Unlike the 3.07 which is vented through the gasket/cover plate, the 3.21 is vented through a spring loaded vent in the center of the LH output shaft. If you should get this 3.21, which was available only on the 66 & 67 Toro\'s and the 67 Eldorado, make sure you get the RH short shaft and bearing support. That shaft is specific to the 3.21, so the 3.07 shaft will *NOT* work with the 3.21. There are other differences, but they are internal.