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)?

  • $\begingroup$ I think you have an error in where your decimal point is on your diameter measurement. A 5/16 screw should fit through, with some clearance, a 5/16 hole which is 0.3125". $\endgroup$
    – Eric S
    Jan 11 '17 at 18:24
  • $\begingroup$ there are tolerance ranges for the diameters which appear to be "+0/-xx" sort (You can find size spec charts all over by google.). 0.3065-0.3125 is a typical spec. $\endgroup$
    – agentp
    Jan 11 '17 at 19:03
  • $\begingroup$ @EricShain That's incorrect, a 5/16" bolt has a Basic Major Diameter of 5/16" or 0.3125" and when considering tolerances, that is the largest allowable size. The smallest allowable hole to allow a 5/16" bolt must be larger than the largest possible bolt. That's why a typical close fit hole for a 5/16" bolt is 0.3230" ( Letter P drill) and a free fit hole is 0.3320" (Letter Q drill). $\endgroup$
    – DLS3141
    Jan 13 '17 at 13:17
  • $\begingroup$ @DLS3141. You're right of course. I should have looked things up first before commenting. $\endgroup$
    – Eric S
    Jan 13 '17 at 14:54

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.

Reference Table including Tolerance Classes

  • 1
    $\begingroup$ do you have a source? There is a nice calculator here amesweb.info/Screws/AsmeUnifiedInchScrewThread.aspx which shows different numbers for the different pitches as well as the different classes. none show a minimum major diameter near as small as .2452 $\endgroup$
    – agentp
    Jan 12 '17 at 15:29
  • $\begingroup$ @agentp Yes, you're correct, your reference does not include UNR threads. I used this reference( efunda.com/designstandards/screws/…) and read the column for the Min Major Dia of UNR threads. For UN threads (UNC or UNF, the correct Min Maj Dia for 1A tolerance is 0.2982") I'll revise my answer. - Thanks $\endgroup$
    – DLS3141
    Jan 12 '17 at 21:01

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