Pistons, connecting rods and the crankshaft

In How an engine makes power, burning fuel pushed a piston down and the crankshaft spun. This lesson looks at the parts that make that happen. Together they’re called the bottom end of the engine, or the rotating assembly:

• The pistons take the push from the burning fuel.
• The connecting rods, or con rods, carry that push down to the crankshaft.
• The crankshaft turns the up-and-down movement of the pistons into the spin that drives the car.

They have a brutal job. The push on a piston can pass two tonnes, many times a second, and each piston has to be stopped and started twice every turn. Yet they must be light, so the engine can rev freely, and fit together with gaps measured in hundredths of a millimetre.

Step through the three parts with the numbered buttons, then watch the graph as you change the engine speed. The engine is a typical 2.0-litre four-cylinder, drawn to scale.

1. Piston. A short, light plug of aluminium, sliding in the cylinder. Its top is the crown, and round its sides sit three piston rings in grooves (the demo shows the top two). The top two seal in the pressure of the burning fuel; the bottom one scrapes oil off the cylinder wall so it doesn’t get burnt. A steel pin called the gudgeon pin runs across the piston and through the top of the rod, letting the rod swing.
2. Connecting rod. A strong steel rod, usually forged. Its small end goes round the gudgeon pin, and its big end wraps round the crankshaft, clamped on by a bolted cap. It is pushed hard on the power stroke, but its toughest job is pulling: twice every turn, it has to stop the piston and fling it back the other way. At high revs, that pull is like hanging well over a tonne from it.
3. Crankshaft. The rod’s big end turns on a crank pin, set off to one side of the crankshaft’s centre, like a pedal on a bike. As the pin goes round, it pulls the rod and piston up and down. The pin’s offset is the crank’s throw, and the piston travels twice that from top to bottom: the stroke, 86 mm (3.4 in) in this engine.

Now try 4. Piston speed. The piston stops dead at the top and the bottom of every stroke, which mechanics call top dead centre (TDC) and bottom dead centre (BDC). In between it moves amazingly fast. At 6,500 rpm, it averages 42 mph (67 km/h) and peaks at over 62 mph (100 km/h), stopping and starting again more than 200 times a second.

The graph isn’t quite symmetrical. Because the rod leans over as the crank turns, the piston covers more than half its stroke, about 58%, in the first quarter turn from the top. It is fastest just before that quarter turn, which is also where the push on the piston has the most leverage on the crank. At the very top and bottom, the rod lines up with the crank and can’t turn it at all.

Pistons in most road cars are cast: melted aluminium poured into a mould. The crown can reach about 300°C (572°F), so a piston is made slightly oval and narrower at the top, and grows to the right shape as it heats up. The lower part, the skirt, keeps the piston upright. As the rod leans, it shoves the piston against one side of the cylinder, and the skirt carries that load on a film of oil.

Connecting rods are usually forged steel with an I-shaped cross-section, which is stiff for its weight. The big end runs on two thin bearing shells, half-moons of soft metal lining the cap and the rod. Like the crankshaft’s other bearings, they never touch the shaft: oil pumped in through holes in the crankshaft holds them apart (see Why engines need oil).

The crankshaft is one piece of cast iron or forged steel running the length of the engine. It turns in main bearings in the block, usually five on a four-cylinder. Between them, the crank pins sit on arms called webs, with heavy counterweights opposite to balance the weight swinging round. On an inline-4, the pins for cylinders 1 and 4 point one way and the pins for 2 and 3 the other, so two pistons always go up while two come down. A pulley on the front drives the belts, and the flywheel bolts to the back.

That balance isn’t perfect. Because the pistons move further in the top half of the turn than the bottom half, a four-cylinder engine still buzzes twice every turn. Many four-cylinder engines of 2.0 litres or more add balance shafts: weighted shafts spinning at twice engine speed to cancel it out.

The size of each cylinder comes from two measurements: the bore, how wide the cylinder is, and the stroke, how far the piston travels. This engine’s are both 86 mm (3.4 in), which is called square.

• Long-stroke engines, with the stroke longer than the bore, have a longer crank throw, so each push has more leverage. They tend to make good torque low down, and are common in everyday and diesel engines.
• Short-stroke engines, with a wide bore and short stroke, have slower pistons at the same revs, so they can rev higher. The wide bore also leaves room for bigger valves. Many sports and racing engines are built this way.

The length of the rod matters too. A longer rod leans over less, so it pushes the piston against the cylinder wall less hard, which cuts friction and wear. But it makes the engine taller and heavier, so road car engines are a compromise.

The bottom end is built to last the life of the engine, and with clean oil it usually does. When it goes wrong, it’s often expensive.

• Worn piston rings. The engine burns oil, showing as blue-grey smoke, and the level drops between services. Pressure leaks past the rings into the crankcase, which is called blow-by, and the engine loses compression and power.
• Piston slap. A hollow rattle from a cold engine that fades as it warms up. The piston is rocking in the cylinder until it expands. Mild slap on some engines is normal; loud slap means wear.
• Big-end knock. A deep knocking from low in the engine that speeds up with the revs and gets louder under load. The big-end bearing has worn, usually after running low on oil, so the rod hammers the crank pin. Stop driving: if the bearing seizes or the rod breaks, it can punch through the side of the block. Mechanics call this “throwing a rod”.
• Over-revving. The pull on the rods grows with the square of the revs, so going from 6,000 to 8,000 rpm nearly doubles it. The engine’s rev limiter stops this under power, but changing down too far in a manual car, such as into 2nd instead of 4th, can spin the engine far past its redline. That can bend valves or break a rod.
• Holed or cracked pistons. Knock, or too little fuel under load, can overheat a piston crown until it melts or cracks. This is a common way for heavily tuned engines to fail.

Most standard bottom ends have a safety margin, but not a huge one. Once a tune pushes power well past standard, especially with a bigger turbo, the pistons and rods are often the weak link.

• Forged pistons and rods. Forged parts are pressed from solid metal, so they’re much stronger than cast ones. H-beam rods are a popular upgrade for high boost. Forged pistons expand more as they heat up, so they’re fitted with bigger gaps and can rattle a little when cold.
• Stroker kits. A crankshaft with a longer throw lengthens the stroke, so every cylinder sweeps more and the engine makes more torque. Try Stroker crank in the demo: the 2.0-litre becomes a 2.2. The pistons move faster at the same revs, so a stroker usually needs a lower redline, and shorter rods or different pistons so the piston doesn’t hit the head.
• Lighter parts. Lighter pistons, rods and flywheels let the engine rev up more quickly and cut the pull on the rods at high revs.
• Balancing. An engine builder weighs every piston and rod and matches them to within a gram or so, then spins the crankshaft and trims it, so the engine runs smoother at high revs.

A built engine, rebuilt with stronger internals, costs thousands, so it’s usually only worth it when you’re chasing far more power than standard. For a mild remap, the standard parts are almost always fine.