M42×4.5 thread — 37.5 mm tap drill, 36.479 mm minor diameter
Coarse
M42×4.5 is a coarse thread: 42 mm across with a 4.5 mm pitch. The bolt’s minor diameter is 36.479 mm, the tap drill 37.5 mm and the stress area 1120.9 mm².
- Outside diameter
- 42 mm
- Pitch
- 4.5 mm (Coarse)
- Pitch diameter
- 39.077 mm
- Minor diameter (bolt)
- 36.479 mm
- Minor diameter (nut)
- 37.129 mm
- Thread height
- 2.761 mm
- Stress area
- 1120.9 mm²
- Threads per inch
- 5.64
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 M42×4.5?
37.5 mm — the 42 mm outside diameter minus the 4.5 mm pitch. Use the nearest drill you have.
Q. What is the minor diameter of M42×4.5?
36.479 mm on the bolt and 37.129 mm in the nut, with the pitch diameter between them at 39.077 mm.
Q. Is it a coarse or a fine thread?
Coarse — plain “M42” means exactly this size.
Q. How much load does it take?
Its stress area is 1120.9 mm². Multiply that by the bolt’s strength grade to get the load it carries.