A helical spline lock for securing an inlet duct to a portion of a combustor body includes a helical spline key having a helical shape defined at least in part by a shape of a combustor body. A first lug is disposed about the combustor body and has a first channel for at least partially receiving and interfacing with the helical spline key. A second lug is disposed about an inlet duct and has a second channel for at least partially receiving and interfacing with the helical spline key. The first lug and the second lug are configured to mate with one another to form a helical keyway from the first and second channels, and the helical spline key is insertable into the keyway to secure the inlet duct to the combustor body.