flexplate has to match the bellhousing. 164 tooth flexplates require a matching bellhousing to place the starter further out, due to the larger diameter of the flexplate. The same holds true for the smaller 157 tooth flex, needing a smaller bell. Example, if you were to use a 157 tooth flex in a 164 tooth bell, the starter would not properly engage the flex, because the starter is placed too far out. The starter itself will work with a 157 or 164 tooth flex (on the appropriate bell of course), because the tooth pitch is the same. C4, C6, T5, and AOD transmissions use one type of starter, with a shallower nose. 3 and 4 speed manual transmission use a starter with a longer nose.