Application of sinker bars aims to prevent longitudinal bending of ordinary sucker rods caused by excessive resistance generated while pumping oil with big pumps, pumping thick oil and pumping in deep wells. Buckling of rods could lead to premature failure of sucker rods, thus amplifying wearing between rod and tube. The sinker bar products, which apply high quality carbon steel or alloy steel, are further classified into three kinds according to API Spec 11B strength levels(first, second and third grade).
| Grade | Minimum Strength of Extension | |
| kpsi | MPa | |
| 1 | 90 | 621 |
| 2 | 90 | 621 |
| 3 | 115 | 793 |
Specifications
| Nominal Size (in) | Rod Length (mm) | Outer Thread Nominal Value (in) | Elevator Neck Diameter (mm) | Minimum Elevator Neck Length (mm) | Sinker Bar Length (mm) |
| 11/4 | 31.75 | 15/16 SR | 22.23 | 101.6 | 7620 9140 |
| 11/4 | 31.75 | 11/16 PR | 22.23 | 101.6 | |
| 13/8 | 34.93 | 15/16 SR | 25.40 | 101.6 | |
| 13/8 | 34.93 | 11/16 PR | 25.40 | 101.6 | |
| 11/2 | 38.1 | 11/16 SR | 25.40 | 101.6 | |
| 15/8 | 41.28 | 13/16 SR | 25.40 | 101.6 | |
| 13/4 | 44.45 | 13/16 SR | 25.40 | 101.6 |
