any 88-91 CRX/Civic axle will fit into any 88-91 CRX/Civic transmission - the difference is at the hub end - the CRX HF and Civic STD have smaller hubs - none of the 88-91 CRX/Civic axles will fit the ZC transmission - it uses the larger diameter 90-93 Integra axles with a ZC (and only ZC) intermediate shaft - the 20 and 21 splines are on the input shaft of the transmission which goes into the clutch disc - has nothing to do with axles - all 88's have 21 spline input shaft and 89-91's have 20 spline