These are critical parts of the combustion process in your BMW motorcycle's engine. They may seem to have a simple job -- sliding back and forth in the cylinder head bore in the block, keeping that process going -- but there's a lot more to them, not to mention plenty of parts that make them up.

For one, as they need to be both lightweight and great at conducting heat to work properly, most pistons are made of a cast aluminum alloy. Secondly, their constituent parts and features range from a head, pin bore, and pin to a skirt and rings, ring grooves, and ring lands. The pin bore is a hole in the side of the piston that receives the pin, a hollow shaft that integrates an engine's connecting rod and the piston.

The skirt, on the other hand, is closest to the crankshaft and helps align the piston as it passes through the bore. Skirts are sometimes profiled to keep the piston as small as possible and give the crankshaft counterweights the clearance they need to run right.

BMW Motorrad Rings

Piston rings create a seal between the piston and cylinder wall to close off the combustion chamber, conduct heat, and return oil to the engine's crankcase; they're commonly made from cast iron, and their configuration and how big they are vary depending on how the engine's designed and what the cylinder's made of. The ring groove, however, is a recessed area on the perimeter of the piston that helps retain the piston ring. Ring lands, on the other hand, are the parallel surfaces of the groove which help in sealing the ring.

The rings you see most often on small engines include a compression ring, a wiper ring, and an oil ring.

The compression ring, you can find in the groove closest to the piston head; it helps keep the combustion chamber from leaking while the engine burns fuel.
The wiper ring is the one with the tapered face in the groove between the compression ring and the oil ring; it's also a seal, but it also helps keep excess oil off the cylinder wall. It even helps capture combustion gases that get past the compression ring.
Finally, the oil ring, found in the groove closest to the crankcase, does the same thing as the wiper ring, but also helps send excess oil back to the oil reservoir in the block.

BMW Motorrad Wristpins

These, also called piston pins or gudgeon pins, connect the heads of pistons to connecting rods. More specifically, they give rods bearings to pivot on during piston movement. You usually find them made of hardened steel, but these days, if your bike's high-performance, they can also be made of tool steel, as it's the most durable and designed to stand up to high horsepower. They're also designed hollow to keep weight to a minimum, with high-performance variants correspondingly even lighter-weight to enable an engine to accelerate a lot quicker.

On stock engines, piston pins get their lubrication and cooling from oil sprayed off the crankshaft as it turns in the block. In systems designed for racing or other high-performance applications, however, there's often a system of tubes directed at the pistons' bottoms which spray the oil directly onto the pins and pin bosses; this helps the pistons produce max horsepower while staving off engine failure caused by heat.

A pin can be held in place one of several ways. Most stock designs are pressed-fit, with the pin being pressed into the rod. On high-performance pistons, however, they're usually fastened with wire clips or aluminum buttons.

The Signs of Problems with Them

A loss of compression
Overly noisy pistons
A seized piston
A pin falling into a piston
A piston locking onto the pin

Pins falling into pistons indicate that they're inadequately lubricated. Pistons locking onto pins, however, can cause engine seizure. Broken connecting rods and a busted block could result in any case. See any of these, and you should run an engine inspection, followed by replacing any of the parts the results recommend changing. You're in luck: you can order what you need, right here.