M39×3 thread — 36 mm tap drill, 35.319 mm minor diameter
Fine
M39×3 is a fine thread: 39 mm across with a 3 mm pitch. The bolt’s minor diameter is 35.319 mm, the tap drill 36 mm and the stress area 1028.36 mm².
- Outside diameter
- 39 mm
- Pitch
- 3 mm (Fine)
- Pitch diameter
- 37.051 mm
- Minor diameter (bolt)
- 35.319 mm
- Minor diameter (nut)
- 35.752 mm
- Thread height
- 1.84 mm
- Stress area
- 1028.36 mm²
- Threads per inch
- 8.47
The thread is a 60° triangle
ISO defines the thread profile as an equilateral 60° triangle. Its height H is 0.866 × pitch, and every other diameter is that triangle cut back by a fixed fraction of H.
The stress area sets the strength
A bolt’s load capacity is worked out from this area, not from the outside diameter. It is the area of a circle whose diameter is the average of the pitch and minor diameters.
Other pitches at this diameter
A tighter pitch cuts shallower, leaving a thicker core. That is why fine threads suit thin walls and places that vibrate.
One size up and down
The 35 coarse threads
Each diameter has one default pitch. When someone just says “M8”, they mean M8×1.25.
How to read this
- The number after M is the outside diameter in mm; the number after × is the pitch in mm.
- H = 0.866 × pitch. The pitch diameter is d − 0.75H, the nut’s minor diameter d − 1.25H.
- Take the tap drill as outside diameter minus pitch, then pick the nearest drill you own.
- Load capacity comes from the stress area, not from the outside diameter.
Frequently asked questions
Q. What drill do I need to tap M39×3?
36 mm — the 39 mm outside diameter minus the 3 mm pitch. Use the nearest drill you have.
Q. What is the minor diameter of M39×3?
35.319 mm on the bolt and 35.752 mm in the nut, with the pitch diameter between them at 37.051 mm.
Q. Is it a coarse or a fine thread?
Fine. There is a coarse pitch for this diameter too, so the pitch must always be written out.
Q. How much load does it take?
Its stress area is 1028.36 mm². Multiply that by the bolt’s strength grade to get the load it carries.