Rust, not mechanical failure, is what kills NA and NB Miatas. The engines and gearboxes run forever; the unibody rots from the inside out, and the structural spots decide whether a car is worth saving.
The critical zones, in order of how fatal they are:
- Front frame rails. The main longitudinal rails rot, often from the inside. This is structural. Rusted-through rails are a serious, expensive repair and a common reason a cheap Miata is actually a parts car.
- Rear subframe mounting points. Where the rear subframe bolts to the chassis, especially just ahead of the rear wheels, rusts and can tear out under load. Check behind and around these mounts. This is structural and can be fatal.
- Rockers and sills. The rocker panels are load-bearing on these cars, with real structure inside. They rot from the rear of the rocker forward, frequently hidden until it breaks through from the inside.
- Floor pans rot from trapped water, often because the drains are blocked (see door and top drainage).
- The battery tray in the trunk corrodes from battery acid and trapped moisture; ugly and worth fixing but not by itself structural.
How to inspect:
- Drop the undertray. Surface rot you can see from outside usually means the inner structure is already gone. A magnet and a screwdriver poke around the frame rails, rocker rears, and subframe mounts tell the real story.
- Poke the front of the rear wheel wells. Holes "held together by underseal" are the classic finding.
Fatal versus fixable: surface scale on non-structural panels is cosmetic. Perforated frame rails, torn subframe mounts, and crumbling rockers are structural and only worth fixing on a car you love, because the labor dwarfs the car's value.