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.

Click on images to enlarge

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

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.

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