On an acetylene rig, what does a grooved nut indicate?

• A. It indicates pressure rating
• B. The left-hand thread is for acetylene use only
• C. It indicates a safety lock
• D. It is decorative

Answer: (B)

The grooved nut signals a left-hand threaded connection used exclusively for acetylene hardware. This deliberately opposite threading helps prevent cross-connecting acetylene equipment with hoses or regulators designed for other gases, which is a critical safety measure since acetylene systems require-compatible fittings. So, the grooved nut isn’t about pressure rating, a safety lock, or decoration—it marks acetylene-use-only hardware by using a left-hand thread.