1-5/8-18 UNEF Thread Dimensions and Tap Drill
1-5/8-18 UNEF is the Unified extra fine thread for the 1-5/8 in (41.27 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.
- Tap drill1-37/64"65% thread, 40.08 mm
- Pitch18 TPI1.411 mm
- Major diameter1.6250 in41.275 mm
- Pitch diameter1.5889 in40.358 mm
- Minor diameter (nut)1.5649 in39.747 mm
- Tensile stress area1.94 in²1250 mm²
Thread profile
1-5/8-18 UNEF basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 1.6250 | 41.275 |
| Pitch P | 0.05556 | 1.411 |
| Pitch diameter d2 = D2 | 1.5889 | 40.358 |
| Minor diameter, internal thread D1 | 1.5649 | 39.747 |
| Minor diameter, external UNR thread d3 | 1.5588 | 39.595 |
| 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 | 0.64° |
| Tensile stress area As | 1.94 in² (1250 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.
1-5/8-18 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 | 1.6235 | 1.6148 | 0.0087 |
| Pitch diameter | 1.5874 | 1.5824 | 0.0050 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 1.578 | 1.565 | 0.0130 |
| Pitch diameter | 1.5954 | 1.5889 | 0.0065 |
Limits of size from ASME B1.1 Table 2.
1-5/8-18 UNEF tap drill size
Tap drill for 1-5/8-18 UNEF: 1-37/64" (1.5781 in, 40.08 mm)
This leaves about 65% thread in the tapped hole.
Published charts rarely list this size. 1-37/64" 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 1.565 and 1.578 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 1-5/8-18 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 |
|---|---|---|---|---|---|
| 1-9/16" | 39.69 | 1.5625 | 87% | no | |
| 40 mm | 40.00 | 1.5748 | 70% | yes | |
| 1-37/64"recommended | 40.08 | 1.5781 | 65% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for 1-5/8 in
ASME B18.2.8 covers sizes up to 1-1/2 in, so there is no standard clearance hole for the 1-5/8 in size. Size the hole from the fit the joint needs.
1-5/8-18 UNEF strength
SAE J429 stops at 1-1/2 in, so the SAE grades give no proof load for 1-5/8-18 UNEF. Bolts this large are made to specifications such as ASTM A307, A449 or A354, whose strengths depend on diameter. For an estimate, multiply the tensile stress area of 1.94 in² by the strength of the material: for example, 60,000 psi gives 116,000 lbf.
Closest metric thread
No metric thread is interchangeable with 1-5/8-18 UNEF. The nearest is M42×1.5, which is 0.73 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 1-5/8-18 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 = 1.6250 − 0.0361 = 1.5889 in
- D1 = d − 1.082532 × P = 1.6250 − 0.0601 = 1.5649 in
- d3 = d − 1.190785 × P = 1.6250 − 0.0662 = 1.5588 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 1.5709² = 1.93808 in²
- Lead angle = arctan(P ÷ (π × d2)) = 0.64°
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.