Pistons, Piston Rings and Connecting Rods

The primary function of the piston in an internal combustion engine is to:

Support the crankshaft Transmit the force of combustion to the connecting rod and crankshaft Control coolant flow Lubricate the cylinder walls only

According to the ILM, pistons must withstand:

Only low temperatures and light loads High temperatures, high combustion pressures and rapid direction changes No mechanical stress Only electrical loads

Most automotive pistons are made from:

Aluminum alloys for light weight and good heat conductivity Cast iron only Plastic composites Glass-filled nylon

The crown of the piston is the:

Lower edge of the skirt Top surface exposed to combustion gases Area where the wrist pin is located Channel for oil control

Piston skirts are designed to:

Support the piston in the bore and control side thrust Hold piston rings in place Seal combustion gases Direct oil to the bearings

The ring lands on a piston are:

Areas that contact the cylinder head Areas between the ring grooves that support the rings Areas that connect to the connecting rod Areas that hold the wrist pin clips

Piston pin bores are located:

In the crankshaft Through the piston boss areas where the wrist pin passes through In the cylinder head In the main bearing caps

The function of compression rings is to:

Control oil on the cylinder walls only Seal combustion gases above the piston and transfer heat to the cylinder walls Support the connecting rod Direct fuel into the combustion chamber

Oil control rings are designed to:

Seal combustion pressure completely Scrape excess oil from the cylinder wall and return it through drain holes Support the piston pin Increase compression ratio

Piston ring end gap must be set correctly so that:

The rings butt tightly when cold Rings have clearance to expand with heat without butting and causing cylinder damage Rings can spin freely out of the groove Oil cannot reach the rings

Common piston ring materials include:

Soft plastic Cast iron or steel with surface treatments such as chrome or moly coatings Glass Lead

The ILM points out that connecting rods:

Connect crankshaft to flywheel only Connect piston to crankshaft and transmit reciprocating motion into rotation Support the cylinder head Carry coolant through the engine

An I-beam connecting rod design is used because it:

Reduces strength Provides a good balance of strength and weight Is easiest to cast Prevents oil flow to the bearings

Connecting rod big-end bearings are typically:

Roller bearings only Replaceable insert bearings fitted between rod and crankshaft journal Solid without any bearing material Located at the small end only

Small-end (wrist pin) connections may use:

Press-fit pins or full-floating pins retained by clips or locks Threaded bolts Glue only Roller chains

A 'full-floating' piston pin is one that:

Is welded to the rod Is free to rotate in both piston and connecting rod bushings Is rigidly fixed to the piston Is only used in diesel engines

Excessive carbon build-up in ring grooves can:

Improve ring sealing Cause rings to stick, leading to poor sealing and oil consumption Reduce engine temperature Increase fuel economy

Piston damage from detonation may appear as:

Clean, sharp edges Pounded ring lands, broken ring grooves or melted areas on the crown Improved ring seating Uniform smooth wear

Scuffing or seizure marks on piston skirts usually indicate:

Excessive clearance and cool operation Insufficient lubrication, overheating or poor clearance Correct operating conditions Normal break-in wear

When installing pistons, the ILM notes that orientation marks such as arrows or 'FRONT' stamps are used to:

Choose paint colour Ensure pistons are installed in the correct direction relative to thrust and valve reliefs Adjust ring end gap Identify cylinder numbers only

Ring side clearance in the groove must be checked because:

Too much clearance improves sealing Too little or too much clearance can lead to ring sticking or pumping of oil and gases It only affects compression on diesel engines It determines coolant level

Tapered and barrel-faced rings are designs intended to:

Reduce contact with the cylinder walls completely Optimize contact pattern and oil control while minimizing friction Prevent any rotation of the ring Eliminate the need for honing

Oil ring expanders in multi-piece oil control rings are used to:

Prevent the ring from contacting the bore Maintain outward pressure of the scraper rails against the cylinder wall Hold the piston in the bore Control combustion pressure

Connecting rod caps must be kept with their original rods because:

They are all identical from the factory They are machined as matched sets and mixing can cause bearing misalignment They differ only in colour They change compression ratio

Rod bolts are often torque-to-yield fasteners, meaning they:

Can be reused indefinitely Are stretched to a precise amount and are often recommended to be replaced rather than reused Are made of plastic Do not clamp the joint

From a parts perspective, piston-related sales opportunities include:

Only cylinder heads Pistons, ring sets, wrist pins, pin bushings, clips and connecting rod bearings Only air filters Only exhaust gaskets

The ILM stresses that ring sets must match:

Vehicle colour Piston groove design, cylinder bore size and intended application (gasoline/diesel) Only the number of cylinders Only engine oil viscosity

Piston ring groove wear can be checked using:

A tire pressure gauge Feeler gauges and the specified side clearance values A voltmeter An air blow gun

Cracked or bent connecting rods can result from:

Normal light-duty operation Hydraulic lock, severe detonation or over-speeding Correct torque procedures Regular oil changes

When fitting new pistons and rings, the ILM indicates that cylinders should be:

Left with a smooth, mirror-like glaze Honed to produce the correct crosshatch pattern for ring seating and oil control Filled with grease Scratched deeply with a file

Connecting rod bearing clearance that is too tight can:

Improve lubrication Lead to overheating, bearing failure and possible seizure Have no effect on operation Increase fuel economy

Excessive connecting rod bearing clearance can cause:

High oil pressure and quiet operation Low oil pressure, knocking noises and reduced bearing life Improved emissions Better starting performance

Piston cooling oil jets (if equipped) are used to:

Spray coolant into the radiator Spray oil at the underside of the piston to remove heat and lubricate Clean the combustion chamber Lubricate the timing belt

The ILM highlights that some diesel pistons use composite designs where:

Crown and skirt are separate pieces joined together to handle high loads and temperatures No rings are needed Only plastic crowns are used Skirts are eliminated

Piston slap noise is most often caused by:

Tight piston-to-wall clearance Excessive clearance between piston skirt and cylinder wall Correct operating temperature Normal valve action

Ring breakage during installation can often be prevented by:

Stretching rings as wide as possible by hand Using proper ring expanders and following correct procedures Hammering rings into place Installing rings upside down intentionally

When ordering piston and ring kits, parts technicians typically need:

Only vehicle paint code Engine family, bore size/oversize, piston design code and fuel type Only odometer reading Only transmission type

The ILM explains that connecting rod length affects:

Only oil capacity Rod angle, piston side thrust and engine characteristics Only tire size Fuel tank capacity