The advertised February introduction of a fully synchronized, four-speed transmission (RPO 651) was postponed until the 1961 model year. This was due to casting problems with the aluminum three-speed transmission case which resulted in technical service bulletins to dealers advising of the potential for differential failure due to external leaks at the front of the transmission's counter gear shaft. The revision of the four-speed transmission designated for 1961 introduction incorporated a cast-iron case and a redesign of the differential pinion shaft to interface with a longer transmission output shaft and a concentric pilot for the revised transmission case. These are among many of the improvements undertaken by Chevrolet by the end of the 1960 model year.