Rings are sized to a specific cylinder bore. They MUST stay round. File-fitting an oversize ring to get the gap right will cause it to get egg-shaped and lose contact with the cylinder wall around most of its circumference. Here's how to check cylinder taper with nothing but an old ring, a feeler gauge, and a calculator:
Place an old ring in the cylinder and square it up against the ridge at the top of the ring travel. Stack up feeler gauge blades to fill the gap and add them up to give you the gap thickness in thousandths of an inch. (For instance, 65 thousandths- - -.065")
Push the ring near the bottom of the cylinder with an inverted piston so it stays square with the bore and measure the gap again. Example: .020"
Subtract bottom gap from top; .065 - .020 = .045"
Divide the difference by 3.14: .045 divided by 3.14 = 14.3
That's a cylinder taper from top to bottom of just under 15 thousandths- - - -three times the allowable taper to get a good re-ring job. The cylinder would need to be rebored at least .020" oversize to get the cylinder wall straight again, and .020" oversize pistons and rings would need to be installed.
Running new piston rings in a tapered bore causes them to flex in and out twice each turn of the crankshaft to maintain contact with the cylinders. At 2500 RPM (normal highway cruise speed) the rings are flexing 5,000 times a minute, or 83 times a second! No wonder they lose tension in a hurry. Reboring the cylinders straight eliminates the ring flex until the cylinders wear tapered again in another 40,000 miles or so.
To make it even more interesting, most cylinder wear happens in the first couple of inches at the top of the piston travel so the rings are flexing very rapidly at the top of the stroke in a tapered cylinder.
Jerry
