Tap drills
5/16-18 tap drill size
The tap drill for 5/16-18 is F, which is 0.2570 inches or 6.53 mm. That leaves about 77 percent thread engagement.
5/16-18 tap drill, clearance and engagement
| Series | UNC (coarse) |
| Major diameter | 0.3125 in (7.94 mm) |
| Threads per inch | 18 |
| Tap drill | F (0.2570 in / 6.53 mm) |
| Thread engagement | 77% |
| Drill for 75% thread | 0.2584 in |
| Clearance, close fit | 21/64 (0.3281 in) |
| Nearest metric drill | 6.5 mm (0.2559 in) |
What 5/16-18 is used for
The coarse thread one size up from 1/4-20, and the usual choice for brackets, machine guards, trailer hardware and light structural work where 1/4 inch is not quite enough.
If you do not have that drill
Letter F is 0.257 in. A 1/4 inch drill at 0.250 is 0.007 under and pushes engagement to about 88 percent, which taps hard in steel. Letter G at 0.261 drops it to about 71 percent and is the easier substitute.
How to tap a 5/16-18 thread
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Drill the hole
Use a F drill, 0.2570 inches or 6.53 mm. Drill square to the surface and clear the swarf on deep holes.
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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.
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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.
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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 5/16-18 tap?
- The tap drill for 5/16-18 is F, which is 0.2570 inches or 6.53 mm. That leaves about 77 percent thread engagement. Letter F is 0.257 in. A 1/4 inch drill at 0.250 is 0.007 under and pushes engagement to about 88 percent, which taps hard in steel. Letter G at 0.261 drops it to about 71 percent and is the easier substitute.
What is the clearance hole for a 5/16-18 bolt?
- 21/64, which is 0.3281 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 5/16-18 tap drill worked out?
- Take the major diameter and subtract the engagement fraction times 1.299 divided by threads per inch: 0.3125 minus 0.75 times 1.299 divided by 18 gives 0.2584 inches. The nearest standard drill is F at 0.2570 inches.
Can I use a metric drill for a 5/16-18 tap?
- The closest standard metric drill is 6.5 mm, which is 0.2559 inches against 0.2570 for F. That changes engagement to about 78 percent, so check that figure is acceptable for your material before substituting.