3-4 UNC Thread Dimensions and Tap Drill

3-4 UNC is the Unified coarse thread for the 3 in (76.20 mm) diameter: 4 threads per inch, a pitch of 0.25000 in (6.350 mm), on a 60° profile. ASME B1.1 lists it in the UNC series.

Tap drill2-3/4"77% thread, 69.85 mm
Pitch4 TPI6.350 mm
Major diameter3.0000 in76.200 mm
Pitch diameter2.8376 in72.076 mm
Minor diameter (nut)2.7294 in69.326 mm
Tensile stress area5.97 in²3850 mm²

Thread profile

3-4 UNC drawn to scale: 27 threads in 6.87 in of length.

Basic profile with the 3-4 UNC dimensions. The nut and bolt share the same pitch diameter.

3-4 UNC basic dimensions

Dimension	in	mm
Major diameter d = D	3.0000	76.200
Pitch P	0.25000	6.350
Pitch diameter d₂ = D₂	2.8376	72.076
Minor diameter, internal thread D₁	2.7294	69.326
Minor diameter, external UNR thread d₃	2.7023	68.639
Height of fundamental triangle H	0.2165	5.499
Thread height, internal (5H/8)	0.1353	3.437
Thread height, external (11H/16)	0.1488	3.781
Crest flat of external thread (P/8)	0.0312	0.794
Crest flat of internal thread (P/4)	0.0625	1.587

Property	Value
Threads per inch	4
Flank angle	60° (30° each side)
Lead angle at the pitch diameter	1.61°
Tensile stress area A_s	5.97 in² (3850 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.

3-4 UNC 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 0.0032 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	2.9968	2.9730	0.0238
Pitch diameter	2.8344	2.8237	0.0107

Internal thread 2B	Max (in)	Min (in)	Tolerance (in)
Minor diameter	2.767	2.729	0.0380
Pitch diameter	2.8515	2.8376	0.0139

Limits of size from ASME B1.1 Table 2.

3-4 UNC tap drill size

Tap drill for 3-4 UNC: 2-3/4" (2.7500 in, 69.85 mm)

This leaves about 77% thread in the tapped hole.

Few charts list a drill for a thread this large, and holes of this size are normally bored rather than drilled. 2-3/4" follows the usual rule of major diameter minus one pitch.

The 2B minor diameter of the finished nut thread must lie between 2.729 and 2.767 in. The recommended drill falls inside that band.

Other drills that work for 3-4 UNC

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
2-3/4"recommended	69.85	2.7500	77%	yes
70 mm	70.00	2.7559	75%	yes
2-13/16"	71.44	2.8125	58%	no

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

Clearance hole for 3 in

ASME B18.2.8 covers sizes up to 1-1/2 in, so there is no standard clearance hole for the 3 in size. Size the hole from the fit the joint needs.

3-4 UNC strength

SAE J429 stops at 1-1/2 in, so the SAE grades give no proof load for 3-4 UNC. 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 5.97 in² by the strength of the material: for example, 60,000 psi gives 358,000 lbf.

How the 3-4 UNC dimensions are calculated

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

P = 0.25000 in = 1 ÷ 4 threads per inch
H = 0.866025 × P = 0.2165 in
d₂ = d − 0.649519 × P = 3.0000 − 0.1624 = 2.8376 in
D₁ = d − 1.082532 × P = 3.0000 − 0.2706 = 2.7294 in
d₃ = d − 1.190785 × P = 3.0000 − 0.2977 = 2.7023 in (external root, 11H/16 deep)
A_s = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 2.7564² = 5.96737 in²
Lead angle = arctan(P ÷ (π × d₂)) = 1.61°

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.