I have quite a few plastic Class 66 bobbins. If you get those actually made by Singer in the 70s and marked Singer, they are OK. If you find aftermarket ones that match the old Singer they are usually OK. But I've also seen them narrower between the rims, burred on the edges, and just sloppily made.
The best bobbins to use in an older machine made for Class 66 bobbins are the metal ones sold by Sew-Classic. Those are really good.
But even better than that are the actual vintage Singer bobbins.
Joe