Precision machined Rexnard lighter weight heat treated steel bottom sprockets for the Ski-doo rev gen4 2021 chassis SLEDS WITH NARROWBOTTOM GEAR ONLY . We machined some gears for the 2021 sleds so you can change your ratios. These gears measure .435" wide at the inside of the spline. These are super tough sprockets made to last and give superior performance with lighter weight! These will not fit the 2021 REV GEN4 TURBO 850 ETEC.