Thread Classes – Basics

Classes were established in order to give manufactures a better understanding of the thread tightness that can be achieved by using a given tap. It must be tight enough to fasten, but loose enough to allow the connection.

When considering the a tap’s class, it is important to understand that this is simply a representation of the tolerances needed in the manufacturing process. It is a given that the tighter the tolerance, the greater the cost will be in achieving those tolerances. This is why it is always best to select a class with the widest tolerance allowed.


When choosing a tap size, it is important to understand that it will rarely cut to it’s given size. Variation in material, machining conditions, and tap life will often times produce a larger size, but sometimes can also be smaller due to shrinkage.

To combat this variation in size, a numbering system was established to designate each series of limitations.

H (HIGH) – Above basic pitch diameter – INCH (Internal)

L (LOW) – Below basic pitch diameter – INCH (External)

D (GROUND, HIGH) – Above basic pitch diameter – METRIC (Internal)

U (GROUND, LOW) – Below basic pitch diameter – METRIC (External)


Lets look at both the basic Inch and Metric Thread Class calls designations


L1 = Basic to Basic minus .0005

H1 = Basic to Basic plus .0005

H2 = Basic plus .0005 to Basic plus .0010

H3 = Basic plus .0010 to Basic plus .0015

H4 = Basic plus .0015 to Basic plus .0020

H5 = Basic plus .0020 to Basic plus .0025

H6 = Basic plus .0025 to Basic plus .0030


U1 = Basic minus .0005 = min. tap P.D.

D1 = Basic plus .0005 = max. tap P.D.

D2 = Basic plus .0010 = max. tap P.D.

D3 = Basic plus .0015 = max. tap P.D.

D4 = Basic plus .0020 = max. tap P.D.

D5 = Basic plus .0025 = max. tap P.D.

D6 = Basic plus .0030 = max. tap P.D.

(P.D = Pitch Diameter)

Looking at the basic class designations makes sense based on their letter. The number designates for each increments the tolerance. These tolerances apply to taps under 1″ in diameter.


As an example, in sizes 1” and smaller, an H1 tap has a tolerance range of from basic to .0005” over basic; an H2 tap from .0005” over basic to .001” over basic, (see chart 1A on this page). In addition, metric threads are also designated in much the same way. The thread tap limits are designated by the letter D (ground, high) above basic pitch diameter, or U (ground, low) below basic pitch diameter. As an example, in sizes M25 and smaller, a D1 tap has a size of .0005” over basic to tap max. P.D.; a D2 tap has a size of .001” over basic to tap max. P.D., (see Chart 1B).

GH Numbers

GH Numbers are listed below. “G” designates Ground Thread. “H” designates the pitch diameter is on high side of basic. These two letters (GH) are followed by a numeral indicating the Tolerance of Pitch diameter oversize.

H1=Basic to Plus .0005  
H2=Basic Plus .0005 to Plus .0010  
H3=Basic Plus .0010 to Plus .0015  
H4=Basic Plus .0015 to Plus .0020  
H5=Basic Plus .0020 to Plus .0025  
H6=Basic Plus .0025 to Plus .0030 H=Above Basic
H7=Basic Plus .0030 to Plus .0035 L=Below Basic

Relation Of Tap Pitch Diameter to Basic Pitch Diameter
American Tap Manufacturers use a series of tap pitch diameter limits. These limits feature a .0005 tolerance in tap sizes #0 Thru 1″ and a .001″ or greater tolerance in tap sizes above 1″ thru 1 1/2″ diameter.


1/4-20. Relationship between Tap Pitch diameter limits and basic nominal pitch diameter.

Basic Pitch Diameter: .2175     GH5.2200

Are you ready to make a selection?

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