[ashtonkem]

While machining down to thousands of an inch is a bit tricky, actually measuring that small isn't that hard. Vernier calipers are cheap (<$20), capable of measuring down to 0.001 inches, and typically have internal jaws for measuring the gap between two surfaces.

Typically the issue isn't the ability to measure to such tight tolerances, the problem is physically getting the calipers into position to do the measurement. Airplanes are full of tight spaces with weird angles, which makes measuring parts without extensive disassembly kind of hard.