#1 find someone with the correct "TIMESERT" thread repair kit. they
use a threaded bushing with timed threads creating a solid high strength
repair which is both one piece , self locking , flush fitting and small OD.
superior to spring type repairs like HELICOIL and REDDOT etc
#2 LOCKTITE (maybe PERMATEX now) makes a thread repair compound
which has a fair amount of strength to it and might do an OK job,it
actually creates new threads out of an epoxy type material.
#3 you might be able to step up to the next size 4mm to 5mm or
even to a fractional size BIG problem here might be the fastener would
no longer match.
I can only testify to repair #1 but have seen all others above in
use with varing success.
BOB FREITAS
MILL VALLEY,CA
and YES I sell the above products