Hot Rods – Bottom End Kits, including kits for the 99-04 Honda TRX 400EX, the 87-06 Yamaha Banshee 350, the 04-09 YFZ 450, the 04-05 Honda TRX 450ER, the 03-06 Kawasaki KFX 400 and the 11-14 Polaris Sportsman 800 4x4 and 6x6. Each kit pairs a forged, single-piece crankshaft with matched main bearings, seals and the gaskets needed to close the cases, so an ATV bottom end goes back together as one engineered assembly instead of a pile of mismatched parts. That matters because the crank and mains carry every bit of load an ATV engine makes, from hard launches in the sand to hours of trail lugging at low rpm, and a tired crank with worn mains is what turns a rebuild into a repeat rebuild. Fresh ATV crankshaft and main bearing kits hold true rod alignment and correct end play, which cuts vibration through the frame and footpegs, sharpens throttle response and keeps oil pressure where it belongs. The result is an engine that revs clean, idles steady and stays quiet under load instead of buzzing your hands numb. Tell us the year, model and displacement of your quad and we will confirm the correct kit fits before you order.