KTM 525 EXC, One of the best do it all KTM RFS motor years made (produced from 2003 to 2007). Early model year issues have mostly been resolved and 2007 was the last year for the model.
While not being named the same, the 520 motors are identical to the later 525s. The RFS motors were introduced in 2000 and except for a few minor updates they were largely unchanged until KTM stopped using them in 2007. Of note is that Beta actually used the KTM RFS motors in some of their bikes up until 2010 IIRC.
I have a 2001 520SX and a 2005 525EXC.
I just raced the 525 last Sunday in an XC. It's a bit of a barge in the woods but it didn't kill me even though it tried.
I love the RFS motors, very robust and with nothing more than good stainless steel valves (I run Kibblewhite) they can easily go 500+ hours with just regular maintenance.
The 520SX while being mechanically identical (except for the early SX heads) is a beast. The ignition is very agressive and the motor definitely has more grunt everywhere than the 525. It also has 4 gears, 2 of which never get used on an MX track. At least not with my limited abilities.
I once had Dave Hopkins build me a very potent 540 using the 05 motor as a base with an early SX head (flows better). It had oversized Kibblewhite valves which were unshrouded for better performance and it used an 8/33 cam. The result was spectacular. The high comp 540 kit mated with the 8/33 cam cancelled out at low RPMs so it felt pretty much stock off the bottom which is to say, just like a tractor. But then it got on the cam in the mid-range and instead of signing off around 7500 RPM like a stock 525 it just kept pulling to the rev limiter. With trail gearing I saw 185kph (115MPH) a few times before backing out of it. And therein lies the problem with the RFS motor. It does not like sustained high RPMs. The lighter SX cranks with roller bearings (SXS and SMR models) could handle it but the heavy EXC crank with roller bearings are ill-suited to that purpose. I completely destroyed that motor chasing the ton on gravel. I a valve seat let go and that was all she wrote.
My 525EXC was/is plated but it was getting long in the tooth as a dual-sport ride. I replaced it with a Beta 500 RR-S. The Beta is a short-stroke motor and is happier spinning fast but I kind of still like the 17 year old RFS bike better in many ways so I doubt I will ever get rid of them. They truly are race bikes that you can just ride and put away wet.