Letter T drill size
A letter T drill is 0.3580 in (9.09 mm). It sits between letter S and letter U in the letter series; the nearest metric drill is 9.1 mm.
Letter T at a glance
Decimal, metric and the nearest sizes in the other series.
Nearest metric drills
Metric drills step by 0.05 mm below 3 mm and by 0.1 mm from 3 mm. Difference is metric minus inch size.
| Metric drill | Inch | Difference | In inches | Hole |
|---|---|---|---|---|
| 9.0 mm | 0.3543" | −0.093 mm | −0.0037" | Smaller |
| 9.1 mm | 0.3583" | +0.007 mm | +0.0003" | Larger |
Nearby inch drills
The next size down and up in each inch series, so you can pick a substitute from the drawer.
| Drill | Series | Inch | mm | Difference |
|---|---|---|---|---|
| #1 | Number | 0.2280" | 5.79 mm | −0.1300" |
| 11/32" | Fractional | 0.3438" | 8.73 mm | −0.0142" |
| Letter S | Letter | 0.3480" | 8.84 mm | −0.0100" |
| Letter T | Letter | 0.3580" | 9.09 mm | this size |
| 23/64" | Fractional | 0.3594" | 9.13 mm | +0.0014" |
| Letter U | Letter | 0.3680" | 9.35 mm | +0.0100" |
Threads a Letter T drill can tap
Threads from the tap tables on this site that this drill can tap, with the thread percentage it leaves (55–85 % shown).
| Thread | Standard drill | Thread with this drill | Suits |
|---|---|---|---|
| M10 × 1 | 9 mm | 70 % | Good in steel |
| M10 × 1.25 | 8.8 mm | 56 % | Hard, tough metals |
Clearance for screws and bolts
Bolts and screws a Letter T hole clears, with the diametral clearance and the gap each side. Metric sizes are compared with ISO 273.
| Fastener | Diameter | Clearance | Per side | ISO 273, metric only |
|---|---|---|---|---|
| 5/16" bolt | 0.3125" | 0.0455" | 0.0227" | – |
| M8 bolt | 8 mm | 1.09 mm | 0.55 mm | Normal fit (normal 9, loose 10 mm) |
Inch clearance is the drill size minus the basic major diameter. A drilled hole usually cuts slightly oversize.
Letter drill index
Every size in the series, holes drawn to scale.
Common questions
What size is a letter T drill?
A letter T drill is 0.3580 in, which is 9.09 mm (9.0932 mm exactly).
What is the metric equivalent of a letter T drill?
The nearest metric drill is 9.1 mm, 0.007 mm larger. The other close size is 9.0 mm.
What thread can I tap with a letter T drill?
It leaves about 70 % thread in M10 × 1 and 56 % in M10 × 1.25.
Which fractional drill is closest to letter T?
23/64 in (0.3594 in) is the closest, 0.0014 in larger.
What does a letter T drill clear?
It clears a M8 bolt with 1.09 mm total clearance (0.55 mm per side); for metric bolts: normal fit.