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What's a piston and how does it work?

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Updated 9 Oct 2019Jason Unrau
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Car engines are complicated beasts with hundreds of moving parts.

One of the most recognisable parts by name, though, is a piston.

Although you’ve heard about its importance, you probably can’t explain what it does, right?

Let’s walk through a piston’s role inside a car engine and what can go wrong with them.

What Is a Piston?

A piston is a metal component that moves inside a cylinder, like a battering ram.

The edge of the piston is fitted with rings that seal it tight against the cylinder wall.

Made of either cast aluminium or cast iron, the piston is attached to the crankshaft by a connecting rod, and it pivots on the connecting rod with a gudgeon pin.

There’s one piston for every cylinder in an engine.

As the piston moves up and down in the cylinder, the connecting rod forces the crankshaft to rotate.

This constant push-pull force translates into power for all your car’s functions, from spinning the alternator and power steering pump to sending torque to your drive wheels.

What Does a Piston Do?

In any internal combustion engine (that’s what all car engines are, by the way), the piston performs four roles on each and every cycle.

  • First, as the piston goes down, it draws air and fuel into the cylinder or combustion chamber.
  • Second, on the upswing, it compresses the air and fuel in the cylinder so it will explode when it ignites.
  • Third, the spark plug ignites the air-fuel mixture, and the force flings the piston back down.
  • Fourth, the piston comes back to the top of the cylinder, expelling the burnt gases (exhaust) out of the cylinder.
  • Then, the cycle repeats over and over, hundreds or thousands of times per minute.

What Can Go Wrong?

As you can imagine, a piston has to be extremely durable to withstand all that energy.

Still, problems can occur that can be minor and annoying or major and catastrophic.

Piston Slap

When the piston is worn and can rock side to side instead of moving vertically, the bottom edge of the piston, or skirt, contacts the cylinder wall.

It’s a sign the piston is worn out.

Burning Oil

It’s a major concern for older vehicles.

An engine that burns oil is an obvious sign that piston rings aren’t sealing the cylinder well and oil is getting into the combustion chamber.

Broken Connecting Rod

Either from a defect, abuse, or lack of lubrication, a connecting rod can break away from the piston or crankshaft.

It usually ends up taking out the rest of the engine as well.

Loose Gudgeon Pin

A rattle from the engine could mean the gudgeon pin, or wrist pin, has excessive play where it attaches to the piston.

Burnt Piston

Improper air-fuel mixture can be extremely unhealthy inside the engine, even causing the high temperatures to melt the top of a piston!

Can a Piston Be Replaced?

The good news is that a piston can be replaced in most cars as long as the engine block itself hasn’t been damaged.

And in nearly every case, it isn’t just one piston that requires replacement – it’s a complete engine rebuild.

For many makes and models, a replacement engine is as cost-effective as an engine rebuild for piston problems.

You can expect repairs to cost between $3,100 and $7,400 for the average engine and easily twice that if it’s a diesel engine.

If you think you’re having piston problems, it’s essential that you get your engine inspected by a qualified mechanic.

You can do this easily by searching and booking a high quality, local mechanic on AutoGuru!

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Written ByJason Unrau

Jason is a Canadian automotive content writer with a background in the auto service industry, but he’s been hooked on cars and mechanics since childhood.

One of his first cars was an ’80 Mazda RX-7 that’s sorely missed to this day. A ’68 Ford Torino GT, a ’66 Ford Country Squire Woodie station wagon, and a ’96 Suzuki GSX-R 750 have spent time in his fleet of cars, bikes, and trucks over the past two decades.

Jason’s pride and joy is under construction – a turbocharged ’88 Mazda RX-7 convertible. Also on his resume is CASCAR official certification.