I think you will find that most PA28s are cleared for spinning (maybe not the newer ones), provided that W&B is within utility category limits.
Problem is loading them within those limits. In a 140 it is often not possible to get the CofG far enough forward to get within utility limits. A 180 with a bigger lump of metal at the front is better for this.
Having said this, they don't actually spin very well and you normally end up in a spiral dive (gives you the opportunity to show to the examiner that you know the difference and the different recovery technique required!).
Regards
Gerard
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