#2-64 UNF Thread Dimensions and Tap Drill

#2-64 UNF is the Unified fine thread for the number 2 size (major diameter 0.086 in, 2.18 mm): 64 threads per inch, a pitch of 0.01562 in (0.397 mm), on a 60° profile. ASME B1.1 lists it in the UNF series. Against the coarse #2-56 UNC it has 64 threads per inch instead of 56, a stress area 6% larger and a lead angle of 3.75° instead of 4.37°.

Tap drill#5079% thread, 1.78 mm
Pitch64 TPI0.397 mm
Major diameter0.0860 in2.184 mm
Pitch diameter0.0759 in1.927 mm
Minor diameter (nut)0.0691 in1.755 mm
Tensile stress area0.00394 in²2.54 mm²
Clearance hole#38normal fit, ASME B18.2.8

Thread profile

#2-64 UNF drawn to scale: 13 threads in 0.20 in of length.

Basic profile with the #2-64 UNF dimensions. The nut and bolt share the same pitch diameter.

#2-64 UNF basic dimensions

Dimension	in	mm
Major diameter d = D	0.0860	2.184
Pitch P	0.01562	0.397
Pitch diameter d₂ = D₂	0.0759	1.927
Minor diameter, internal thread D₁	0.0691	1.755
Minor diameter, external UNR thread d₃	0.0674	1.712
Height of fundamental triangle H	0.0135	0.344
Thread height, internal (5H/8)	0.0085	0.215
Thread height, external (11H/16)	0.0093	0.236
Crest flat of external thread (P/8)	0.0020	0.050
Crest flat of internal thread (P/4)	0.0039	0.099

Property	Value
Threads per inch	64
Flank angle	60° (30° each side)
Lead angle at the pitch diameter	3.75°
Tensile stress area A_s	0.00394 in² (2.54 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.

#2-64 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 0.0006 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	0.0854	0.0816	0.0038
Pitch diameter	0.0753	0.0733	0.0020

Internal thread 2B	Max (in)	Min (in)	Tolerance (in)
Minor diameter	0.0752	0.0691	0.0061
Pitch diameter	0.0786	0.0759	0.0027

Limits of size from ASME B1.1 Table 2.

#2-64 UNF tap drill size

Tap drill for #2-64 UNF: #50 (0.0700 in, 1.78 mm)

This leaves about 79% thread in the tapped hole.

#50 is the drill listed for #2-64 UNF in standard tap drill charts. It is 0.0160 in under the 0.0860 in major diameter.

The 2B minor diameter of the finished nut thread must lie between 0.069 and 0.075 in. The recommended drill falls inside that band.

Other drills that work for #2-64 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
#50recommended	1.78	0.0700	79%	yes
1.8 mm	1.80	0.0709	75%	yes
#49	1.85	0.0730	64%	yes

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

Clearance hole for #2

The clearance hole depends only on the 0.0860 in diameter, not on the thread series. The normal fit, a #38 drill, is the usual choice for general assembly.

Fit	Drill	Hole Ø (in)	Hole Ø (mm)	Clearance on Ø (in)
Close	3/32 in	0.0938	2.38	0.0078
Normal	#38	0.1015	2.58	0.0155
Loose	#32	0.1160	2.95	0.0300

Nominal drill sizes from ASME B18.2.8.

#2-64 UNF strength

SAE J429 starts at 1/4 in, so the SAE grades give no proof load for #2-64 UNF. For an estimate, multiply the tensile stress area of 0.00394 in² by the strength of the material: for example, 60,000 psi gives 236 lbf.

#2 pitches compared

All standard pitches on the #2 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.

#2 pitches at the same scale with crests aligned; #2-64 UNF is outlined.

Thread	TPI	Pitch Ø (in)	Minor Ø D1 (in)	Stress area (in²)	Tap drill
#2-56 UNC	56	0.0744	0.0667	0.00370	#50
#2-64 UNF	64	0.0759	0.0691	0.00394	#50

When to choose a coarse or a fine thread

How the #2-64 UNF dimensions are calculated

Every basic dimension follows from the major diameter and the pitch:

P = 0.01562 in = 1 ÷ 64 threads per inch
H = 0.866025 × P = 0.0135 in
d₂ = d − 0.649519 × P = 0.0860 − 0.0101 = 0.0759 in
D₁ = d − 1.082532 × P = 0.0860 − 0.0169 = 0.0691 in
d₃ = d − 1.190785 × P = 0.0860 − 0.0186 = 0.0674 in (external root, 11H/16 deep)
A_s = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 0.0708² = 0.00393 in²
Lead angle = arctan(P ÷ (π × d₂)) = 3.75°

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.