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 really a parts car.
- Rear subframe mounting points. Where the rear subframe bolts to the chassis, especially just ahead of the rear wheels. It rusts and can tear out under load. Structural, and it can be fatal.
- Rockers and sills. The rockers are load-bearing on these cars. They rot from the rear of the rocker forward, often hidden until it breaks through from the inside.
- Floor pans. They rot from trapped water, often because the drains are blocked.
- The battery tray in the trunk corrodes from acid and trapped moisture. Ugly and worth fixing, but not structural by itself.
How I inspect one before buying:
- 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, the rocker rears and the 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 labour dwarfs the car's value.