It's not always the bore wear as much as exceeding the allowable bore taper that leads to blow by. Taper is the difference in measurement of the bore at different intervals from the deck to the bottom. "Re-ringing" a bore to the loose side can still seal well (for a time) as long as there's not much appreciable taper.

BTW, Some motors can have (cold) slap and still seal well, btw. Old timers used to flip pistons that had pin offests around so the rod angle was less extreme on the power stroke, they were noisy as heck when you first fired them up but they made a little more power up top from the reduced friction. Lots of 68-69 six packs were set up that way along with cut down (-1/2" to 5/8") pins to save weight.