Think they are 1.6mm m4 if I remember correctly
ok, but they must be longer than 1.6mm. M5 starts at 8mm length; i suppose M4 would too, but if they're too long, it's quite easy to cut them and clean the start of the thread.
Try this thread maybe: http://www.ecigssa.co.za/threads/common-screw-sizes-orings-for-rba-rta-rda.1405/