M39×1.5 Thread Dimensions and Tap Drill

M39×1.5 is an ISO metric fine-pitch thread: the 39 mm diameter with a 1.5 mm pitch (16.9 threads per inch) instead of the coarse 4 mm of M39×4. The thread is 1.35 mm shallower, so the tensile stress area rises from 976 to 1110 mm² (14% more) and the lead angle falls from 2.00° to 0.72°.

Tap drill37.5 mm77% thread
Pitch1.5 mm16.9 TPI
Major diameter39.000 mm1.5354 in
Pitch diameter38.026 mm1.4971 in
Minor diameter (nut)37.376 mm1.4715 in
Tensile stress area1110 mm²1.72 in²
Clearance hole42 mmmedium fit, ISO 273

Thread profile

M39×1.5 drawn to scale: 60 threads in 89.3 mm of length.

Basic profile with the M39×1.5 dimensions. The nut and bolt share the same pitch diameter.

M39×1.5 basic dimensions

Dimension	mm	in
Major diameter d = D	39.000	1.5354
Pitch P	1.500	0.0591
Pitch diameter d₂ = D₂	38.026	1.4971
Minor diameter, internal thread D₁	37.376	1.4715
Minor diameter, external thread d₃	37.160	1.4630
Height of fundamental triangle H	1.299	0.0511
Thread height, internal (5H/8)	0.812	0.0320
Thread height, external (17H/24)	0.920	0.0362
Crest flat of external thread (P/8)	0.188	0.0074
Crest flat of internal thread (P/4)	0.375	0.0148

Property	Value
Threads per inch	16.93
Flank angle	60° (30° each side)
Lead angle at the pitch diameter	0.72°
Tensile stress area A_s	1110 mm² (1.72 in²)

Basic (nominal) sizes per ISO 724, before tolerances. d₃ is the root diameter of a bolt with the rounded root that ISO 898-1 uses for the stress area.

M39×1.5 tolerance limits (6g bolt, 6H nut)

6g and 6H are the general-purpose tolerance classes for commercial screws and nuts. The 6g screw has a fundamental deviation of 32 µm, so even at its largest it sits 0.032 mm below the basic size and leaves room for a thin plating.

External thread 6g	Max (mm)	Min (mm)	Tolerance (mm)
Major diameter d	38.968	38.732	0.236
Pitch diameter d₂	37.994	37.844	0.150

Internal thread 6H	Max (mm)	Min (mm)	Tolerance (mm)
Minor diameter D₁	37.676	37.376	0.300
Pitch diameter D₂	38.226	38.026	0.200

Computed from the ISO 965-1 deviations and tolerances for normal length of engagement; the results match the limits tabulated in ISO 965-2.

M39×1.5 tap drill size

Tap drill for M39×1.5: 37.5 mm

This leaves about 77% thread in the tapped hole.

The rule for metric tap drills is diameter minus pitch: 39 − 1.5 = 37.5 mm, which is a stocked drill size.

The 6H minor diameter of the finished nut thread must lie between 37.376 and 37.676 mm. The recommended drill falls inside that band.

Other drills that work for M39×1.5

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 6H limits
1-15/32" (1.4688 in)	37.31	1.4688	87%	no
37.5 mmrecommended	37.50	1.4764	77%	yes
38 mm	38.00	1.4961	51%	no

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

Clearance hole for M39

The clearance hole depends only on the 39 mm diameter, not on the pitch. The medium series, 42 mm, is the usual choice for general assembly.

Series	Hole Ø (mm)	Hole Ø (in)	Clearance on Ø (mm)
Fine (H12)	40	1.5748	1
Medium (H13)	42	1.6535	3
Coarse (H14)	45	1.7717	6

Hole diameters from ISO 273.

M39×1.5 strength

ISO 898-1 tabulates fine-pitch loads only for a selected set of threads from M8×1 to M39×3, so it gives no proof or breaking loads for M39×1.5. For an estimate, multiply the tensile stress area of 1110 mm² by the stress of the material: for example, 800 MPa gives 888 kN.

M39 pitches compared

All standard pitches on the M39 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.

M39 pitches at the same scale with crests aligned; M39×1.5 is outlined.

Thread	Pitch (mm)	Pitch Ø (mm)	Minor Ø D1 (mm)	Stress area (mm²)	Tap drill
M39×4	4	36.402	34.670	976	35 mm
M39×3	3	37.051	35.752	1030	36 mm
M39×2	2	37.701	36.835	1080	37 mm
M39×1.5	1.5	38.026	37.376	1110	37.5 mm

When to choose a coarse or a fine thread

Closest inch thread

No inch thread is interchangeable with M39×1.5. The nearest is 1-9/16-18 UNEF, which is 0.69 mm larger in diameter with a pitch of 1.411 mm against 1.500 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)
M39×1.5 (this thread)	39.000	1.500	16.93	—
1-9/16-18 UNEF	39.688	1.411	18.00	+0.688
1-1/2-18 UNEF	38.100	1.411	18.00	-0.900
1-5/8-18 UNEF	41.275	1.411	18.00	+2.275

Identify a thread from measurements

How the M39×1.5 dimensions are calculated

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

P = 1.5 mm
H = 0.866025 × P = 1.299 mm
d₂ = d − 0.649519 × P = 39 − 0.974 = 38.026 mm
D₁ = d − 1.082532 × P = 39 − 1.624 = 37.376 mm
d₃ = d − 1.226869 × P = 39 − 1.840 = 37.160 mm (external root, 17H/24 deep)
A_s = π/4 × ((d₂ + d₃) ÷ 2)² = π/4 × 37.593² = 1109.93 mm²
Lead angle = arctan(P ÷ (π × d₂)) = 0.72°

Related sizes and tools

Sources: ISO 68-1 and ISO 724 (profile and basic dimensions), ISO 261 (diameter and pitch series), ISO 965-1 (tolerances), ISO 273 (clearance holes), ISO 898-1 (mechanical properties) and tap drill tables based on ISO 2306. How this site checks its data. Last reviewed 29 September 2026.