The WR500 was introduced in 1991, billed as a 1992 model, and was produced until late 1993. It was an updated version of the aged, but venerable YZ490. The WR500 engine is the same mill that powered the YZ490, unchanged except for the ignition stator, 38mm round-slide Mikuni, new combustion chamber, pipe and silencer. The ignition features a lighting coil (35-watt), less advance (1.7mm from 2.0mm) and the same flywheel as the 490. Compression dropped from 6.9:1 to 6.64:1 to get rid of the pinging. Also, there is a new lower engine mount to cut vibration. Overall gearing is a tad taller (14/50), but the internal ratios are unchanged from the YZ490. Basically, the chassis is a '91 YZ250 unit, modified to accept the larger engine and strengthened to deal with the added power. But the main difference from the 250 frame is that the WR500 frame has a completely removable subframe (a Yamaha first!). The 250 only has the left side backstay removable (the WR500 also has the removable backstay for quick airbox and shock service and removing). Weight bias is different from the 250, and the head angle is kicked out (from 27.5 to 28.15). Linkage rates are identical to the 250, but the shock has different spring rate and valving. The WR500 is stiffer at both ends, and the Deltabox swingarm is different also. It´s 20mm longer and has snail adjusters. Besides the 3.43-gallon gas tank, which is laminated to hold the tank stickers better, and left side air scoop, the WR500 gets plastics identical to the '91/92 WR250. Model nomenclature for the 1992 is WR500ZD, and for the 1993 is WR500ZE.