Fittings that are marked LH on the outside wall indicate what type of threads?

Fittings marked with "LH" on the outside wall indicate left-hand threads. This designation is crucial in applications where different thread types are used to prevent the assembly of incompatible components. Left-hand threads are designed to tighten counterclockwise, which is the opposite of standard right-hand threads that tighten clockwise.

The use of left-hand threads is commonly found in specific applications, such as gas fittings or specialized mechanical equipment, where it's necessary to differentiate from standard right-hand threads to avoid accidental cross-threading or loosening during operation. Recognizing the designation and the corresponding direction for tightening is key for ensuring proper assembly and safety in the installation process.