ASME B18.6.3 Type 3 Square Recessed Flat Countersunk

Dimensions of ASME B18.6.3 Type 3 Square Recessed Flat Countersunk Head Machine Screws

Nominal Size or Basic Screw Diameter	Length, L	Head Diameter, A		Head Height, H, Ref.	Recess Across Flats, M, Ref.	Recess Depth, T, Ref.	Recess Size	Recess Penetration Gaging Depth, P		Protrusion Above Gaging Diameter, F		Gaging Diameter, G
Nominal Size or Basic Screw Diameter	Length, L	Max	Min	Head Height, H, Ref.	Recess Across Flats, M, Ref.	Recess Depth, T, Ref.	Recess Size	Max	Min	Max	Min	Gaging Diameter, G
2	1/8	0.162	0.144	0.051	0.050	0.057	00	0.033	0.028	0.029	0.017	0.124
3	1/8	0.187	0.167	0.059	0.070	0.066	0	0.038	0.028	0.031	0.018	0.148
4	3/16	0.212	0.191	0.067	0.070	0.066	0	0.038	0.028	0.032	0.019	0.172
5	3/16	0.237	0.215	0.075	0.091	0.96	1S	0.055	0.040	0.034	0.020	0.196
6	3/16	0.262	0.238	0.083	0.091	0.96	1S	0.055	0.040	0.036	0.021	0.220
8	1/4	0.312	0.285	0.100	0.112	0.115	2S	0.063	0.048	0.039	0.023	0.267
10	5/16	0.362	0.333	0.116	0.112	0.127	2R	0.075	0.060	0.042	0.025	0.313
12	3/8	0.412	0.380	0.132	0.133	0.158	3R	0.095	0.080	0.045	0.027	0.362
1/4	7/16	0.477	0.442	0.153	0.133	0.158	3R	0.095	0.080	0.050	0.029	0.424
5/16	1/2	0.597	0.556	0.191	0.191	0.194	4R	0.100	0.085	0.057	0.034	0.539
3/8	9/16	0.717	0.670	0.230	0.191	0.194	4R	0.100	0.085	0.065	0.039	0.653

Notes :

Where specifying nominal size in decimals, zeros preceding the decimals and in the fourth decimal place shall be omitted.
Screws of these length and shorter shall have undercut heads.
Tabulated values determined from formula for maximum H in Appendix A.
“R” in the recess size tabulation means regular depth recess and the “S” means short depth recess.
Square recesses incorporate a slight taper on the sides of the recess. This taper can result in loss of penetration gaging depth on finished fasteners due to the build up of plating in the recesses. The recess penetration dimensions specified here for finished (plated) products. Manufacture should be advised that they should not use all this tolerancein the heading process, or an out of tolerance condition could exist after plating.
No tolerance for gaging diameter is given. If the gaging diameter of the gage used differs from tabulated value, the protrusion will be affected accordingly and the proper protusion values must be recalculated using the formulas.