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  • An interference engine is one that has pistons slamming into the valves when the timing belt snaps.
  • Every engine with a timing chain is also an interference engine, even old domestic V8 engines.
  • A broken timing belt on a freewheeling engine will simply stop the engine. But if this happens to an interference engine, major engine damage can occur.
  • For the average driver, it can be hard to identify an interference engine at first glance. Fortunately, the internet and a couple of legitimate sources can make the job easier.

There’s a lot of automotive jargon that can easily confuse the average driver, especially when it comes to engine design.

While there might be some terms that you won’t need to think about, there are a few that you need to understand because they play a crucial part in vehicle maintenance.

Out of the many terms you need to look for, one of the most important ones is the “interference engine.”

What Is an Interference Engine?

An interference engine is one that has pistons slamming into the valves when the timing belt snaps.

The pistons in any engine will stay in motion even when the timing belt is broken and the engine stops producing power. When this happens on an interference engine, even as the engine spools down at the moment of failure because the camshafts either stop or become badly out of time, some of the valves will make contact with the pistons, which causes the valves to bend, or sometimes to even break off, which destroys the engine to the point that the whole engine must be replaced.

snapped off exhaust valve of a car
When the timing belt is broken in an interference engine, some of the valves will make contact with the pistons, which destroys the engine. This was the case with this Kia in the photos. One exhaust valve bent and the other one had its head snapped off and you can see what happened to the piston. It made more sense to replace this engine than to replace the head and the one piston. | Image Source: Richard McCuistian

A broken belt can easily lead to major engine damage in an interference design, and every gearhead knows that engine repair is nothing but costly.

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So how come a lot of drivers still prefer an interference engine over a non-interference one?

The answer is simple: performance.

Interference vs. Non-Interference Engine

Compared to a non-interference or freewheeling engine, an interference engine has larger valves that open further into the combustion chamber, which means it “breathes” better than its counterpart.

An interference engine also has a smaller combustion chamber that helps achieve the highest compression possible.

If you’re new to the concept of engine design, a higher compression ratio means more power.

Interference engines also use less fuel and generate less emissions, so it’s no wonder why a lot of drivers would risk having this type of engine even when there’s an increased risk of engine damage.

But what about a freewheeling engine?

A non-interference (free spinning) engine is designed so that even if the toothed belt that drives the camshaft fails, the valves will never make contact with the piston.

A freewheeling engine causes no internal damage should the drive belt break while the engine is running. This is because it doesn’t let the pistons travel far enough to hit the valves.

A broken timing belt on a freewheeling engine will simply stop the engine. Replacing the belt and re-timing the engine should get the affected vehicle running as it should.

, What Are Interference Engines?

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: The late 1990s 2.2L Toyota Camry engines were non-interference engines, as is the Chrysler-Dodge 2.4L. Ford Escorts were redesigned in 1983 to make them non-interference engines.

Valve Timing Is Key

Valve timing is extremely crucial in an interference engine because it prevents the pistons and valves from colliding.

Precise valve timing is possible only when the timing belt does its job.

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The timing belt synchronizes the rotation of the crankshaft and camshaft, controlling the operation of the pistons and valves in the process.

During the combustion process, the valves are responsible for air-fuel intake while the exhaust gases push the pistons down.

But what does all of this mean? Let’s get a bit more specific.

Camshaft timing is stated in the angle of the crankshaft in relation to the top dead center (TDC) or bottom dead center (BDC).

In a four-stroke cycle engine, the intake valve should open slightly before the piston reaches TDC and goes down on the intake stroke. This ensures that the valve is fully open as the piston travels downward.

Once the piston reaches BDC, the intake valve closes.

As the piston travels down on the power stroke, the exhaust valve opens to release combustion pressure. The exhaust valve will only close once the piston has traveled past TDC and is starting down on the intake stroke.

Managing the Risks of Major Engine Damage

In an interference engine, the timing belt is the only thing that’s keeping your vehicle from suffering major damage.

In most cases, timing belts have a service life of anywhere between 60,000 and 100,000 miles. Make sure to closely follow timing belt replacement intervals recommended by the manufacturer. It’s important that you decide when the belt is replaced rather than letting the belt decide.

Aside from exceeding the manufacturer’s recommended interval, the timing belt should also be replaced if it has been contaminated with coolant or engine oil and if the belt has broken teeth.

Your vehicle also has its way of telling you that there’s something wrong with the timing belt. The most common symptoms you should keep an eye out for include the following:

  • Misfiring
  • Ticking noises
  • Oil leaks
  • Unusual engine activity (typically between 2,000 to 4,000 rpm)
  • Increased tailpipe emissions
  • Engine won’t turn over
See also  Common Questions Asked by Motorists

How to Determine If Your Vehicle Has an Interference Engine

For the average driver, it can be hard to identify an interference engine at first glance. Fortunately, the internet and a couple of legitimate sources can make the job easier.

You can try cross-checking your engine specifications with a list from Google to know whether or not you have an interference engine.

You can also consult a professional mechanic to know what type of engine your vehicle has.

Timing Belt Replacement Guidelines

In an interference engine, regular maintenance is the only way for the pros to outweigh the cons. The timing belt might seem like a simple rubber part, but it holds so much power in an engine, and ignoring it should never be an option.

Below are generic steps on how to replace a timing belt in most vehicles, but it’s also a good idea to consult your owner’s manual for specific instructions.

Step 1: Make sure the engine is cooled down before removing the necessary parts to avoid injury.

Step 2: Once you have access to the timing belt and marks, check for signs of damage. If the timing belt is still intact, rotate the engine until the crankshaft and camshaft marks are aligned.

Step 3: Loosen or remove the tensioner to remove the timing belt.

Step 4: Install the new timing belt. Most manufacturers recommend replacing other components, including the tensioner assembly, water pump, camshaft oil seal/s, and the front crankshaft seal.

Step 5: Rotate the engine for several revolutions to verify that the camshaft timing is correct.

Step 6: Check for any leaks before putting the other components back together.

Step 7: Complete the engine reassembly process and conduct a test drive.

About The Authors
Written By Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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