The motors have shafts that are 1/2" [12.7mm] in diameter with a 5mm wide keyslot that is 12mm long (NEMA 23) and 20mm long (NEMA 34) and 3mm deep. They are 3mm deep because a 5x5mm key would seat down 2.5mm, but an extra 0.5mm is given for clearance.
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[https://www.teknic.com/files/downloads/clearpath_user_manual.pdf#page=163 NEMA 23 drawing]
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[https://www.teknic.com/files/downloads/clearpath_user_manual.pdf#page=162 NEMA 34 drawing]