We know inch fasteners use different naming system from metric fasteners. Not only are they different in measuring units, but also different in engineering philosophy.

 

First of all, imperial system uses gauge numbers to differentiate sizes.

Below is an example of inch system threads (UNC), followed by a table of metric machine threads. 

Nominal Size (UNC)	Major Diameter (in.)	conversion in mm
No.6	0.138	3.5052
No.8	0.164	4.1656
No.10	0.190	4.826
No.12	0.216	5.4864
1/4	0.250	6.35
5/16	0.3125	7.937

 

Nominal Size (Metric)	Major Diameter (mm)
M2.5	2.5
M3	3.0
M3.5	3.5
M4	4.0
M5	5.0
M6	6.0
M8	8.0

Here, a question may come up to some people's mind: "are M8 threads equivalent to 5/16 threads? "

They look very close but not compatible.

Let us look at the engineering philosophies of the two systems.

"Steel bolt/nut assemblies using DIN 6914 bolts and DIN 6915 nuts are designed to comply with the requirement that if an adequately lubricated assembly fails due to overtightening, stripping of the thread occurs rather than fracture of the bolt."  It says in DIN 6915:1999-12, section 4.technical delivery conditions.

 "In the design of structural joints, good engineering suggests that, if a bolt/nut combination should fail due to overtightening during installation or overstressing in service, effort must be made to assure failure by bolt fracture and not by thread stripping" It says in Inch Fastener Standards 7th edition Section H-9.

Obviously, the two engineering perspectives conflict.  But according to the context, we can tell that Inch fasteners tend to have bigger threads to avoid thread stripping than metric fasteners, even though their major diamters are almost the same.

The pitch of M8 threads is 1.25mm.

And, TPI (Threads Per Inch) of 5/16 UNC threads is 18, which can be converted to 1.41mm per pitch.

 

Therefore, there is no chance to fit a M8 bolt to a 5/16 nut.