Pistons and piston rings

Pistons and Piston Rings The piston is a straight-line driving part in the engine. It provides a seal between the combustion chamber and the crankcase. The piston is subjected to heat, pressure, and friction. The piston must be lightweight, strong, and properly fitted.

Pistons and Piston Rings The piston assembly is made from a light weight Aluminum Alloy. It can travel at more than 1000 RPM (Revolutions Per Minute). Must withstand pressures up to 2,000 PSI. Must withstand heat up to 1,200 F.

Pistons and Piston Rings Low compression, blow-by, oil pumping, and fouled spark plugs all are potential signs of the need for piston and ring service.

Pistons and Piston Rings Pistons expand during operation. Therefore, a specific amount of clearance between the piston skirt and the cylinder wall must be present to allow for lubrication and piston expansion. Too much clearance results in “piston slap” which will harm the piston and the rings. Piston slap noise will be worse during cold starting.

Pistons and Piston Rings Piston heads may have several different shapes, depending on the type of small engine use. The piston head receives the brunt of combustion heat and expands more than the rest of the piston. The skirt is the area below the bottom of the lower ring groove. To allow for expansion, the piston head diameter is usually less than the skirt diameter. The skirt actually guides the piston and keeps it from tipping from side to side.

Pistons and Piston Rings Piston rings create a seal between the piston and cylinder walls. Most four-stroke engines use 3 rings. The area that separates the piston rings is called the Land.

Pistons and Piston Rings There are two types of piston rings: Compression Oil Control

Pistons and Piston Rings The top two rings are compression rings. Compression rings create a seal between the piston and cylinder walls. Compression rings are made of cast iron.

Pistons and Piston Rings The bottom ring is the Oil Control Ring. Oil Control Rings remove excess oil from the cylinder wall. This is done thru a light scraping motion against the cylinder walls. Oil trapped by the ring flows down inside the piston where it drops into the crankcase.

Pistons and Piston Rings The inside diameter of a piston ring is smaller than the piston’s diameter. Therefore, the piston ring must be expanded to get it installed in a ring groove.

Pistons and Piston Rings The amount of ring end gap is critical. Too much end gap will allow the gasses to leak between ring ends. Too little end gap will result in breaking the ring and scoring the cylinder wall. Ring end gap is measured with a feeler gauge.

Pistons and Piston Rings The piston pin connects the piston to the connecting rod. Snap Rings or E-Clips are used to secure free floating piston pins.

Pistons and Piston Rings The piston and crankshaft are connected by a connecting rod. The piston and piston pin push on the connecting rod to turn the crankshaft. The connecting rod has an I-Beam construction, which saves weight and makes it stronger.

Pistons and Piston Rings The upper end of the connecting rod has a hole thru which the piston pin is passed. The lower end contains a large bearing that fits around the crankshaft journal. Bearings are smooth sliding surfaces that reduce friction between moving parts. Our shop engines use Rod Metal bearings.