Regardless of the quality of the construction, these components are poorly mated for one exceedingly simple reason: The threads are entirely incompatible. Through-hull fittings use straight, or NPS, threads, while in-line ball valves rely on tapered, or NPT, threads. The two may initially seem to fit; however, it's difficult to obtain more than two or three threads of engagement. Instead, use purpose-made, flanged, marine sea valves, all of which have the appropriate threads. And never-repeat, never-use a standard gate valve (similar to what's used with garden hoses) on a boat. If you have one, replace it.