This picture shows the wear into the cylinder wall caused by the top ring. The ring does the wear; but what causes the wear is a bit more involved than just the ring being there.
I had started to hone the cylinder to clean up the glazing, and to prepare the cylinder wall surface for the new rings. While looking things over after a couple of passes, it dawned on me that this was worth a picture or two for the gallery. And would offer an opportunity to explain a few things.
If you look around the top of the bore, you will see a uniform line of demarcation above the dark area. That line of demarcation is where the top edge of the top compression ring stops when the piston reaches top dead center.
The area above the line showing where the top ring stops is known as the "ridge". It is an area of the cylinder bore that does not get worn by anything: nothing touches it as the piston goes up and down. It is a common practice to remove that ridge using a tool called a ridge reamer during the teardown process. Removing the ridge facilitates the removal of the piston, according to the general rules of procedure. I used to do that. I no longer do it. I want to preserve the ridge AS IS; so that I can accurately measure the wear of the bore at the top of the piston travel. Removing the piston with the ridge intact may not be as easy, but keeping the ridge there is assistive in taking measurements and making evaluations.
Those two long thin oval patches of dark area above the line of demarcation, located in the "ridge" area, are not uncommon things to see. When running a glaze breaker or honing stones over the surface of the cylinder, any low spots around the bore show up as dark spots. The depth of the low spots may only be a tenth of a thou of an inch [0.0001in]or less; meaning that they look worse than they are. As the honing proceeds, such dark spots in the ridge area usually disappear because the variations in the surface are made to disappear by the smoothing and leveling action of the honing stones.
The dark area below the ridge is a totally different matter. That dark area can be several thousanths of an inch deep, depending mostly on the miles on the motor. But sometimes poor, or nonexistent, maintenance [AKA lack of proper and timely oil changes] can accelerate the wear.
With the ridge at the top of the cylinder intact, the original bore size can be measured; as can be the accurate depth of the dark area of top ring wear.
This wear area is known as "belling". If the "belling" depth is minor...only a thou or two in depth...and if the overall wear down the cylinder wall is also only a matter of a couple of thou...whether that be "egg" or "taper": then the cylinders do not necessarily HAVE to be bored oversize. A honing to restore uniformness of the bore may be able to restore the roundness and straightness of the bore, and remove the belling [or minimize it], and accomplish all that with the removal of only a few thousandths of an inch of cylinder wall metal.
As already said, I do not remove the ridge with a reamer during the teardown procedure. The ridge gives me a very easily identifiable and quantifiable measurement of the amount and depth of the worst wear in the cylinder.
The common misconception is that the ridge is where the wear is; and that if you remove the ridge, then you are okay. The truth is the opposite: the ridge shows you what the cylinder diameter was to start with. Then, using an accurate bore gauge, you can measure and compare the rest of the bore to that diameter.
In other words, keeping the ridge there gives you a very quick comparison view of the least wear diameter right above the area of the worst wear: the belling area.
Also, having the ridge still there, assists the honing process: the metal up top there in the ridge ring, representing the original bore diameter, helps guide and direct the hone to maintain straightness while honing. Honing, as a procedure, by its nature and methodology, follows the bore as it is; and slowly enlarges the diameter...to hopefully restore the roundness and trueness of the cylinder. The ridge ring acts as a top end guide for the hone stones.
It is a win-win. Once the honing is completed, and the surface of the cylinder walls are properly 'roughened' up for a good cross-hatch pattern, the ridge will actually be diminished: after all, I AM grinding [HONING] away some of the metal, albeit very slowly.
Looking closely inside the belling area, some pitting is evident. This motor sat for several years before I purchased the vehicle it was in: it had been involved in a side collision. So, beyond the belling wear, I have some corrosion pitting to take care of as well.
With the next picture, I will get into the explanation of the cause of 'belling'.
**** to be continued and completed