Standard Square Drives for Socket Tools ASA B5.38-1958

General Engineering Reviews | Engineering Applications and Design

The following is the standard size and dimensions for American square tool drives per ASA B5.38.

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American square tool drives per ASA B5.38

Note: All dimensions are in inches

External Drive End
Drive
Nominal Size
Design
Am
Bm
Max
Cm
Dm
Em
M in
Fm
Max
Rm
Max
Max
Min
Max
Min
Max
Min

1/⁄4

A

0.252

0.247

0.330

0.312

0.265

0.165

0.153

0.078

0.015

3/ 8

A

0.377

0.372

0.500

0.438

0.406

0.227

0.215

0.156

0.031

1/ 2

A

0.502

0.497

0.665

0.625

0.531

0.321

0.309

0.187

0.031

5/8

A

0.627

0.622

0.834

0.656

0.594

0.321

0.309

0.187

0.047

3/4

B

0.752

0.747

1.000

0.938

0.750

0.415

0.403

0.216

0.047

1

B

1.002

0.997

1.340

1.125

1.000

0.602

0.590

0.234

0.063

1 1/2

B

1.503

1.498

1.968

1.625

1.562

0.653

0.641

0.310

0.094

Square Drive Chamfer

Internal Drive End
Drive
Nominal Size
Design
Af
Bf
Min
Df
Ef
Min
Rf
Max
Max
Min
Max
Min

1⁄4

A

0.258

0.253

0.335

0.159

0.147

0.090

3⁄8

A

0.383

0.378

0.505

0.221

0.209

0.170

1⁄2

A

0.508

0.503

0.670

0.315

0.303

0.201

5⁄8

A

0.633

0.628

0.839

0.315

0.303

0.201

3⁄4

B

0.758

0.753

1.005

0.409

0.397

0.216

0.047

1

B

1.009

1.004

1.350

0.596

0.584

0.234

0.062

1 1/⁄2

B

1.510

1.505

1.983

0.647

0.635

0.310

0.125

  • The radius (Rm) located at the shoulder of male tang precludes use of minimum diameter crosshole
    in socket (Ef), unless female drive end is chamfered (shown as optional).
  • If female drive end is not chamfered, socket cross-hole diameter (Ef) is increased to compensate
    for fillet radius Rm, max.
  • Minimum clearance across flats male to female is 0.001 inch through 3⁄4-inch size; 0.002 inch in 1-
    and 11⁄2-inch sizes. For impact wrenches Am should be held as close to maximum as practical.
  • Cf, min. for both designs A and B should be equal to Cm, max.