Which coupling nut is appropriate for use with 1/2-inch aluminum oil lines?

The AN-818-8 coupling nut is the appropriate choice for use with 1/2-inch aluminum oil lines. This is because the AN-818 series coupling nuts are designed to fit specific sizes of tubing, and in this case, the number indicates the size compatibility. Specifically, the number "8" corresponds to a 1/2-inch nominal size.

Each coupling nut in the series is designated for different tubing sizes; therefore, selecting the correct nut that matches the tubing is crucial for ensuring a proper fit and seal. In contexts involving fluid systems, using a coupling nut that is not designed for the nominal size of the tubing could lead to leaks or failures.

Other options such as the AN-818-6, AN-818-7, and AN-818-9 correspond to different tubing sizes, meaning they would not provide the correct fit for 1/2-inch aluminum oil lines. The emphasis on the accuracy of size is key in aviation maintenance and safety, where proper fittings are critical for avoiding system failures.