Letter S drill size
A letter S drill is 0.3480 in (8.84 mm). It sits between letter R and letter T in the letter series; the nearest metric drill is 8.8 mm.
Letter S 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 |
|---|---|---|---|---|
| 8.8 mm | 0.3465" | −0.039 mm | −0.0015" | Smaller |
| 8.9 mm | 0.3504" | +0.061 mm | +0.0024" | 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 | −0.1200" |
| Letter R | Letter | 0.3390" | 8.61 mm | −0.0090" |
| 11/32" | Fractional | 0.3438" | 8.73 mm | −0.0042" |
| Letter S | Letter | 0.3480" | 8.84 mm | this size |
| Letter T | Letter | 0.3580" | 9.09 mm | +0.0100" |
| 23/64" | Fractional | 0.3594" | 9.13 mm | +0.0114" |
Threads a Letter S 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 × 1.25 | 8.8 mm | 71 % | Good in steel |
| M10 × 1.5 | 8.5 mm | 60 % | Hard, tough metals |
Clearance for screws and bolts
Bolts and screws a Letter S 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" | 0.0355" | 0.0177" | – |
| M8 bolt | 8 mm | 0.84 mm | 0.42 mm | Close fit (close 8.4, normal 9 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 S drill?
A letter S drill is 0.3480 in, which is 8.84 mm (8.8392 mm exactly).
What is the metric equivalent of a letter S drill?
The nearest metric drill is 8.8 mm, 0.039 mm smaller. The other close size is 8.9 mm.
What thread can I tap with a letter S drill?
It leaves about 71 % thread in M10 × 1.25 and 60 % in M10 × 1.5.
Which fractional drill is closest to letter S?
11/32 in (0.3438 in) is the closest, 0.0042 in smaller.
What does a letter S drill clear?
It clears a M8 bolt with 0.84 mm total clearance (0.42 mm per side); for metric bolts: close fit.