Quite often, when stovebolt owners have an engine apart, the decision must be made, "Do I need to go for a full overhaul, with rebored cylinders, new pistons, and the whole works, or will a new set of rings do the job?" Here's a few things we used to consider back when in-the-frame overhauls were common.
First and foremost- - - - -how much wear does the engine have? Before the days of paper-element air filters and positive crankcase ventilation, engines ate a lot of dirt, and dirt wears out rings, and cylinders. It's not uncommon to find enough ring ridge at the top of a cylinder to keep pistons from being removed without reaming the area above the normal ring travel. If there's enough ring ridge to make it difficult to remove the pistons, it's probably time for a rebore. Here's why: Rings must expand and contract to maintain contact with tapered cylinder walls as the pistons travel from top to bottom and back, and the major wear happens in the top 1 1/2 inches of the cylinder. Unless there's a very slight amount of cylinder taper, the rings will lose their tension quickly as they attempt to stay in contact with the cylinder walls. At 2,000 RPM, the rings are changing diameter 66 times per second in a tapered cylinder! If the cylinder is straight, rings don't have to expand and contract- - - -they just ride up and down in a hole that never changes size. I used the rule of thumb that if a cylinder had .005" or less of taper, I'd do a glaze-breaking hone job and re-ring it. Beyond that, it's time to yank the block out and get out the boring bar! The cleanup after the hone job involved HOT soapy water and a scrub brush, (Tide is good)followed by a boiling-water rinse, several times. Hone grit in the cylinder pores has worn out many piston rings long before their time!
Here's a quick way to estimate cylinder taper if you don't have a set of inside micrometers: Pull a compression ring off a piston, and use the top of the piston to square it up in the bore right at the top of the ring travel. Measure the ring end gap by stacking up several feeler gauge blades until you get a snug fit in the gap. Write the dimension down. Push the ring to the bottom of its travel, and measure the gap again. Subtract the bottom measurement from the top, and divide the difference by 3. The result is the cylinder taper! This works because of the old circumference of a circle formula, "Circumference equals Pi times diameter". By measuring the difference in circumference (ring gap) and dividing by 3 (close enough to 3.14 for estimating), you get the difference in diameter, or cylinder taper! Quick, easy, and no precision measuring tools involved! If it's less than .005", re-ring. Otherwise, re-bore!
Jerry
