The OT-EM3215BLDC motor (SparkFun part ROB-20441) provides M1.6 threaded mounting holes on the rotor face for attaching a hub or fixture. The manufacturer’s schematic labels these as 通孔 (through-holes) extending entirely through the thin rotor faceplate. As a result, using screws longer than intended risks scraping the copper windings as Scott Bezek notes in the SmartKnob BOM:
Screw: M1.6x3 (countersunk). … Beware: long screws may scrape internal motor stator windings!
Judging by the SmartKnob CAD files, I think the effective thread engagement appears to be only about 1.5 mm in the rotor face itself. It would be nice to confirm this value and place it on the product page for the ROB-20441 and ROB-22132 as a heads-up.