Pressed-sleeve cylinders have been around since Hector was a pup and continue to work well. They're inexpensive to build and can be easily overbored or relined in the event of damage, or to facilitate the installation of big-bore kits. On the downside, they don't transfer heat as well as they might, but for most of us that's a minor concern.
Plated cylinders are lighter, cool a bit better and are more resistant to wear than the sleeve type. Unfortunately, they can't be repaired without very specialized equipment and are slightly more expensive to manufacture. For the most part, they're found only on bikes that place a premium on performance, such as hard-core sportbikes, pure race motorcycles and cutting-edge, performance-oriented cruisers like the Suzuki Boulevard M109R.
As a rule, most motorcycle engines use removable cylinders that are fastened to the crankcase with studs and bolts, just as they have been since the early 1900s. It works, but makes for a flexible crankcase assembly and a ready-made path for leaks. A better idea would be casting the cylinder "in-block"-meaning that the upper crankcase half and the cylinders are made in one piece. Casting in-block makes for a rigid, leak-free and more compact cylinder assembly and eases the installation of water-cooling passages and cam drives. Honda is a proponent of in-block construction, with the old CX 500/650 series, the Magnas and, of course, Gold Wing/Valkyries being the examples that come most readily to mind.
While there are many good reasons why manufacturers don't want to adapt this method (primarily cost and complication), I can't help but think that somewhere down the road we'll be seeing lots more of them
Rings
Piston rings are used to create a gas-tight seal between the cylinder and the piston to assist in keeping the piston cool (about one third of the piston's heat is transferred through the rings to the cylinder wall) and to control cylinder wall lubrication.
Common practice is to use two compression rings, locating them as close as possible to the top of the piston to seal the combustion chamber, and one oil control ring positioned just below them to prevent lubricating oil from migrating into the combustion chamber. The rings are split to ease assembly and to allow for thermal expansion. Like the piston, they take a real beating, so they're normally made of a hard yet elastic material like ductile cast iron or molybdenum steel alloy. To enhance longevity, rings are often plated with chromium, nitrided or, in some cases, wear a ceramic coating.
Modern rings are incredibly well made and durable. When I began riding, rings lasted anywhere from 10,000 to maybe 30,000 miles before they needed replacement, and many riders did the job on their own beneath the shade of the nearest tree. Nowadays, they'll last anywhere from 100K to 200K, and I'd bet the majority of you have never even held one in your hand. Progress is a wonderful thing.
Cylinder Head
Engines are a lot like people in that they breathe through their heads. At times they're also leaky, cantankerous and obstinate, which makes them a lot like me, but I digress.
Cylinder heads have three main components: the intake port, which allows fresh mixture to flow into the engine; the exhaust port, which is where it exits; and the combustion chamber, which is the recessed area machined into the head where ignition and combustion take place (although there's one design, called a Heron Head, that locates the combustion chamber in the piston crown). Because the head controls flow and combustion it's where the bulk of an engine's torque and horsepower are created, so engineers and tuners go to great lengths to get the design just right.