Threads / Drawing guide
A class needs
its standard and limits.
Use the complete callout to find the applicable acceptance limits. Nominal dimensions alone cannot establish a fit.
Unified · 2A / 2B
A identifies external threads; B identifies internal threads. The preceding number belongs to the class designation. Keep it with the size and thread series.
1/4-28 UNF-2B
Metric · 6g / 6H
The number is the tolerance grade; the letter identifies the tolerance position. Letter case matters: lowercase is external, uppercase internal. Do not turn a class label into a generic material or coating recommendation.
M10 × 1.5-6H
To obtain an inspection limit
- Record the complete size, pitch, thread system, external/internal class and handedness.
- Read the drawing’s referenced standard and edition, engagement requirements and coating condition.
- Obtain the applicable limit tables and gauging requirements for that exact callout.
- Compare measured or gauged results with those limits, not the nominal values in a size directory.
This page explains notation. Paid tolerance tables were not reviewed; no maximum/minimum diameters or default class recommendations are supplied.