---
type: "thread"
name: "#8-36 UNF"
designation: "#8-36 UNF"
series: "unf"
series_name: "Unified Fine (UNF)"
system: "unified"
size_label: "#8"
tpi: 36
pitch_in: 0.027778
pitch_mm: 0.7056
basic_major_dia_in: 0.164
basic_major_dia_mm: 4.1656
basic_pitch_dia_in: 0.146
basic_pitch_dia_mm: 3.7084
basic_minor_dia_in: 0.1339
basic_minor_dia_mm: 3.4011
tap_drill: "#29"
tap_drill_dia_in: 0.136
tap_drill_dia_mm: 3.4544
tap_drill_thread_pct: 77.6
clearance_normal_drill: "#9"
tensile_stress_area_sqin: 0.014727
tensile_stress_area_sqmm: 9.501
internal_class_assumed: "2B"
standard: "ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)"
verified: "August 1, 2026"
url: https://shopmath.org/threads/unf/8-36
---

# #8-36 UNF Thread — Dimensions, Tap Drill & Clearance

The tap drill for #8-36 UNF is #29 — 0.1360 in (3.454 mm) — which leaves 77.6% thread engagement with a cut tap. Basic pitch diameter is 0.1460 in (3.708 mm) and basic minor diameter is 0.1339 in (3.401 mm). Tapped-hole figures on this page assume a class 2B thread; drill sizes for 50–85% engagement are in the tap drill table below.

- **Series**: Unified Fine (UNF)
- **Designation**: #8-36 UNF
- **Threads per inch**: 36
- **Tap drill**: #29
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for #8-36 UNF

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.1640 | 4.166 |
| Threads per inch | 36 | — |
| Pitch | 0.0278 | 0.706 |
| Basic pitch diameter | 0.1460 | 3.708 |
| Basic minor diameter | 0.1339 | 3.401 |
| External thread root diameter | 0.1309 | 3.325 |
| Sharp V thread height (H) | 0.0241 | 0.612 |

_Basic dimensions are the theoretical values before any tolerance class is applied._

## Tap Drill Sizes for #8-36 UNF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a #29 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.1460 | 3.708 | #26 | 0.1470 | 3.734 | 47.1 |
| 60% | 0.1423 | 3.614 | #27 | 0.1440 | 3.658 | 55.4 |
| 65% | 0.1405 | 3.569 | #28 | 0.1405 | 3.569 | 65.1 |
| 70% | 0.1387 | 3.523 | #28 | 0.1405 | 3.569 | 65.1 |
| 75% (default) | 0.1369 | 3.477 | #29 | 0.1360 | 3.454 | 77.6 |
| 80% | 0.1351 | 3.432 | #29 | 0.1360 | 3.454 | 77.6 |
| 85% | 0.1333 | 3.386 | #29 | 0.1360 | 3.454 | 77.6 |

_Target hole Ø is the theoretical hole for the stated engagement. The drill column is the nearest size in the standard number/letter/fractional/metric index, and actual thread % is what that real drill produces._

## Clearance Hole Sizes for #8-36 UNF

The through-hole a #8-36 UNF fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | #15 | 0.1811 | 4.600 | Located joints, dowelled assemblies |
| Normal | #9 | 0.1969 | 5.001 | General assembly — the default |
| Loose | #4 | 0.2126 | 5.400 | Slotted or adjustable joints, weldments |

_Clearance holes per ISO 273 (metric) and ASME B18.2.8 (inch)._

## Thread Class Limits for #8-36 UNF

Manufacturing limits for classes 2B, 3B, 2A, 3A. A gauge reading outside these bands is a reject, not a judgement call.

### External thread (bolt / screw) limits

| Limit | Class 2A (in) | Class 2A (mm) | Class 3A (in) | Class 3A (mm) |
| --- | --- | --- | --- | --- |
| Major Ø max | 0.1632 | 4.145 | 0.1640 | 4.166 |
| Major Ø min | 0.1577 | 4.006 | 0.1585 | 4.026 |
| Pitch Ø max | 0.1451 | 3.686 | 0.1460 | 3.708 |
| Pitch Ø min | 0.1423 | 3.614 | 0.1439 | 3.655 |
| Pitch Ø tolerance | 0.0028 | 0.071 | 0.0021 | 0.053 |
| Minor Ø max | 0.1271 | 3.228 | 0.1279 | 3.249 |
| Minor Ø min | 0.1243 | 3.157 | 0.1258 | 3.195 |
| Allowance | 0.0008 | 0.020 | 0.0000 | 0.000 |

### Internal thread (nut / tapped hole) limits

| Limit | Class 2B (in) | Class 2B (mm) | Class 3B (in) | Class 3B (mm) |
| --- | --- | --- | --- | --- |
| Major Ø max | 0.1717 | 4.361 | 0.1707 | 4.336 |
| Major Ø min | 0.1640 | 4.166 | 0.1640 | 4.166 |
| Pitch Ø max | 0.1496 | 3.800 | 0.1487 | 3.777 |
| Pitch Ø min | 0.1460 | 3.708 | 0.1460 | 3.708 |
| Pitch Ø tolerance | 0.0036 | 0.091 | 0.0027 | 0.069 |
| Minor Ø max | 0.1416 | 3.597 | 0.1416 | 3.597 |
| Minor Ø min | 0.1339 | 3.401 | 0.1339 | 3.401 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of #8-36 UNF

