Born in 2000
The four-stroke that went racing
Back in the late nineties, a four-stroke dirt bike meant a big trail thumper, and KTM already built those: the LC4. What KTM wanted was a four-stroke you could race. So it bought the struggling Husaberg company to learn how to build one, and for 2000 it rolled out the 400 and the 520 in SX motocross and EXC enduro trim. Racing four-strokes, so the project got its name: RFS.
Single overhead cam, four valves, rocker arms. The 520 SX ran a four-speed box and weighed 240 pounds with an empty tank on Dirt Bike's scale. The EXC enduros got a wide-ratio six-speed and an electric starter, and KTM claimed they weighed the same 259 pounds as Yamaha's YZ400F motocrosser. Cycle World rode them in 1999 and signed off with one line: "The future of dirtbikes has arrived."
Then it went racing. Giovanni Sala won the world enduro championship on the 400, and Joël Smets took the 520 SX to his fourth 500cc world motocross title in 2000.
What made it great
Crazy fast, easy to ride
Dirt Bike's verdict on the 520 fit in five words: "The 520 is crazy fast." Next to a Yamaha 426 it was faster, torquier and revved higher. And it was no handful. Dirt Rider called the 525 SX motor "super-torquey with massive power, yet is very easy to control."
For 2003 the family grew. The 520 became the 525, same displacement with a fatter torque curve, and a 450 arrived as the US and Europe let 450 four-strokes race in the 250 classes. The off-road 450 was a 400 with a longer stroke. The 450 SX got a more rpm-friendly bore and stroke of its own. And the little 250 RFS enduro got a head of its own, with smaller valves.
Through 2007 the RFS powered SX, MXC, EXC, XC, XC-W and SMR models. It was right at home in Baja: in 2005 a 525 MXC won the San Felipe 250 and ended American Honda's ten-race SCORE win streak.
The ruling class of off-road four-strokesDirt Rider on the 520 and 525, 2008
Reading the parts books
The 525 is a 520 in disguise
Every KTM part number is an address: project, group, item, version. Project 590 is the RFS. It's on the 2000 520's cylinder and crankshaft, and on hundreds of parts around them. Line up the cylinder, crank and piston of five RFS bikes and watch the numbers repeat.
Look up a 2005 525 EXC and the book hands you a 520. The cylinder reads "CYLINDER 95MM 520 RACING 00", the crank "CRANKSHAFT CMPL. 520 RAC.2001", and the piston carries the 2000 520's number. The 450 EXC is the cleverest of the lot: the 400's 89 mm cylinder on the 520's crank, with a piston of its own. The 2000 520 book even gives the strokes, 64 mm for the 400 crank and 72 mm for the 520.
Here's the giveaway. The 520 and the 525 are the same size: 510 cc, from a 95 mm bore and a 72 mm stroke. Same engine, new badge. And that 510 has worn a lot of badges: 520 SX, EXC and MXC, then 525 SX, EXC, MXC, XC, XC-W and a 525 supermoto. It even went four-wheeling, in KTM's 525 XC quad and the Polaris Outlaw 525. Dirt Wheels said it straight: "its engine was 510cc and called a 525." Bore it out to 100 mm on the same 72 mm stroke and you get the 565 cc 560 SMR.
| Bike | Cylinder | Crankshaft | Piston |
|---|---|---|---|
| 2000 400 EXC | 595·30·005·000 | 595·30·018·100 | 595·30·007·000 |
| 2001 520 EXC | 590·30·005·000 | 590·30·018·200 | 590·30·007·000 |
| 2005 450 EXC | 595·30·005·000 | 590·30·018·200 | 594·30·007·000 |
| 2005 525 EXC | 590·30·005·000 | 590·30·018·200 | 590·30·007·000 |
| 2007 525 XC‑W | 590·30·005·000 | 590·30·118·000 | 590·30·007·000 |
2007 and 2008
A new engine, an old soul
Six model years apart, a 2001 520 EXC and a 2007 525 XC-W still share 158 engine part numbers. Then the next generation arrived. The 2007 450 SX-F got the new RF4 engine, and Dirt Rider said it made "the old SOHC RFS motor seem, well, enduro." For 2008 the 450 and 530 enduros moved to the new XC4 engine. The 2007 525 XC-W lists 189 engine parts with the RFS project number. The 2008 530 XCR-W lists 40. The RFS itself rolled on in KTM's 525 XC quad, still a 510cc single overhead cam with four titanium valves.
Still in the parts books
The RFS never left
RFS parts in the 2027 books
- Cam-chain tensioner 590·36·003·000, "TIMINGCHAIN TENSIONER CPL.RAC.": the 2000 400 SX, 400 EXC and 520 EXC, and the 2027 450 SX-F and 450 Rally Replica. 440 bikes in all.
- Valve-stem seals 590·36·027·000, "Valve-stem sealing Racing": 865 bikes from 2000 to 2027, right up to the 690 Enduro R and the 1390 Super Adventure S EVO.
- Clutch intermediate disc 590·32·010·100, "INTERMEDIATE DISC 1MM 2002": 1,132 bikes, two-strokes and 100 GASGAS bikes included.
- Oil screen 590·38·016·200: 780 bikes, 2000 to 2027.
Why they're still out there
Riders won't let them go
Dirt Bike said it best: "To this day, there are riders who will not stop riding their KTM 520s and 525s." Dirt Wheels called the RFS motor "one of the most reliable, powerful, lightweight and well-respected powerplants in the world." Fast, light and tough. That's a bike you keep.
And keeping one alive is easier than it looks. One engine family, eight model years, and the same numbers turning up again and again: the 520's cylinder and piston in the 2007 525, the 400's cylinder in the 450 EXC, one camshaft number in the books from 2000 to 2007. Every RFS that gets parted out is a donor for a whole garage of them.
The RFS crowd
25 bikes in under an hour
We asked RFS owners to show us their bikes. In less than an hour, 25 brilliant bikes landed in the comments. Not bad for a machine KTM drew up last century.
Enduros on mountain ridges and under snowy peaks. Supermotos on street tyres. A rally build with a tall screen at a farm gate. A night racer in checkered gear, climbing rocks. And one engine split wide open, because every RFS owner knows that story too.
The crowd goes way past our page. In 2015 six South African riders signed up for the Amageza rally as Team 525. One rode a 2007 525 XC-W. Some had never ridden a rally before. One was told he'd do better on a 525 than on his 990.
That's the RFS crowd. They ride them, race them, fix them, and swap tips and parts to keep each other going. Twenty-plus years on, this bike still has a fan club. Here are some of the bikes they sent.
