Data for 1/2-26 UNS thread:
Inch Units
External thread
0.5000-26 UNS-2A
Allowance,0.0011
Major Diameter Max,0.4989
Major Diameter Min,0.4921
Pitch Diameter Max,0.4739
Pitch Diameter Min,0.4701
Minor Diameter Max,0.4573
Minor Diameter Min (Ref),0.4451
Root Radius Max,0.0056
Root Radius Min,--
Flat At Root,0.0096
Thread Height (Basic),0.0236
Internal thread
0.5000-26 UNS-2B
Allowance,--
Minor Diameter Min,0.458
Minor Diameter Max,0.467
Pitch Diameter Min,0.4750
Pitch Diameter Max,0.4799
Major Diameter Min,0.5000
Major Diameter Max (Ref),0.5104
Flat At Root,0.0048
Thread Height (Basic),0.0208
Pitch,0.0385
Lead,0.0385
Starts,1
Lead Angle,1.476
Included Angle,60.0000
If you don't have a thread micrometer here are the dimensions for using thread wires:
Wire dia - 0.0222
Max dia - .5072
Min dia - .5034