Metric Thread Tolerances

Tolerance class format

A tolerance class has a number (grade) and a letter. The number indicates the tolerance grade; the letter indicates position relative to the basic profile. For external threads (bolts) common letters are g (clearance) or e (looser). For internal threads (nuts) H is common.

Common classes

Fit and interchangeability

6g/6H gives a medium fit: bolts and nuts from different sources usually assemble without issue. Tighter classes (e.g. 4h6h) are used where precision or fatigue life matters. Our Thread Identifier and size chart help you match diameter and pitch; tolerance is rarely needed for everyday identification.

FAQ

What does 6H mean on a metric thread?

6H is a tolerance class for internal (nut) threads. The number is the grade (tolerance level); the letter H is for internal. 6H is a common general-purpose class.

What is the difference between 6g and 6H?

6g is for external (bolt) threads; 6H is for internal (nut) threads. Together they give a medium fit. g is above the zero line (smaller OD); H is above zero (larger ID).

Do I need to specify tolerance for normal bolts?

For most general fasteners, standard commercial tolerance (e.g. 6g/6H) is assumed. Specify tighter tolerance only when fit or interchangeability is critical.