Many of my taper bored engines lose the pinch, at least what you can feel with the glow plug in place, in about 10 hours or so of use. That doesn't mean it doesn't have any taper, just that the piston no longer has an interferance fit at the top. They still have excellent compression and seem to run this way a lot longer than it took to wear the piston enough to lose it's pinch. In fact they run a lot better this way, smoother and more power.