Counterbore, countersink & clearance

A socket head cap screw seats in a counterbore just over its head diameter — an M6 SHCS has a Ø10 head, so it takes an ~Ø11 counterbore deep enough to sink the head below flush, over a clearance hole picked from the close/normal/loose series. This page lists counterbore, countersink, and clearance-hole dimensions by fastener size so you can read (or check) the callout without hunting through a machinery handbook.

Drawings often specify these implicitly — “C’BORE FOR M6 SHCS” — and the inspector needs the numbers the note implies. That implied dimension is exactly what this table gives you.

Fastener size

3 matching rows highlighted.

Counterbore

Size	Standard	Close hole	Free hole	C'bore dia	C'bore depth	Unit
M3	iso_4762	3.4	3.6	6.5	3.5	mm
M4	iso_4762	4.5	4.8	8	4.8	mm
M5	iso_4762	5.5	5.8	10	5.8	mm
M6	iso_4762	6.6	7	11	6.8	mm
M8	iso_4762	9	10	15	8.8	mm
M10	iso_4762	11	12	18	10.8	mm
M12	iso_4762	13.5	14.5	20	12.8	mm
M14	iso_4762	15.5	16.5	24	14.8	mm
M16	iso_4762	17.5	18.5	26	16.8	mm
M20	iso_4762	22	24	33	21	mm
M24	iso_4762	26	28	40	25	mm
#0	asme_b18_3	0.067	0.073	0.125	0.060	in
#2	asme_b18_3	0.094	0.106	0.188	0.086	in
#4	asme_b18_3	0.125	0.136	0.219	0.112	in
#6	asme_b18_3	0.154	0.170	0.281	0.138	in
#8	asme_b18_3	0.180	0.194	0.313	0.164	in
#10	asme_b18_3	0.206	0.221	0.375	0.190	in
1/4"	asme_b18_3	0.266	0.281	0.438	0.250	in
5/16"	asme_b18_3	0.328	0.339	0.531	0.312	in
3/8"	asme_b18_3	0.390	0.403	0.625	0.375	in
7/16"	asme_b18_3	0.453	0.468	0.719	0.438	in
1/2"	asme_b18_3	0.516	0.531	0.813	0.500	in
5/8"	asme_b18_3	0.641	0.656	1.000	0.625	in
3/4"	asme_b18_3	0.766	0.781	1.188	0.750	in
1"	asme_b18_3	1.016	1.031	1.625	1.000	in

Countersink

Size	Standard	Angle	Head dia	C'sink depth	Unit
M3	iso_10642	90°	6.72	1.86	mm
M4	iso_10642	90°	8.96	2.48	mm
M5	iso_10642	90°	11.2	3.1	mm
M6	iso_10642	90°	13.44	3.72	mm
M8	iso_10642	90°	17.92	4.96	mm
M10	iso_10642	90°	22.4	6.2	mm
M12	iso_10642	90°	26.88	7.44	mm
M14	iso_10642	90°	30.8	8.4	mm
M16	iso_10642	90°	33.6	8.8	mm
M20	iso_10642	90°	40.32	10.16	mm
#4	asme_b18_3	82°	0.225	0.083	in
#6	asme_b18_3	82°	0.279	0.097	in
#8	asme_b18_3	82°	0.332	0.112	in
#10	asme_b18_3	82°	0.385	0.127	in
1/4"	asme_b18_3	82°	0.507	0.161	in
5/16"	asme_b18_3	82°	0.635	0.198	in
3/8"	asme_b18_3	82°	0.762	0.234	in
1/2"	asme_b18_3	82°	0.874	0.251	in
5/8"	asme_b18_3	82°	1.088	0.324	in
3/4"	asme_b18_3	82°	1.327	0.396	in

Clearance holes

Size	Standard	Close	Medium	Free	Unit
M1.6	iso_273	1.7	1.8	2	mm
M2	iso_273	2.2	2.4	2.6	mm
M2.5	iso_273	2.7	2.9	3.1	mm
M3	iso_273	3.2	3.4	3.6	mm
M4	iso_273	4.3	4.5	4.8	mm
M5	iso_273	5.3	5.5	5.8	mm
M6	iso_273	6.4	6.6	7	mm
M8	iso_273	8.4	9	10	mm
M10	iso_273	10.5	11	12	mm
M12	iso_273	13	13.5	14.5	mm
M14	iso_273	15	15.5	16.5	mm
M16	iso_273	17	17.5	18.5	mm
M18	iso_273	19	20	21	mm
M20	iso_273	21	22	24	mm
M22	iso_273	23	24	26	mm
M24	iso_273	25	26	28	mm
M27	iso_273	28	30	32	mm
M30	iso_273	31	33	35	mm

Close, normal, or loose clearance — which do I use?

The clearance series trades assembly forgiveness against location control. Closefits suit doweled or precisely located joints where the screw shouldn’t float; normal is the default for general work; looseabsorbs stack-up across patterns of many fasteners or painted/coated parts. If the drawing just says “drill thru for M6”, normal clearance is the customary reading — but confirm against the note or your customer’s drawing-interpretation flow-down when position tolerances are tight.