Letter R drill size
A letter R drill is 0.3390 in (8.61 mm). It sits between letter Q and letter S in the letter series; the nearest metric drill is 8.6 mm.
Letter R 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 |
|---|---|---|---|---|
| 8.6 mm | 0.3386" | −0.011 mm | −0.0004" | Smaller |
| 8.7 mm | 0.3425" | +0.089 mm | +0.0035" | 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 | −0.1110" |
| 21/64" | Fractional | 0.3281" | 8.33 mm | −0.0109" |
| Letter Q | Letter | 0.3320" | 8.43 mm | −0.0070" |
| Letter R | Letter | 0.3390" | 8.61 mm | this size |
| 11/32" | Fractional | 0.3438" | 8.73 mm | +0.0047" |
| Letter S | Letter | 0.3480" | 8.84 mm | +0.0090" |
Threads a Letter R 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 × 1.5 | 8.5 mm | 71 % | Good in steel |
| 3/8-24 UNF | Q | 67 % | Good in steel |
Clearance for screws and bolts
Bolts and screws a Letter R 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" | 0.0265" | 0.0133" | – |
| M8 bolt | 8 mm | 0.61 mm | 0.31 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 R drill?
A letter R drill is 0.3390 in, which is 8.61 mm (8.6106 mm exactly).
What is the metric equivalent of a letter R drill?
The nearest metric drill is 8.6 mm, 0.011 mm smaller. The other close size is 8.7 mm.
What thread can I tap with a letter R drill?
It leaves about 71 % thread in M10 × 1.5 and 67 % in 3/8-24 UNF.
Which fractional drill is closest to letter R?
11/32 in (0.3438 in) is the closest, 0.0047 in larger.
What does a letter R drill clear?
It clears a M8 bolt with 0.61 mm total clearance (0.31 mm per side); for metric bolts: close fit.