Letter Y drill size
A letter Y drill is 0.4040 in (10.26 mm). It sits between letter X and letter Z in the letter series; the nearest metric drill is 10.3 mm.
Letter Y 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 |
|---|---|---|---|---|
| 10.2 mm | 0.4016" | −0.062 mm | −0.0024" | Smaller |
| 10.3 mm | 0.4055" | +0.038 mm | +0.0015" | 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 | −0.1760" |
| 25/64" | Fractional | 0.3906" | 9.92 mm | −0.0134" |
| Letter X | Letter | 0.3970" | 10.08 mm | −0.0070" |
| Letter Y | Letter | 0.4040" | 10.26 mm | this size |
| 13/32" | Fractional | 0.4062" | 10.32 mm | +0.0022" |
| Letter Z | Letter | 0.4130" | 10.49 mm | +0.0090" |
Threads a Letter Y 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 |
|---|---|---|---|
| M12 × 1.75 | 10.2 mm | 76 % | Soft metals, plastics |
Clearance for screws and bolts
Bolts and screws a Letter Y 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 |
|---|---|---|---|---|
| 3/8" bolt | 0.3750" | 0.0290" | 0.0145" | – |
| M10 bolt | 10 mm | 0.26 mm | 0.13 mm | Tighter than close fit (10.5 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 Y drill?
A letter Y drill is 0.4040 in, which is 10.26 mm (10.2616 mm exactly).
What is the metric equivalent of a letter Y drill?
The nearest metric drill is 10.3 mm, 0.038 mm larger. The other close size is 10.2 mm.
What thread can I tap with a letter Y drill?
It leaves about 76 % thread in M12 × 1.75.
Which fractional drill is closest to letter Y?
13/32 in (0.4062 in) is the closest, 0.0022 in larger.
What does a letter Y drill clear?
It clears a M10 bolt with 0.26 mm total clearance (0.13 mm per side); for metric bolts: tighter than close fit.