Not sure if this will help but - see the Article from Wikipedia in the attached link. There are various 'conversion tables' in there which might solve your question.
In engineering you can use items around the workshop such as posters on info boards or their may already be drill and hole sizes for tap for example a 6.8mm drill bit for a M8 tap on a technical drawing or bend tolerences for the materials thickness.
M 14 1.25
The instrument "Metalic tap" is generally used for length measurement in India
There are size on size weldolets. The hold on the run pipe will necessarily be undersize (either hot tap or torch ) because it's not possible to cut the same size hole in the run without damaging the base material.
For a cut tap the correct drill size is a #17 drill, which has a diameter of .173 inches.
2,3mm
Use 7/64 as tap drill and 9/64 as clearance drill
Drill Jigs are used to tap holes.
Google "tap drill sizes" and you'll get 19400 results. The formula for that calculation is searchable in Machinery's Handbook, but there are simple charts you will get in your search that can give you the answer for any size you need to know.
Drill and tap it for what?
1/8, tpi=28, pitch=0.0357, major dia.=0.383, minor dia.=0.3372, tap drill size=11/32 (8.73mm)
It is not a drill bit, it is called a Tap. You drill a hole slightly smaller than the Tap and then screw it into the hole.
The hole would have no metal to cut the threads in if the hole were drilled the same diameter as the tap.
Common references list the tap drill for a #12-24 screw as #17 or .173"
acquire correct tap from hardware or automotive supplier. Drill smaller hole through EGR port boss on head. Drill larger hole , size stated on tap to same depth as on intake port. then tap. If metric thread tap cannot be acquired a tapered pipe thread tap will also work. Threads are very close and will seal because of taper.
drill off the head remove whatever it held down use vicegrips to remove the rest becarful not to drill to deep or brake the saft off or youll have to drill and tap out the hole for another size screw