This is the 5th gear on the countershaft for the 5 speed NV4500 transmission in a Chevrolet. This gear has 51 teeth, 6 1/2" diameter and a 6.34 low ratio. This gear is used when the 5th gear on the mainshaft has 19 teeth. You can identify this transmission in a Chevy by the cast "C-16812" or "C-18157". This transmission has an iron case with an aluminum top cover! This transmission usually comes in the 1990-95 Chevy 3/4 ton or 1 ton gas and diesel trucks.