- In 2004, Yamaha introduced an aluminum alloy frame supported by a steel backbone to provide a superior power-to-weight ratio with a dry weight of 190 lbs. Yamaha offered the YZ125 ready to race directly out of the crate with such racing features as ProTaper aluminum handlebars, gripper seat and titanium foot pegs. There was a removable aluminum rear subframe made with square-section tubing to further reduce the bike's weight.
- Yamaha enlarged the Mikuni TMX carburetor from 32 mm on the air-cooled models to 38 mm on the liquid-cooled versions. Its small 125 cc displacement prevented Yamaha from matching the engine with a four-speed transmission, so the motorcycle maker used five- and six-speed transmissions. The 2009 and 2010 versions matched the engine with a close-ratio six-speed transmission. The engine also featured the motorcycle maker's patented Yamaha Power Valve System, or YPVS, with a reed-valve space for better off-the-line acceleration and mid-range power. It also had a 10-row-core, large-capacity radiator with four louvers for better cooling.
- The Yamaha YZ125 patterned its brake system from the YZ250F, with a 9.8-inch single front disc brake and a 9.6-inch rear disc brake. Yamaha's Speed-Sensitive System featured 48 mm KYB inverted forks with a piston speed control system to control the damping force. The travel range of the forks was 11.8 inches. The rear suspension featured an adjustable ultra-lightweight titanium KYB shock with 12.4 inches of travel. The rear also featured a fully locked oil rebound system for better rear wheel control. The front tire mounted to a 21-inch wheel and the rear tire to a 19-inch wheel. Original factory YZ125s were equipped with Dunlop knobby tires.
- Yamaha sought to provide riders with greater comfort and the ability to have more freedom to shift their weight on the bike by designing a gripper-style seat and fuel tank. The motorcycle maker widened and cleated the titanium foot pegs for better grip as well. Lightweight box-head bolts reduced body weight and made it easier for owners to perform maintenance. To enhance the bike's appearance, Yamaha accented the drive chain with gold plates. It also provided a wide fuel tank filler cap on the 2.1-gallon tank, an exhaust silencer and molded handlebar grips.
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