Guide To Four Stroke Motorcycle Engines, Part 2
The power stroke is the one phase when the engine is generating power. This power results from the violently burning air-fuel mixture which forces the piston down the bore, moving the rod that applies this force to the crankshaft.
After the power stroke the cylinder is filled with burned gases that must be expelled somehow. Another valve is opened to permit the exhaust gases to exit as the piston comes up on its last stroke in 4-phase cycle. The piston now begins another set of strokes so it can get to its next power stroke and do its job.
The cylinder is sometimes called the barrel or "jug" and it is bolted to the crankcase section. Most cylinders are made of aluminum fins with replaceable steel or iron liners in which the piston travels. Some cylinders are simply made of cast iron, but these are a bit heavy. Other cylinders are made of aluminum with a chrome-plated bore, but the chrome flakes off. This type is not easily re-bored. Aluminum Cylinders are attached to the lower end cases either by long studs that also hold the cylinder head or by short studs that are attached to a flange around the bottom of the cylinder. Either system seems to work, but if the long studs are over-tightened, cylinder distortion and loss of power may result.
In nearly all motorcycles on the road today the piston is an aluminum casting. A few of the high performance engines do have stronger forged aluminum pistons. Pistons are nearly perfectly round, being only slightly oblong to compensate for different expansion rates at different places on the piston. The dimension measured at the pin section will usually be several thousandths of an inch smaller than the measurement through 90° away from the pin direction.
They are tapered slightly for the same reason. These tapered and cam-ground pistons insure less wear and quieter engines when the engine is starting cold. The crown or top of a piston may have various configurations. Most are slightly domed or flat, while others have a tall "pop up" section for increased compression in hemispherical combustion chambers.
Four stroke pistons often have notches cut or cast into the crowns to insure that the pistons and valves don't hit one another.
The popular ring set-up for four stroke engines has two straight rail compression rings at the top and a wider cast iron oil-control ring at the bottom. Most compression rings are chrome-plated, but some are plain cast iron. Holes drilled into the piston at the oil ring land enable the oil-control ring to relieve itself of extra oil that it scrapes from the cylinder walls. The oil control rings have openings that allow oil from the cylinder walls to pass through the holes in the ring land. The second compression ring often helps oil control also. Sometimes the lower edge of the ring is beveled to enable it to wipe away some of the oil from the wall.
Pin and Retaining System
Pistons are attached to the connecting rods by wrist pins or "grudgeon pins," as the British call them. These pins are centered and retained in the piston by snap rings or locking clips. The Brits refer to these items as "circlips".