There are eight chrome-plated setscrews securing the four side curtain escutcheons to the doors of the XK120 OTS. XK120 Explored shows the threads of these setscrews to be No. 6 ANF.
At one of the eight screw locations, the setscrew will not screw all the way down. The threads on all eight setscrews seem to be fine, and I believe all eight setscrews are original to the car. Thus, I suspect the female threads inside the door (at that one location) are cross-threaded.
My question: Can I use a No. 6 UNF (40 TPI) tapered tap to the chase these No. 6 ANF threads?
First check with another screw that the threads are indeed 40 tpi.
You are in the USA so your local hardware store will have nuts if you want to check by trying your screw on one.
Then if it fits right, yes, you can use a # 6 UNF tap to chase them.
Go gently in tightening, because with crossed threads, you are not really repairing them, you are just making them fit.
The American National Standard Series (N-Series) was last defined in ANSI/ASME B1.1:1935 and has been obsolete since 1949. In ANSI/ASME B1.1:1949 when the American National Standard Series (N-Series) was made obsolete the N-Series screw thread data was moved to the Nonmandatory Appendix. When the standard was revised again as: ANSI/ASME B1.1:1960 the American National Standard Series was intended to be forgotten and was removed completely from the standard. ANSI/ASME B1.1 has since been revised in 1974; 1982, 1989 and 2003.
https://www.ring-plug-thread-gages.com/ti-N-vs-UN.htm
Rob, thanks for the technical detail and advice. Where I live, everything in No. 6 UNF seems to be obsolete and unavailable at the retail level. I may end up purchasing a bag of 100 No. 6 UNF nuts from McMaster-Carr just to verify the thread size of my chrome setscrews. Fortunately, it’s nothing critical or urgent.
Hold the tap against a screw to compare thread pitches. Run your tap into one of the good holes and see if it feels right.
