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Originally Posted by Woodz428
I ran an Automotive machine shop for several years in the late 60s into the mid 70s and again in the later 80s. It is my recollection that most of the wear is at the TOP of the cylinder( that's why the lip). The heat ,etc. makes the wear more prominent at the top. Piston skirts of aluminum are going to wear themselves away much faster than it could ever wear the wall. That's why they went to aluminum from the cast iron they used to use for pistons. The rings, always under extreme tension to keep a seal, are much harder and do much wearing. If you examine a worn cylinder closely, you see that the wear starts at the top and tapers off to less wear as you go down, hence the word taper. The wear in a cylinder, while not exclusively in the ring travel area, is mostly confined to that area. I will credit some of the newer ring coatings for some of the reduction in wear also. But the low tension rings have more overall impact on wear reduction.
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Hard to argue with your logic. I know the top of the cylinder is where you get the ridge line (still at original bore above where the ring stops at TDC). What I have seen (granted much less experiance than you) is that the wear at the top of the bore is even, as the bore is still fairly round even though it is worn. As you go down the bore, the wear is not even. The cylinders are out of round. Very little wear when you measure in a line drawn front to back of the engine across the cylinders. However when you measure perpendicular to that line there is much more wear. The wear will also be on the side of the cylinder that the rod thrusts the piston into, if you measure the radious from the center of the bore. I base this on a 327 375 HP Vette engine that the bore was worn 0.030" in the perpendicular plane, with 0.025" of that on the thrust side. The shop said I would have to bore it 0.050" just to get the cylinders round again. This engine had seen a lot of WOT high RPM duty. So it may have had much more side thrust wear than the average family car. In fact when I think about it, almost every engine I pulled appart, back in my puppy days, had been abused and ran hard.
It would be logical that the maximum wear from the side thrust caused by the rod angle would be part way down, where the crank is turned 90 deg from TDC. I had assumed the piston caused this wear, but your arguement that the piston is a soft aluminum and rings are harder makes perfect sense. I have to agree that the rings are doing the wearing in the bore. I said the bottom of the bore was out of round, when I should have said partway down the bore.
I will point out that there is a ridge line at the top of the cylinders where the rings never travel, however I have never seen one at the bottom of a cylinder. The compression rings are designed to allow cylinder pressure to go under the ring and push the ring outward into the bores (I have not explained this well as it if fairly compicated to explain). There is more pressure in the cylinder when the piston is at the top than when it is at the bottom. Therefore the rings push harder against the cylinder wall at the top than at the bottom. I believe this is why there is no ridge at the bottom. This factor, I suspect is significantly more than the tension in the rings at WOT. I had originally discounted the lower tension, but less tension would make a bigger differance at idle and low power, where most family cars spend the vast majority of their time. So you have convinced me that the low tension rings has made a difference. Also this supports your statement that your experiance showed the most wear at the top of the bore. The side thrust wear may not show up in mildly operated engines.
I looked it up and the 4.6 Modular, and the sleeves are ductile iron as others have stated. I was flat wrong, when I said steel. However, I do have to wonder if ductile iron is more wear resistant than cast iron. I think Barry_R indicated it is, but I'm not certain that was what he ment.
Thanks to all for the conversation. I learned quite a bit.