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.01473 | 9.50 |

The effective cross-section used to size a #8-36 UNF fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

## Other Thread Pitches in the Same Size as #8-36 UNF

A #8 fastener is not always #8-36 UNF. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [#8-32 UNC](https://shopmath.org/threads/unc/8-32.md) | Unified Coarse (UNC) | 32 TPI | #29 |

## Sources and Method for #8-36 UNF

### Basic dimensions

- **Source**: NBS Handbook H28 (1969) Part I §§6-7 (public domain, 17 U.S.C. 105); normative modern reference ASME B1.1; series membership: Series membership: threadlib UIS_thread.csv (BSD-3-Clause); cross-checked against NBS H28 (1969) Part I Table 2.21; clearance holes: ISO 273 (metric) and ASME B18.2.8 (inch) via FreeCAD FeatureHole.cpp (LGPL-2.1+), cross-checked against bd_warehouse clearance_hole_sizes.csv (Apache-2.0)
- **Method**: computed
- **Verified against**: NBS H28 (1969) Part I, Table 2.21 (public domain); cross-checked against FreeCAD CAM thread tables (LGPL-2.1+) — 1064 checks passed at 0.0001 in

### Tap drill sizes

- **Source**: Machinery's Handbook %-thread formula (geometry-derived); nearest-standard-drill-to-75% rule verified at 36/39 published chart rows in docs/research/data-sourcing.md §3.4
- **Method**: computed
- **Verified against**: Wikipedia Unified Thread Standard, preferred cutting tap drill size columns (CC BY-SA 4.0) — 36/39 published UNC+UNF rows reproduced

### Class limits

- **Source**: NBS Handbook H28 (1969) Part I §§6-7 (public domain, 17 U.S.C. 105); normative modern reference ASME B1.1
- **Method**: computed
- **Verified against**: NBS H28 (1969) Part I, Table 2.21 (public domain); cross-checked against FreeCAD CAM thread tables (LGPL-2.1+) — 1064 checks passed at 0.0001 in

### Formulas

**Tap drill hole diameter, inch threads**

```text
hole Ø  =  D − (%thread ÷ 100) × (1.29904 ÷ n)

  D = basic major diameter (in)
  n = threads per inch
```

_1.29904 = 2 × 0.64952, twice the engaged thread height per side on the 60° Unified profile._

**Actual thread engagement of the chosen drill**

```text
%thread  =  (D − drill Ø) × n ÷ 1.29904 × 100
```

_The drill column rounds to a real drill; this is the engagement that drill actually leaves._

**Tensile stress area, Unified inch threads**

```text
At  =  0.7854 × (D − 0.9743 ÷ n)²
```

_ASME B1.1 / NBS H28 form. Reproduces the stored value exactly for this thread._

### Assumptions

- The headline tap drill assumes 75% thread engagement, the general shop default. Every engagement from 50% to 85% is in the tap drill table — change the assumption and the number moves.
- Tap drill figures are for cut taps. Drill diameters are nominal; a worn or run-out drill cuts oversize and reduces engagement further.
- Tapped-hole figures assume a class 2B internal thread, the general-purpose fit.
- Dataset scope: Le = 1D for UNC/UNF, 9P for UNEF — determined empirically against H28 Table 2.21, resolving data-sourcing.md open item 5. EXCLUDED: form (roll) tap drills, class 1A/1B limits, class 1AR allowances, constant-pitch UN series.

**Data last verified**: August 1, 2026

## Common Questions About #8-36 UNF

### What is the tap drill size for #8-36 UNF?

The tap drill for #8-36 UNF is #29 — 0.1360 in (3.454 mm). That leaves 77.6% thread engagement with a cut tap.

### What is the pitch diameter of #8-36 UNF?

The basic pitch diameter of #8-36 UNF is 0.1460 in (3.708 mm). For a class 2B thread it runs 0.1460 in (3.708 mm) to 0.1496 in (3.800 mm).

### What is the minor diameter of #8-36 UNF?

The basic minor diameter of #8-36 UNF is 0.1339 in (3.401 mm). That is the theoretical root of the internal thread, not the drilled hole size — see the tap drill table for the hole to drill.

### What is the thread pitch of #8-36 UNF?

#8-36 UNF has 36 threads per inch, a pitch of 0.0278 in (0.706 mm).

### What size clearance hole does a #8-36 UNF fastener need?

close fit #15 (0.1811 in (4.600 mm)), normal fit #9 (0.1969 in (5.001 mm)), loose fit #4 (0.2126 in (5.400 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of #8-36 UNF?

The tensile stress area of #8-36 UNF is 0.01473 in² (9.50 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Fine (UNF): [#6-40 UNF](https://shopmath.org/threads/unf/6-40.md) — tap drill #33
- Larger in Unified Fine (UNF): [#10-32 UNF](https://shopmath.org/threads/unf/10-32.md) — tap drill #21
- All Unified Fine (UNF) sizes: [Unified Fine (UNF) thread chart](https://shopmath.org/threads/unf.md)
- Every thread series: [Screw thread reference](https://shopmath.org/threads.md)
- Cross-series chart: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)

---

*Source: [ShopMath](https://shopmath.org/threads/unf/8-36) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*