These are Heidt's 31 Spline Axles for use with 9" Ford 60" track width rear end housings. These rear axles have Chevy 4-3/4" bolt pattern with 1/2" studs and will give you 1/2" offect for your rear brakes. These axles are commonly use on 1967-69 Camaro and Fierbirds, 1968-72 Nova and 1970-81 Camaro and Fierbird applications.
