Tested Dirt Bike Products - Thumper Racing KTM 570 Four-stroke Big-bore Kit - Experts on Dirtbikes at Dirt Rider Magazine

It can be a headache riding with a group of KTM riders. All those KTM 525 pilots whining about how underpowered and slow their bikes are ... not! We couldn't imagine why we would want a faster KTM 525; the machines are rockets when stock, and quiet. Yet KTM has a 570 kit in its Hard Parts catalog (probably under the subhead, "You've got to be kidding!") that consists of a 540 big-bore kit and a stroker crank. Thumper Racing offers the same-size displacement increase with the bore alone, and it costs $690 if you send in your cylinder to be sleeved.Frankly, we expected the Thumper 570 to be fast but less fun and ridable than the stocker. Our 540 experience had us convinced the bike would accelerate so hard that suspension action would be negatively impacted. We couldn't have been more wrong. The 525 engine seems smooth—until you ride it back-to-back with the Thumper 570. The 570 kit probably doesn't boost overall power much, but it does add a bunch of torque to the lower rpm ranges. The result is an engine even smoother and more docile than a stock 525 but with more meat to the power. Total dial-a-wheelie thrust is available at any rpm. Note that we say wheelie rather than wheelspin; this kit does boost grunt and make you grin like a loon when you twist the handle, but it is even more civil than a stocker. Does that mean we think you need it? This kit should be under, "You've got to be kidding!" in the Thumper catalog, too, but if we had our 525 apart, we'd have trouble thinking of reasons not to put in the bigger piston. Of course, there are folks who still believe in the "too much is just enough" theory. A 6-foot 5-inch, 240-pound test rider who tried the 570 loved the Thumper because it mimicked his XR650R's grunt but with a lighter overall package. He actually checked into buying our test bike. We wouldn't blame any horsepower junkie for going big.DR Tested: 9.5Thumper Racing: 800/259-5186; www.thumperracingusa.com