Basically, the head broke off of the small plastic air bleed screw in the neck on top of the radiator, and it no longer seals. What size is this screw (or the drain screw, same thing) so i can just get a bolt and some silicone to seal it, or where can I get another one? This is one of the last things keeping me from driving my newly swapped turbo, and it has to be something infuriating like that.

 

I don't know what size it is but when I broke mine I took it to AutoZone and they had it in one of those aftermarket packs. Worked fine.

 

10x1.25

 

I got an M10 and M12, both 1.25 pitch, and it was the M12 that fit. So just as a correction, its M12 x 1.25