The critical coupling thickness of API threads

Table C.7—Critical Thickness for Couplings with  API Threads
Label 1	Critical Thickness for Couplings
			Special Clearance
	NU	EU	EU	BC	BC	LC	SC
1	2	3	4	5	6	7	8
1.05	4.29	5.36	—	—	—	—	—
1.315	5.36	6.55	—	—	—	—	—
1.66	6.07	6.1	—	—	—	—	—
1.9	4.98	6.38	—	—	—	—	—
2 3/8	7.72	7.62	5.69	—	—	—	—
2 7/8	9.65	9.09	6.45	—	—	—	—
3 1/2	11.46	11.53	7.47	—	—	—	—
4	11.53	11.63	—	—	—	—	—
4 1/2	11.05	12.52	—	6.58	11.35	12.04	11.74
5	—	—	—	6.76	12.17	12.98	12.47
5 1/2	—	—	—	6.81	12.28	13.06	12.57
6 5/8	—	—	—	6.96	11.91	12.9	12.32
7	—	—	—	7.11	13.46	14.43	13.72
7 5/8	—	—	—	8.84	13.61	14.55	13.87
8 5/8	—	—	—	8.94	15.29	16.43	15.54
9 5/8	—	—	—	8.94	15.29	16.69	15.6
10 3/4	—	—	—	8.94	15.29	—	15.7
11 3/4	—	—	—	—	15.29	—	15.7
13 3/8	—	—	—	—	15.29	—	15.7
16	—	—	—	—	16.94	—	16.05
18 5/8	—	—	—	—	21.69	—	20.8
20	—	—	—	—	16.94	17.09	16.1
NOTE The coupling blank thickness is greater than tHose indicated above, due to thread height and manufacturing allowance to avoid black crest threads.