M1.8×0.35 thread — 1.45 mm tap drill, 1.371 mm minor diameter
Coarse
M1.8×0.35 is a coarse thread: 1.8 mm across with a 0.35 mm pitch. The bolt’s minor diameter is 1.371 mm, the tap drill 1.45 mm and the stress area 1.7 mm².
- Outside diameter
- 1.8 mm
- Pitch
- 0.35 mm (Coarse)
- Pitch diameter
- 1.573 mm
- Minor diameter (bolt)
- 1.371 mm
- Minor diameter (nut)
- 1.421 mm
- Thread height
- 0.215 mm
- Stress area
- 1.7 mm²
- Threads per inch
- 72.57
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.
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 M1.8×0.35?
1.45 mm — the 1.8 mm outside diameter minus the 0.35 mm pitch. Use the nearest drill you have.
Q. What is the minor diameter of M1.8×0.35?
1.371 mm on the bolt and 1.421 mm in the nut, with the pitch diameter between them at 1.573 mm.
Q. Is it a coarse or a fine thread?
Coarse — plain “M1.8” means exactly this size.
Q. How much load does it take?
Its stress area is 1.7 mm². Multiply that by the bolt’s strength grade to get the load it carries.