The first thing that you notice as a difference between this picture and the last one is that the belling on the minor thrust side is two tiered, or two levels. What causes that is the fact that on the minor thrust side, the piston is never pushed into the cylinder wall with as much pressure as the major thrust side sees....which is WHY one side is the MAJOR thrust side and one is the MINOR thrust side.
So how does THAT make for a difference in the actual belling of the cylinder wall? I'll come back to that, and answer that, as the explanation progresses.
What causes the belling? Several things do.
...the fact that the rings are under tension outwards...trying to expand out against the cylinder wall. That is part of how the rings seal the gap between the piston and the cylinder wall: the effort by the rings to open up back up to their original size when made...
...said outward tension of the rings being GREATLY increased by the added pressure of combustion gases that do get down behind the top ring [and to a lesser extent, the second ring]. This CONSIDERABLE outward pressure on the compression rings...at its highest pressure values right after TDC on the power stroke...exerts a LOT of outward pressure on the rings during a period of little piston travel.
...meaning that, during the last 20 degrees of crankshaft rotation up to top dead center and away from top dead center, the piston is only going up and down a few thousandths of an inch,
...combine that with the fact that the piston does actually "rock" a little bit on the wristpin while it is up there near top dead center,
...combine that further still with the various sideways thrust loadings on the piston as it approaches TDC and leaves TDC for the four different strokes of the Otto cycle [these said sideloads being what gives rise to the delineation of major thrust side and minor thrust side],
...and you get a fair amount of oscillating, rotating, pressurized rubbing going on up and down during a VERY short distance of piston movement.
Those are the BASIC causes of belling. Which means that in any reciprocating piston engine that has rings on the upper ends of the pistons, you will get some belling wear of the cylinder walls at TDC.
A few caveats before continuing:
...this narrative is austere: I am writing it more as a summarization. A book it is not. And a book would not be free for the reading.
...because this is done in a summarization format, I am not bringing up and discussing all the little fine points, side issues, special circumstances, what-ifs, and how-abouts, that some might get all worked up over. A multitude of books, and a mountain of articles, have been written to satisfy the obsessive compulsives so concerned with such. Some are actually good and worthy of reading. Many are the modern version of the old carnival sideshow snake oil salesman. Or worse yet, they actually believe what they are shovelling. Discerning which is which is not always easy.
Studying the theory of operation, analyzing the whats, the hows and the whys; combined with some success analysis and failure analysis, ie seeing if it actually works in the real world; makes the discernment easier.
I concentrate my efforts on those things that I have determined, via that methodology, to be the most important and crucial to achieve my objectives.
...I say this now because some of the things I will be bringing up as I expand the causes of belling discussion will be contrarian to the latest fads of motor building.
Which is just fine by me: I don't do fads.
I intend to continue and expand the discussion on belling in a new gallery that I have been working on for some time now. Which is still not finished. But, I wanted to mention two things here and now that will be part of that discussion: ring placement and skirt length.
And those two items are the 'fad' items alluded to above.
I call them 'fads' to be sarcastic about it. I do understand the reasons for the shorter, lighter weight pistons that are the current rage. And I understand the reasons for moving the rings higher up the sides of the pistons...in part because they have to do that because the pistons are shorter...and in part to reduce the quench areas where unburned fuel can hide down the sides of the piston crowns, which is so terrible and bad for emissions.
I've read the reports and tests that show how those things are just 'oh so marvelous'.
And I go: "uh huh......yeah, right".
...shorter pistons....and the resulting shorter piston skirts...are NOT necessarily more stable in the bores.
...and those higher located rings are exposed to a lot more of the heat of combustion than when located further down the side.
Combine MORE piston rocking and much HOTTER rings and you get MORE belling.
Especially in a boosted motor.
****** to be continued ******