13/16-20 UNEF Thread Dimensions and Tap Drill
13/16-20 UNEF is the Unified extra fine thread for the 13/16 in (20.64 mm) diameter: 20 threads per inch, a pitch of 0.05000 in (1.270 mm), on a 60° profile. ASME B1.1 lists it in the UNEF series as a secondary size.
- Tap drill49/64"72% thread, 19.45 mm
- Pitch20 TPI1.270 mm
- Major diameter0.8125 in20.637 mm
- Pitch diameter0.7800 in19.813 mm
- Minor diameter (nut)0.7584 in19.263 mm
- Tensile stress area0.458 in²295 mm²
Thread profile
13/16-20 UNEF basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 0.8125 | 20.637 |
| Pitch P | 0.05000 | 1.270 |
| Pitch diameter d2 = D2 | 0.7800 | 19.813 |
| Minor diameter, internal thread D1 | 0.7584 | 19.263 |
| Minor diameter, external UNR thread d3 | 0.7530 | 19.125 |
| Height of fundamental triangle H | 0.0433 | 1.100 |
| Thread height, internal (5H/8) | 0.0271 | 0.687 |
| Thread height, external (11H/16) | 0.0298 | 0.756 |
| Crest flat of external thread (P/8) | 0.0063 | 0.159 |
| Crest flat of internal thread (P/4) | 0.0125 | 0.318 |
| Property | Value |
|---|---|
| Threads per inch | 20 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 1.17° |
| Tensile stress area As | 0.458 in² (295 mm²) |
Basic (nominal) sizes per ASME B1.1, before tolerances. A UN external thread with a flat root has the same basic minor diameter as the nut (D1); d3 applies to the rounded UNR root.
13/16-20 UNEF limits of size (class 2A bolt, 2B nut)
Classes 2A and 2B are the general-purpose fits for commercial bolts, screws and nuts. The 2A external thread has an allowance of 0.0013 in, so its maximum size is that much below basic; the 2B nut starts at the basic size.
| External thread 2A | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Major diameter | 0.8112 | 0.8031 | 0.0081 |
| Pitch diameter | 0.7787 | 0.7743 | 0.0044 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 0.770 | 0.758 | 0.0120 |
| Pitch diameter | 0.7858 | 0.7800 | 0.0058 |
Limits of size from ASME B1.1 Table 2.
13/16-20 UNEF tap drill size
Tap drill for 13/16-20 UNEF: 49/64" (0.7656 in, 19.45 mm)
This leaves about 72% thread in the tapped hole.
49/64" is the drill listed for 13/16-20 UNEF in standard tap drill charts. It is 0.0469 in under the 0.8125 in major diameter.
The 2B minor diameter of the finished nut thread must lie between 0.758 and 0.770 in. The recommended drill falls inside that band.
Other drills that work for 13/16-20 UNEF
A larger drill cuts less thread and taps more easily; a smaller one gives a fuller thread but loads the tap harder.
| Drill | mm | in | % thread | Within 2B limits | Thread engagement |
|---|---|---|---|---|---|
| 49/64"recommended | 19.45 | 0.7656 | 72% | yes | |
| 19.5 mm | 19.50 | 0.7677 | 69% | yes |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for 13/16 in
ASME B18.2.8 does not list the 13/16 in size, so there is no standard clearance hole for the 13/16 in size. Size the hole from the fit the joint needs.
13/16-20 UNEF proof and breaking loads
Loads for steel bolts by SAE grade, from the tensile stress area of 0.458 in². The proof load is the highest load the bolt must carry without permanent set.
| SAE grade | Proof stress (psi) | Min tensile strength (psi) | Proof load (lbf) | Min breaking load (lbf) | Proof load (kN) |
|---|---|---|---|---|---|
| Grade 2 | 33,000 | 60,000 | 15,100 | 27,500 | 67.2 |
| Grade 5 | 85,000 | 120,000 | 38,900 | 55,000 | 173 |
| Grade 8 | 120,000 | 150,000 | 55,000 | 68,700 | 244 |
Strengths from SAE J429 for this diameter; stress area from ASME B1.1. Loads rounded to three significant figures.
Closest metric thread
No metric thread is interchangeable with 13/16-20 UNEF. The nearest is M20×1.5, which is 0.64 mm smaller in diameter with a pitch of 1.500 mm against 1.270 mm. A nut may start on the wrong thread and then bind or strip, so identify by both diameter and pitch.
How the 13/16-20 UNEF dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 0.05000 in = 1 ÷ 20 threads per inch
- H = 0.866025 × P = 0.0433 in
- d2 = d − 0.649519 × P = 0.8125 − 0.0325 = 0.7800 in
- D1 = d − 1.082532 × P = 0.8125 − 0.0541 = 0.7584 in
- d3 = d − 1.190785 × P = 0.8125 − 0.0595 = 0.7530 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 0.7638² = 0.45818 in²
- Lead angle = arctan(P ÷ (π × d2)) = 1.17°
Related sizes and tools
Sources: ASME B1.1 (profile, series, limits of size and stress area), ASME B18.2.8 (clearance holes), SAE J429 (mechanical properties) and the tap drill tables of Machinery’s Handbook. How this site checks its data. Last reviewed 29 September 2026.