Here are the 4 types of starter bushings, showing the various sizes and comparing them to one another:Upper Left: 6 volt transmission case, to be used with a 6 volt starter. Often called a straight 6 bushing.Upper Right: 12 volt transmission case, to be used with a 6 volt starter. Often called a 12-to-6 bushing.Lower Left: 12 volt transmission case, to be used with a 12 volt starter. Often called a straight 12 bushing.Lower Right: 6 volt transmission case, to be used with a 12 volt starter. Often called a 6-to-12 bushing.