Tap drills
3/8-16 tap drill size
The tap drill for 3/8-16 is 5/16, which is 0.3125 inches or 7.94 mm. That leaves about 77 percent thread engagement.
3/8-16 tap drill, clearance and engagement
| Series | UNC (coarse) |
| Major diameter | 0.3750 in (9.53 mm) |
| Threads per inch | 16 |
| Tap drill | 5/16 (0.3125 in / 7.94 mm) |
| Thread engagement | 77% |
| Drill for 75% thread | 0.3141 in |
| Clearance, close fit | 25/64 (0.3906 in) |
| Nearest metric drill | 8 mm (0.3150 in) |
What 3/8-16 is used for
Structural inch bolting: machine bases, motor mounts, heavy brackets, trailer and towing hardware. The point where a tap handle stops being enough and a tapping head starts to help.
If you do not have that drill
The tap drill is a plain 5/16 at 0.3125, so no substitution is normally needed. Letter O at 0.316 lowers engagement to about 72 percent for stainless.
How to tap a 3/8-16 thread
-
Drill the hole
Use a 5/16 drill, 0.3125 inches or 7.94 mm. Drill square to the surface and clear the swarf on deep holes.
-
Start the tap square
Start with a taper tap and check it is square in both axes within the first two turns. A tap started crooked will cut a crooked thread all the way down and cannot be corrected.
-
Cut and break the chip
Turn forward about half a turn, then back a quarter turn to break the chip. Use cutting fluid on steel and stainless; aluminium and brass usually need none, though a wax stick helps in aluminium.
-
Check the fit
Run the mating fastener in by hand. It should turn freely without side play. If it binds, the hole was undersized or the tap wandered.
Common questions
What size drill do I need for a 3/8-16 tap?
- The tap drill for 3/8-16 is 5/16, which is 0.3125 inches or 7.94 mm. That leaves about 77 percent thread engagement. The tap drill is a plain 5/16 at 0.3125, so no substitution is normally needed. Letter O at 0.316 lowers engagement to about 72 percent for stainless.
What is the clearance hole for a 3/8-16 bolt?
- 25/64, which is 0.3906 inches. That is a close fit. A normal fit is one drill size larger and a loose fit two, and the looser classes are the sensible choice when the parts have to line up without reaming.
How is the 3/8-16 tap drill worked out?
- Take the major diameter and subtract the engagement fraction times 1.299 divided by threads per inch: 0.3750 minus 0.75 times 1.299 divided by 16 gives 0.3141 inches. The nearest standard drill is 5/16 at 0.3125 inches.
Can I use a metric drill for a 3/8-16 tap?
- The closest standard metric drill is 8 mm, which is 0.3150 inches against 0.3125 for 5/16. That changes engagement to about 74 percent, so check that figure is acceptable for your material before substituting.