In the cylinder, do the compression and oil rings make contact with the cylinder walls? Does the piston itself make contact with the cylinder walls?

Sorry about the basic question but I'm slowly putting all this together.

 

For your first question. Yes, the compression and oil rings touch the cylinder walls. The compression ring keeps the compressed gas and exhaust gases from leaking into the engine block and the oil ring, which is below the compression rings holds oil in it to lubricate the cylinder walls. The oil ring picks up oil every time the piston moves down into the block. For your second question. No the piston should not touch the cylinder walls.

 

This piston should ride on a very very thin layer of oil. That is what should separate the skirts from the walls. The oil rings help control this from actually getting into the 'combustion area'. That is why oil rings are often compound, with holes and channels passing to holes in the ring land to bleed excess oil back into the crankcase. There is usually a fair amount of force pressing the rings against the cylinder walls, this helps maintain compression and the control of the oil on the walls. When you see a car going down the road belching lots of smoke, that usually is a telltale sign that at least the oil ring has lost its tension and is no longer forming this tight seal, thus excessive oil is getting up into the combustion chamber. As for the pistion, it is inevitable that it will have some contact. Every engine I've ever torn down has shown some kind of evidence of contact between piston and cylinder walls, but it should be minimal, even after 100k miles or so. If you think about how a crank rotates and the angle of the rod either pushing or pulling on the piston, there are 'side' loads that are pulling/pushing the piston against the cylinder wall. That is the nature of the beast. There is a lot more to learn about this, more than I can go over hear.

If you are really interested in getting a more detailed understanding about engines I'd suggest doing what I had my kids do so they could understand it better. I just picked up an old lawn mower engine and had them go through every piece of it. We took the sawzall out and sectioned it so they could see everything working when turning it over etc. They learned a lot from that project.

 

Thanks guys for all the info, it actually helps me a lot to visualize the sytem better. That is a pretty good idea about ripping apart an old lawnmower; think I'll take a drive around on garbage nite and see what I can find.