Internal Thread Basics

MAJOR DIAMETER – ROOT: Must clear the major diameter of the mating external thread part.

MINOR DIAMETER – CREST: Controls the strength or shear of the thread.

PITCH DIAMETER: Controls the fit of the mating parts.

GO Thread Plug Gages

• Plus tolerance on OD (Outer or Major Diameter), and PD (Pitch Diameter.
• Functional check; checks minimum pitch diameter, minimum major diameter, flank angles, and lead.
• Engages for full length of thread free and easy.

NOGO, NOTGO, HI Thread Plug Gages

• Minus tolerance on OD (Major Diameter), and PD (Pitch Diameter)
• Checks one feature only which is the maximum functional pitch diameter.
• Engagement

Inch: Acceptable when gage does not enter the product more than three complete turns. Thin or soft material, or a small number of threads may require altering three-turn limit. Per ANSI B1.2-1983

Metric: Acceptable when a definite drag from contact with the product material results on or before the second turn of entry. Per ANSI B1.16M-1984