Pistons

Piston A piston is a cylindrical engine component that slides back and forth in the cylinder bore by forces produced during the combustion process. The piston acts as a movable end of the combustion chamber. The stationary end of the combustion chamber is the cylinder head

Piston features Piston head, Piston pin bore, Piston pin, Skirt, Ring grooves, Ring lands, and Piston rings

The piston head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal engine operation. A piston pin bore is a through hole in the side of the piston perpendicular to piston travel that receives the piston pin. A piston pin is a hollow shaft that connects the small end of the connecting rod to the piston. The skirt of a piston is the portion of the piston closest to the crankshaft that helps align the piston as it moves in the cylinder bore.

A ring groove is a recessed area located around the perimeter of the piston that is used to retain a piston ring. Ring lands are the two parallel surfaces of the ring groove which function as the sealing surface for the piston ring. A piston ring is an expandable split ring used to provide a seal between the piston an the cylinder wall.

Piston Design ConsiderationsPistons must:

Contain cylinder pressureTransmit the pressure created by combustion to force on the connecting rodProvide a place for oil control and compression rings to be located

Aluminum expands when heated, and proper clearance must be provided to maintain free piston movement in the cylinder bore. Insufficient clearance can cause the piston to seize in the cylinder. Excessive clearance can cause a loss of compression and an increase in piston noise

Piston Design ConsiderationsPistons must:

Be rigid enough to not deform under the tremendous pressures and forces encounteredBe ductile enough to absorb pressure peaks and not shatterRetain the proper shape under the extreme temperatures encountered.

Piston ConstructionMaterials

Cast iron (used in very old engines)Cast aluminum (most common)Forged aluminumHypereutectic alloys (high silicon content aluminum)Carbon Graphite (being tested)

Piston Construction

Piston ConstructionPiston head

RoundApprox. .040 undersizedRing groovesRing landsDrain holes and slotsWrist pin boss

Piston ConstructionReinforcing struts cast pistons contain steel struts to control expansion and aid in wrist pin supportLock ring grooves for full-floating piston pinsPiston skirt

Full skirtPartial skirt

Piston ConstructionCam ground pistons.

Most pistons are cam ground, this means they are not perfectly round.The diameter is larger on the non-thrust sides, because they tend to expand more as the temperature of the piston increases.Piston taper pistons may be tapered from top to bottom

Piston ConstructionPiston Pin Offset

The piston pin is offset towards the major thrust side of the piston to reduce piston wear, and piston noiseThe major thrust side is the side that is loaded during the power eventThe minor thrust side is the side loaded during the compression event

Major/Minor Thrust

Piston DirectionPistons should be positioned so arrow, dot or other marking faces the front of the engine (except reverse rotation engines)

Piston Head DesignPistons may have a dished, flat or domed design

Piston InspectionAbnormal skirt wearScoringMelted aluminum or pits in the head indicates detonationCracks check ring lands, pin boss, and skirt closelyPin bore check for scoring and wear