think they're a left hand thread
Mostly correct Andy, however when buying spares at jumbles look closely at them - there are right hand thread ones out there, I found this out after trying for a while to get one started, suspected I'd got a bad thread on it (takes a while for pennies to drop these days).
Bought a new one (retainer) and went in fine, the odd one belongs a spare hub I have so maybe some models where right handed, sure someone will inform us.
I don't have a tool but the making of one is on the to do list, thinking of a big washer with a nut welded to it for a socket and for little pegs from somewhere.
Also I reckon more than a few of us would admit to the savagery of hammer and punch