9/16-18 UNF Thread Dimensions and Tap Drill

9/16-18 UNF is the Unified fine thread for the 9/16 in (14.29 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 UNF series. Against the coarse 9/16-12 UNC it has 18 threads per inch instead of 12, a stress area 12% larger and a lead angle of 1.92° instead of 2.99°.

9/16-18 is the pedal thread on adult bicycles (left pedal left-handed) and the straight thread of the SAE -6 O-ring port.

Tap drill33/64"65% thread, 13.10 mm
Pitch18 TPI1.411 mm
Major diameter0.5625 in14.287 mm
Pitch diameter0.5264 in13.371 mm
Minor diameter (nut)0.5024 in12.760 mm
Tensile stress area0.203 in²131 mm²

Thread profile

9/16-18 UNF drawn to scale: 23 threads in 1.29 in of length.

Basic profile with the 9/16-18 UNF dimensions. The nut and bolt share the same pitch diameter.

9/16-18 UNF basic dimensions

Dimension	in	mm
Major diameter d = D	0.5625	14.287
Pitch P	0.05556	1.411
Pitch diameter d₂ = D₂	0.5264	13.371
Minor diameter, internal thread D₁	0.5024	12.760
Minor diameter, external UNR thread d₃	0.4963	12.607
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	1.92°
Tensile stress area A_s	0.203 in² (131 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.

9/16-18 UNF 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 0.0014 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	0.5611	0.5524	0.0087
Pitch diameter	0.5250	0.5205	0.0045

Internal thread 2B	Max (in)	Min (in)	Tolerance (in)
Minor diameter	0.515	0.502	0.0130
Pitch diameter	0.5323	0.5264	0.0059

Limits of size from ASME B1.1 Table 2.

9/16-18 UNF tap drill size

Tap drill for 9/16-18 UNF: 33/64" (0.5156 in, 13.10 mm)

This leaves about 65% thread in the tapped hole.

33/64" is the drill listed for 9/16-18 UNF in standard tap drill charts. It is 0.0469 in under the 0.5625 in major diameter.

The 2B minor diameter of the finished nut thread must lie between 0.502 and 0.515 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 9/16-18 UNF

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
1/2"	12.70	0.5000	87%	no
33/64"recommended	13.10	0.5156	65%	no
13.1 mm	13.10	0.5157	65%	no

% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.

Clearance hole for 9/16 in

ASME B18.2.8 does not list the 9/16 in size, so there is no standard clearance hole for the 9/16 in size. Size the hole from the fit the joint needs.

9/16-18 UNF proof and breaking loads

Loads for steel bolts by SAE grade, from the tensile stress area of 0.203 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	55,000	74,000	11,200	15,000	49.7
Grade 5	85,000	120,000	17,300	24,400	76.8
Grade 8	120,000	150,000	24,400	30,500	108

Strengths from SAE J429 for this diameter; stress area from ASME B1.1. Loads rounded to three significant figures.

9/16 in pitches compared

All standard pitches on the 9/16 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.

9/16 in pitches at the same scale with crests aligned; 9/16-18 UNF is outlined.

Thread	TPI	Pitch Ø (in)	Minor Ø D1 (in)	Stress area (in²)	Tap drill
9/16-12 UNC	12	0.5084	0.4723	0.182	31/64"
9/16-18 UNF	18	0.5264	0.5024	0.203	33/64"
9/16-24 UNEF	24	0.5354	0.5174	0.214	33/64"

When to choose a coarse or a fine thread

Closest metric thread

No metric thread is interchangeable with 9/16-18 UNF. The nearest is M14×1.5, which is 0.29 mm smaller 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.

Thread	Major Ø (mm)	Pitch (mm)	TPI	Ø difference (mm)
9/16-18 UNF (this thread)	14.287	1.411	18.00	—
M14×1.5	14.000	1.500	16.93	-0.287
M14×1.25	14.000	1.250	20.32	-0.287
M14×1	14.000	1.000	25.40	-0.287

Identify a thread from measurements

How the 9/16-18 UNF 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 = 0.5625 − 0.0361 = 0.5264 in
D₁ = d − 1.082532 × P = 0.5625 − 0.0601 = 0.5024 in
d₃ = d − 1.190785 × P = 0.5625 − 0.0662 = 0.4963 in (external root, 11H/16 deep)
A_s = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 0.5084² = 0.20298 in²
Lead angle = arctan(P ÷ (π × d₂)) = 1.92°

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.