1-3/8-18 UNEF Thread Dimensions and Tap Drill
1-3/8-18 UNEF is the Unified extra fine thread for the 1-3/8 in (34.92 mm) diameter: 18 threads per inch, a pitch of 0.05556 in (1.411 mm), on a 60° profile. ASME B1.1 lists it in the UNEF series. Against the coarse 1-3/8-6 UNC it has 18 threads per inch instead of 6, a stress area 19% larger and a lead angle of 0.76° instead of 2.40°.
- Tap drill1-21/64"65% thread, 33.73 mm
- Pitch18 TPI1.411 mm
- Major diameter1.3750 in34.925 mm
- Pitch diameter1.3389 in34.008 mm
- Minor diameter (nut)1.3149 in33.397 mm
- Tensile stress area1.37 in²884 mm²
- Clearance hole1-1/2 innormal fit, ASME B18.2.8
Thread profile
1-3/8-18 UNEF basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 1.3750 | 34.925 |
| Pitch P | 0.05556 | 1.411 |
| Pitch diameter d2 = D2 | 1.3389 | 34.008 |
| Minor diameter, internal thread D1 | 1.3149 | 33.397 |
| Minor diameter, external UNR thread d3 | 1.3088 | 33.245 |
| Height of fundamental triangle H | 0.0481 | 1.222 |
| Thread height, internal (5H/8) | 0.0301 | 0.764 |
| Thread height, external (11H/16) | 0.0331 | 0.840 |
| Crest flat of external thread (P/8) | 0.0069 | 0.176 |
| Crest flat of internal thread (P/4) | 0.0139 | 0.353 |
| Property | Value |
|---|---|
| Threads per inch | 18 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 0.76° |
| Tensile stress area As | 1.37 in² (884 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.
1-3/8-18 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 0.0015 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 | 1.3735 | 1.3648 | 0.0087 |
| Pitch diameter | 1.3374 | 1.3325 | 0.0049 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 1.328 | 1.315 | 0.0130 |
| Pitch diameter | 1.3453 | 1.3389 | 0.0064 |
Limits of size from ASME B1.1 Table 2.
1-3/8-18 UNEF tap drill size
Tap drill for 1-3/8-18 UNEF: 1-21/64" (1.3281 in, 33.73 mm)
This leaves about 65% thread in the tapped hole.
Published charts rarely list this size. 1-21/64" is the nominal diameter minus 3/64 in, the same convention charts use for 9/16-18 and 5/8-18.
The 2B minor diameter of the finished nut thread must lie between 1.315 and 1.328 in. The charted drill is slightly under that band: the tap trims the crests to size, at the cost of higher tapping torque. A drill inside the band is the safer choice in tough materials.
Other drills that work for 1-3/8-18 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 |
|---|---|---|---|---|---|
| 1-5/16" | 33.34 | 1.3125 | 87% | no | |
| 33.5 mm | 33.50 | 1.3189 | 78% | yes | |
| 1-21/64"recommended | 33.73 | 1.3281 | 65% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for 1-3/8 in
The clearance hole depends only on the 1.3750 in diameter, not on the thread series. The normal fit, a 1-1/2 in drill, is the usual choice for general assembly.
| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Clearance on Ø (in) |
|---|---|---|---|---|
| Close | 1-7/16 in | 1.4375 | 36.51 | 0.0625 |
| Normal | 1-1/2 in | 1.5000 | 38.10 | 0.1250 |
| Loose | 1-39/64 in | 1.6094 | 40.88 | 0.2344 |
Nominal drill sizes from ASME B18.2.8.
1-3/8-18 UNEF proof and breaking loads
Loads for steel bolts by SAE grade, from the tensile stress area of 1.37 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 | 45,200 | 82,200 | 201 |
| Grade 5 | 74,000 | 105,000 | 101,000 | 144,000 | 451 |
| Grade 8 | 120,000 | 150,000 | 164,000 | 206,000 | 731 |
Strengths from SAE J429 for this diameter; stress area from ASME B1.1. Loads rounded to three significant figures.
1-3/8 in pitches compared
All standard pitches on the 1-3/8 in diameter. A finer pitch gives a shallower thread, a larger core and a smaller lead angle; a coarser pitch is faster to assemble and more tolerant of damage and plating.
| Thread | TPI | Pitch Ø (in) | Minor Ø D1 (in) | Stress area (in²) | Tap drill |
|---|---|---|---|---|---|
| 1-3/8-6 UNC | 6 | 1.2667 | 1.1946 | 1.155 | 1-7/32" |
| 1-3/8-12 UNF | 12 | 1.3209 | 1.2848 | 1.315 | 1-19/64" |
| 1-3/8-18 UNEF | 18 | 1.3389 | 1.3149 | 1.37 | 1-21/64" |
Closest metric thread
No metric thread is interchangeable with 1-3/8-18 UNEF. The nearest is M36×1.5, which is 1.08 mm larger in diameter with a pitch of 1.500 mm against 1.411 mm. A nut may start on the wrong thread and then bind or strip, so identify by both diameter and pitch.
How the 1-3/8-18 UNEF dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 0.05556 in = 1 ÷ 18 threads per inch
- H = 0.866025 × P = 0.0481 in
- d2 = d − 0.649519 × P = 1.3750 − 0.0361 = 1.3389 in
- D1 = d − 1.082532 × P = 1.3750 − 0.0601 = 1.3149 in
- d3 = d − 1.190785 × P = 1.3750 − 0.0662 = 1.3088 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 1.3209² = 1.37029 in²
- Lead angle = arctan(P ÷ (π × d2)) = 0.76°
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.