1. D. Romberger Ag Power & Technology
Piston & Piston Rings
D. Romberger
Ag Power & Technology

2. PISTON RINGS & LUBRICATION
Ring Grooves
The clearance between the piston ring and the ring lands is critical for proper rotation, flexing, sealing of piston rings, and routing of combustion gases. The depth of the ring groove must provide sufficient clearance for the piston ring to compress into the ring groove without contacting the piston.
Piston windows are a series of small holes machined into the oil ring groove surface of the piston. The oil ring collects excess oil from the cylinder bore. Piston windows allow oil in the oil ring groove to drain into the oil reservoir.
PISTON RINGS & LUBRICATION

3. COMMON PISTON RING ORIENTATION
Compression Ring
Wiper Ring
Oil Ring
Currently, most piston ring design improvements are dic-tated by stricter emission control standards. Each pis-ton ring design and ring set are specifically designed to maximize efficiency and reduce oil consumption. Engine manufacturers provide piston ring assembly charts to ensure the proper ring type and position.
Compression Ring-Seals piston against cylinder wall to form a seal
Wiper Ring-Move oil from bottom of piston to top of piston
Oil Ring-Collect excess oil from cylinder walls and return to reservoir

4. When Briggs & Stratton introduced the first aluminum alloy engine for consumer use in 1953, one of the major engineering challenges was material selection for the piston. Use of an aluminum alloy piston in an aluminum alloy cylinder bore posed a problem, as metals with similar hardness and other properties tend to adhere to each other when thermally stressed. This can cause severe scuffing, metal transfer, and eventual engine seizure. Plating of aluminum alloy pistons for use in aluminum alloy cylinder bores eliminated the problem. Aluminum alloy pistons are commonly plated with a chromium alloy to provide a hardened piston running surface. An unplated aluminum alloy piston can provide adequate performance in cast iron cylinder bores. A newer process, iron plating, creates the dissimilar metal requirements of the piston and cylinder bore. Iron plating greatly reduces the environmental concerns present with the chromium alloy process.
Why can’t an alumninum alloy piston be used in an alumninum cylinder bore?

5. Purpose of Piston Rings:
Free Piston Ring Gap:
Piston rings seal the combustion chamber, transferring heat to the cylinder wall and controlling oil consumption. A piston ring seals the combustion chamber through inherent and applied pressure. Inherent pressure is the internal spring force that expands a piston ring based on the design and properties of the material used. Inherent pressure requires a significant force needed to compress a piston ring to a smaller diameter. Inherent pressure is determined by the uncompressed or free piston ring gap. Free piston ring gap is the distance between the two ends of a piston ring in an uncompressed state. Typically, the greater the free piston ring gap, the more force the piston ring applies when compressed in the cylinder bore.

6. A piston ring expander prevents improper stressing or bending of the piston ring during installation. A piston ring compressor is used to install the piston and piston rings into the cylinder bore
Proper installation of a piston ring is critical for optimum engine performance. Any improper stressing or bending of the ring during installation can cause serious problems. All piston rings should be installed using a ring expander. A ring expander is a tool that expands the piston ring uniformly and causes no permanent distortion to the piston ring. Use of hands to install pistons rings is not recommended. The piston ring should be expanded only enough to slide over the head of the piston. A ring compressor is a tool that is used to compress piston rings for installation in the cylinder bore. Distortion of the piston ring dimensions can remove the intentional positive or negative twist causing increased oil consumption, scuffing, and/or eventual failure.