The fun of being an apprentice starts with a steep learning curve. Being sent to the nearest supply house for two 1/2-inch male x flare fittings and flare nuts seemed simple enough until I returned to the job site where we were running a 1/2-inch soft copper (type-L) gas line. Imagine my surprise when the flare nuts did not fit over the tubing! Turned out we referred to copper tubing by its ID (internal diameter) and flare nuts/fittings by the tubing OD (outside diameter). Lesson learned.
Back then, we did a few things wrong when making flared joints. Aside from occasionally forgetting to slide the flare nut over the copper before making the flare (D’oh!), flares too small or large, over-tightening the flare nuts, failure to ream the copper, using flare fittings that had been mixed in with other fittings with the flare surface scratched or dinged and testing for leaks with a lit match. When I questioned the practice of using a lit match, the experienced techs at the shop said it was perfectly safe and to prove that, they deliberately set up a leak. A small leak, which would momentarily ignite and then go back out because there wasn’t enough gas leaking to support a constant flame. I bought a small plastic bottle of liquid leak detector and stopped using matches for leak testing.