Not sure, compare stroke specs 400/520, if the stroke is the same, it should work, providing the rod/wrist pin is the same on the 520 piston. Also check timing specs to make sure the cam to crank timing will be the same.
As indicated by Phatty, the stroke is 6mm shorter on the 400. Looks like you would have to install a 520 crank, at that rate it probly better to sell the 400. Shouldnt be hard to get a good $$$ for it.
KTM400MAM, being that you ride your bike in the desert and some MXcross here and there do you like the 4spd in your SX, or would you rather have the MXC,EXC 6spd trannies.
Is it possible to change the trannies out with out to much trouble and cost?
Whats the reason to want to change the displacement from the 400 to the 520, speed in the desert, or just more power?
Have you guy's riden any of the new smallbore 4-strokes yet?
I ride/race mainly mx, but occasionally I do some play riding in the desert or mountains or local hills around So. Cal. I like the 4 speed for the most part but I will eventually put 5th and 6th gears in. The 2001 400sx's come with 6 speeds already.
I figure while I have the cases split, why not drop a bigger piston in it and end up with a 454cc motor. Not too much power, but just right. Erik P.S., since you are a local
O.C. rider, drop me an e-mail if you want to go riding sometime.