Myles-
I believe 1/8"-27 NPT pipe tap. It is similar to 3/8"-24 which what you'd have with 3/8" fine thread. Those nipples are hard to get back on, I nearly messed up one of my a-arms trying to get things put back together. I may have one because of that fiasco but I'm not positive.
Randy
edit: Just checked, yes I did buy this tap to fix up the same problem - so if you'd like to borrow it, pm me.
edit #2: Just checked the team.net archives to refresh my memory. I think the original was 1/8-28 British Standard Pipe, but the complication is that it is pretty exotic and the nipples you get here are 1/8-27 National Pipe Thread. The thread pitch is close enough that I think it will mate fine. Someone with better technical knowledge may have a more correct story - but I know I bought the tap I have to fix this problem, and it did!