Sounds like your solenoid isn't kicking the bendix (gear) out to engage the flywheel and your starter is just spinning. If you had teeth missing I would think that you would hear a horrible noise upon engagement. Where did you buy and what brand starter are these? I replaced one (world parts - Beck)on my 240 last spring only to replace again a month later. The problem with the new starter was that the bushing in the bendix snout had fallen out causing a horrible noise upon starting. Try banging on the solenoid before starting see if that helps. It works in pinch sometimes. If your solenoid is bad then either switch to a different brand or get a new solenoid and replace yourself. I have had multiple starter failures from napa starters on other cars before. Good Luck.
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92 244 lowered, Bilsteins, IPD sways, Unitek cam/header, TSW Revo 18x8's - 94 965 Niiice MONEY PIT! - 76 Datsun 280Z lowered, modified head, cam, headers, intake