Compression and combustion pressure is highest in the first inch of the piston travel at the top of the cylinder. That pressure gets behind the top piston ring and pushes it tight against the cylinder wall. That pressure, even more than the tension of the ring itself, is what seals the ring against the cylinder wall and keeps hot gases from blowing past the rings. It also promotes wear. At 2000 RPM, those rings reach the top of the cylinder 120 times a second. The cylinder wears tapered, with the maximum wear happening in the first inch from the top.
Here's a quick way to measure cylinder taper wear: Remove one of the rings from a piston, and square it up snug against the ring ridge. Stack up several feeler gauge blades to measure the ring gap. Record the measurement. Use a piston to push the ring to the bottom of its travel and measure the gap again. It will be noticeably smaller because there will be little or no wear near the bottom. Subtract the lower measurement from the upper, and divide by 3.14 (Pi). The result will be cylinder taper in thousandths of an inch. For example: Upper gap: .040" Lower gap: .025" Difference: .015" .015/3.14 = .0047". That's right on the ragged edge of too much wear- - - - -doing a re-ring job on a cylinder with over .005" of taper wear will result in very short ring life- - - - -the ring must flex too much to maintain contact with the cylinder.
Reboring the cylinders and installing oversize pistons and rings eliminates the taper. Most stovebolt engines can handle overbore sizes of .060" or even .080".
Jerry
