Where should joint lubricant be applied when assembling threaded joints?

• A. Only to the female threads of the joint
• B. Only to the male threads of the joint
• C. On both male and female threads
• D. On the joint housing exterior

Answer: (B)

Joint lubricant is used to reduce friction where the threads engage so the joint can be tightened to the correct torque and seat properly without galling. In threaded assemblies, apply the lubricant to the male threads only. This standardizes the friction at the interface and helps ensure the joint seals reliably without altering the seal surface or drawing lubricant into the seal area. Lubricating the female threads can contaminate the sealing surfaces or affect how the joint seats, and lubricating the exterior of the joint housing doesn’t influence the thread engagement or seal. Therefore, applying lubricant to the male threads only is the correct practice.