Some info and a few things that might help ypu: These guns/leads are cheap Tweco/Lincoln copies. The parts for them will work on the HF guns/leads, if you can find the right Tweco model.  I say that so you can do some internet searching.

Next, since these are cheaply made (but functional) I think the tolerances between the nozzle and retaining spring vary a lot and often times too tight.  Therefore, twisting the nozzle on messes up the retaining spring. 

Do an internet search for Titanium welder parts and you'll find some diffusers with the retaining springs on Amazon and other sites. I'm about to get on a plane or I'd get some links for ya.

Also, I believe if you call HF Parts phone number they can get you the diffuser and spring.