Like Jerry I've never had a problem with superglue or silicone retained motors, either in 2mm or 4mm NG, as he says most of these motors do not have much torque and so are unlikely to tear themselves free. I have, however, just gone slightly crazy and taken out a mortgage for a small Faulhaber and threaded retaining ring so I may soon be basking in securely soldered model engineering respectability. Jerry - nice idea with the one - sided Terrier gearbox, by the way.
Alex.