Cylinder taper of more than .005" from top to bottom of the piston ring travel is my personal limit for a re-ring job. As a cylinder wears tapered, the rings must constantly flex to maintain contact with the cylinder walls. at 2500 RPM, that means the rings are flexing 5,000 times a minute, or 83 times a second. In a rebored, straight cylinder, they flex- - - - -ZERO times a second. Which one will give longer ring life?
You can get a pretty good check on cylinder taper by inserting a used piston ring into a cylinder, and squaring it up against the ring ridge at the top. Then stack up feeler gauge blades and measure the ring gap. Push the ring to the bottom of the cylinder and measure the gap again. Subtract the bottom measurement and divide the difference by Pi (3.14). The result is the cylinder taper in thousandths of an inch. For instance:
Top gap = .090"
Bottom gap = .065"
Difference = .025" Divide by Pi = .008"- - - -too much taper to re-ring!
Jerry
