Yes #20 but if you're doing one you might as well do both sides the set of them is $30 or so. Mine popped out on the same side and I just pushed it right back in. Make sure you remove the flywheel correctly and line up you stator timing marks and seat you flywheel all the way and torque the retainer nut so you don't run into the flywheel issues I ran into. If you replace both the nut that holds that gear on the opposite side is a reverse thread (righty loosey lefty tighty) so be mindful of that when disassembling to get to the other seal.