A sloppy, notchy Miata shifter is almost always worn turret components, not the gearbox. Rebuild the shifter with fresh nylon bushings, a new boot seal, and proper grease and it tightens right up for cheap.
What wears. The shift lever pivots in the turret on a plastic ball cup and is retained by a spring and cap. Below, the lever tip sits in nylon bushings that engage the shift rail
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