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Summary
  • The top dead center refers to the point where the engine’s cylinder one reaches its highest position during the compression stroke of the four-stroke cycle.
  • Every cylinder has a different TDC position. To find the TDC, identify the companion cylinders and determine where the #1 cylinder is on your engine.
  • TDC makes it easier to align replacement parts that rely on synchronizing their movements with various engine parts like the crankshaft, camshafts, and pistons.

Read on to learn all about top dead center (TDC), why you need it, and how you can find it.

What Is TDC?

The top dead center is the point where the engine’s cylinder one reaches its highest position during the compression stroke of the four-stroke cycle. During this part of the cycle, the piston goes up the combustion chamber, compressing the mixture of air and fuel inside the chamber.

dead center diagram
Finding TDC is important if you’re doing something like a cylinder leakage test, which requires the cylinder being tested to be on top dead center compression stroke. When installing a distributor you also need to find #1 TDC compression stroke.| Image Source: Richard McCuistian

How to Find Top Dead Center

Let’s start by establishing the fact that every cylinder has a TDC position, and there are diagnostic reasons for finding TDC on any cylinder. But each cylinder has a companion cylinder that reaches top dead center (except on engines with 3 or 5 cylinders). The companion cylinder will be on the exhaust stroke when its companion is on compression stroke.

Take note of companion cylinders

Companion cylinders on inline 4 cylinder engines are typically 1 and 4, and 2 and 3. Inline 6 cylinder engines will typically pair 1 and 6, 2 and 5, and 3 and 4. As for V6 engines, companion cylinders depend on the firing order.

tools to find the dead center
There are tools available such as the one shown in the photo to specifically find TDC on any cylinder. But you don’t necessarily need a tool. | Image Source: Richard McCuistian
cylinder 1 mark
Number one cylinder and its companion are typically the easiest to find because there’s a zero mark on most engines. It won’t always look like this (see photo), but when you find the mark, it’ll be on the crank pulley or balancer. Sometimes it’s just a pointer on the cover and a painted notch on the crank pulley. Sometimes the numbers are on the pulley or balancer and sometimes they’re on the timing cover like you see in the photo. When you line this mark up, #1 and its companion are at TDC, one on compression stroke and the other on exhaust stroke. | Image Source: Richard McCuistian

Find the #1 cylinder

You’ll need to research where the #1 cylinder is on your engine. On I4 and I6 engines, the #1 cylinder will be the one nearest the belts, regardless of whether the engine is mounted linear (belts behind the radiator) or transversely (sideways in the engine bay). One notable exception to this rule would be the I4 engines in Renaults, which tend to put #1 cylinder on the flywheel end of the engine, but those cars are just about all gone now.

On Ford V8s, #1 will be the front plug on the passenger side bank. On GM and Dodge V8s, #1 will be the front plug on the driver side bank. If you have a transverse V6 or some other make, find out where #1 is before you do anything else.

Next Steps

Remove the coil or wire, remove the #1 plug, put a rubber cork or a whistle attachment in the spark plug hole, and have an assistant “bump” the starter in very short increments so you’re moving the engine only a few degrees with each “bump.”

When the cork pops out or the whistle sounds, stop. Use a strong flashlight to find the mark on the pulley (notch or zero number). 

Since the cork will almost always pop out before that cylinder reaches TDC, you should be able to use a wrench on the crankshaft bolt or on the alternator pulley bolt (if you can get to it and the belt doesn’t slip) to turn the engine very slowly until the zero mark is aligned with the pointer or notch. If you’re doing this on an engine stand, use a wrench for the whole task.

Important Note: If the mark has already passed the zero when you find it, don’t ever turn any engine backward to line it up. You can cause some engines to jump time that way.

When you get the crank zero mark lined up using this method, you’ve found TDC on cylinder #1. You can also use this same mark and technique to find TDC on the companion cylinder to #1, whichever it may be. Just move your cork or whistle to that cylinder and repeat the process. Again, the companion will be a different hole on different engine platforms, so be prepared to do your research.

Finding TDC on another cylinder

But what if you want to find TDC on another cylinder that’s NOT cylinder #1 or its companion? Well, that’s where the spring-loaded TDC tools come in handy. But even if you don’t have those, you can use the firing order to find the next one, but you’ll need to mark the balancer with a protractor. 

If, for example, you’ve found #1 on a Ford 4.0L, the firing order is 1-3-2-5-4-6, and there are 120 degrees between each TDC position on a 6 cylinder. There is 180 degrees between each TDC position on a 4 cylinder and 90 degrees between each TDC on a V8.

We get those degree numbers by dividing 720 degrees (a full 4 stroke cycle) by the number of cylinders the engine has.

Finding TDC in Engines With Hard-To-Reach Spark Plug Holes

Start by removing the spark plug from cylinder one and finding the crankshaft pulley.

Some engines have been designed to cover their timing marks. You must remove the engine valve cover to find these marks. There are several bolts that attach the valve cover to the engine. Locate these bolts and take them off.

Next, carefully remove the head gasket with care to avoid damaging this fragile part. Otherwise, you might have to replace its seal. After removing the gasket, lift the engine valve cover. You should be able to see the camshaft or camshafts, if your vehicle has a dual overhead cam engine.

Now that the engine valve cover is out of the way, search for the timing marks on the camshaft gears. There will be notches, pins, stamps, or other clear indicators.

Use the ratchet and socket to rotate the crankshaft pulley until the timing marks on the crankshaft pulley and camshaft gears align. In most cases, the marks either point toward each other or away from each other once the engine is in top dead center alignment. If you aren’t sure about their alignment, consult your vehicle’s service manual on how to move the crankshaft pulleys into TDC.

zoom in shot of a car engine and camshaft
The TDC makes it easier to align replacement parts that rely on synchronizing their movements with various engine parts like the crankshaft, camshafts, and pistons.

Why Is Top Dead Center Important?

There are many maintenance tasks and repair jobs that require the engine to be in its dead-center position. TDC makes it easier to align replacement parts that rely on synchronizing their movements with various engine parts like the crankshaft, camshafts, and pistons.

Some of the engine-related jobs that require TDC include:

  • installing a distributor
  • performing a cylinder leakage test
  • setting the ignition timing
  • phasing in a new cam sink
  • degreeing the camshaft

Once you become familiar with the process, putting the engine in top dead center position becomes straightforward. If you don’t have the time, skill, or confidence to perform the task yourself, you can always bring your vehicle to a trusted mechanic who can do the job. But it never hurts to know how to do it, especially since you can save on labor costs if you can put your vehicle’s engine in TDC.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com 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 CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

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.

File Under : Engine , DIY
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