I wonder, do fine and course screws have different diameter if they are the same standard size? For example, what is the outer diameter for 5/16″-18 and 5/16″-24 if I measure them with a caliper? Is it possible that a 5/16″-24 has a diameter of 0.3015inch diameter (which I measured)?
The Basic Major Diameter for both 5/16" -18 UNC and 5/16"-24 UNF threads is 0.3125". The depth of the threads and thus the pitch diameters are different 0.2764" for UNC and 0.2854" for UNF. The limits for those diameters will depend on the tolerance class for the bolts in question. The minimum Major Diameter for a 5/16" UNC bolt varies with its tolerance class as follows:
- Min Maj Dia for 5/16 - 18 UNC Class 1A = 0.2982"
- Min Maj Dia for 5/16 - 18 UNC Class 2A = 0.3026"
Min Maj Dia for 5/16 - 18 UNC Class 3A = 0.3038"
Min Maj Dia for 5/16 - 24 UNF Class 1A = 0.3006"
- Min Maj Dia for 5/16 - 24 UNF Class 2A = 0.3042"
- Min Maj Dia for 5/16 - 24 UNF Class 3A = 0.3053"
So, while the Basic Major Diameter of both coarse and fine thread 5/16" fasteners is the same, in reality the diameters can certainly be different within the allowable tolerances. The diameter of the fastener which you measured at 0.3015" fits within the acceptable range for Class 1A fasteners.