Cool. Thanks for looking that up. I believe the part numbers are different because the L uses the automatic decompressor and the R doesn't. You lose the automatic decompressor when you put in an after market cam. I've also checked on the valve sizes for both bikes. According to this website they are the same. http://bigborethumpers.com/valves.html So it looks like the only performance differences between the two is the lower weight of the XR600R, A higher compression ratio (The R is 9:1 and the L is 8.3:1), the ignition curve (which I believe is used because of the higher compression ratio), charging system, and possibly a lighter flywheel. Can you tell me what the ignition specs are for the XR600R? The L's is: advance start 8 degrees/2300 RPM stop at 28 degrees/4000 RPM. There has been a lot of speculation that an XR600R CDI will help performance on an L. I think it will but only with a High compression piston. It would be nice to know what the start and stop are so you could duplicate it with an after market programmable ignition system when you go with a higher compression piston.