Read thread diameters, pitch and lead separately
- Major diameter
The arrow spans the external crest envelope. Diameter alone does not identify pitch, form, taper or compatible mating parts.
- Pitch diameterThis term
Dashed lines represent the reference surface through the thread profile. Correct outside diameter does not establish an acceptable thread fit.
- Pitch
Pitch is the axial spacing of corresponding profile points. Use the same unit and complete designation for both mating parts.
- Thread root
The highlighted groove bottom is the root. Root shape and damage can affect strength without changing the nominal crest diameter.
- Thread lead
Lead is the axial advance per revolution. Check the number of starts; a multiple-start thread has a different lead-to-pitch relationship.
The largest diameter, reference pitch diameter, groove root and axial spacing describe different features. Lead measures one complete revolution and equals pitch only for a single start.
Simplified external-thread profiles and dimensional arrows, not gauge geometry. The full thread standard defines the actual profile and measurement method.