Hello and help please. I'm trying to get a newly machined 316 stud 3/4" BSW into a cast iron threaded hole which has got a bit rusty. I've cleaned it out as as best I can,is repeatedly. Access is not good as it's under 2cm of fibreglass. The original stud goes in ok-ish. The new stud threads look good inasmuch as a nut freely runs along it. The other same sized stud that goes into the other hole gets stuck at the same place but goes into it's own hole ok. I suspect that the rust in the hole is the problem. I have seen articles about making a makeshift thread cleaning tap by cutting grooves into a similarly threaded bolt. My main question is: does that work? I haven't really got time to order a proper tap as this is holding up work that needs to be done by the end of the weekend. Any other tips or tricks - anything to dissolve loose surface rust? I can't get even a small wire brush in there, just a toothbrush & rags on a screwdriver, which I've been using. Many thanks.