The single most impactful bushing job on an NA/NB is the rear end: the subframe (often called the "power plant frame" area and rear subframe) bushings and the differential mount bushings. When they perish you get a clunk on throttle on/off and a loose, disconnected rear.
Why it matters:
- The rear subframe carries the whole rear suspension and diff. Its mounting bushings age, crack, and let the assembly shift under load. The result is a clunk when you get on or off the throttle and a rear that steps out unpredictably.
- The differential nose and mount bushings absorb driveline shock. Worn, they let the diff move, producing a distinct thunk during gear changes and throttle transitions that owners often misdiagnose as a broken mount or bad U-joint.
Fixes and options:
- Refresh the rubber with OEM or upgraded bushings for a quiet, compliant result. This is the right call for a street car.
- Solid or urethane diff and subframe bushings kill the clunk and tighten the rear decisively, at the cost of some noise, vibration, and harshness transmitted into the cabin. Popular with track and autocross owners.
- Inspect the subframe mounting points on the chassis while you are in there. This is a prime rust zone on NA/NB, and a bushing that looks bad may actually be sitting in a rusty, deforming mount. Fix structure before you chase noise.
If your Miata clunks on every throttle transition and the diff and PPF checks out, worn subframe and diff bushings are the prime suspect.