What kind of thread does the acetylene hose have?

• A. Right-hand thread
• B. National pipe thread
• C. Left-hand thread
• D. Metric thread

Answer: (C)

Acetylene hose connections use left-hand threads as a safety feature. This unusual threading direction helps prevent accidentally connecting acetylene hose and fittings to oxygen equipment, since the two gases require different fittings and could be dangerous if mixed. Only components with the matching left-hand thread will fit, reducing the risk of a dangerous cross-connection. The other options don’t fit because right-hand threads are the standard for many fittings but would not prevent cross-connection; National pipe thread is for rigid piping rather than hose fittings; and metric threads are not the standard for U.S. welding gear.