this recent idea is that I heard I may not be able to interchange the cranks sucessfully from the XR650L becase the flywheel end might be different. Plus the simplicity of the operation. So if I do the XR600R bottom/ XL600R top, what piston would be right? Edit: this just in: a little more researching led to this discovery: Model / Bore / Stroke / Compression Ratio XL600/100/75/8.5:1 XR650L/100/82/8.3:1 XR600R/97/80/9.0:1 SO, if I used the XR crank for it's 628-650+ displacement (again, depending on how far overbored I go), I could use an 11:1 wiseco piston made for an XL and still have decent compression, given that the piston does not actually go above the cylinder, if that is even possible. Is that even a concern? It makes more sense to me that they all go the same height and the distance between the top of the piston and the piston pin is to make TDC within the same cylinder height, right?