Top Dead Center (TDC) What You Need To Know

Top Dead Center (TDC) for any internal combustion engine refers to the point when the piston is at the absolute top of its stroke.

A piston can be at Top Dead Center (TDC) on the compression stroke and on the exhaust stroke.

As a general reference point, or when installing a distributor, Top Dead Center (TDC) on the compression stroke is required.

Usually it can be found by looking at timing marks but on some engines these marks may be lost or hidden.

Fortunately there’s an easy way to discover top dead center without the use of any timing marks.

Being able to accurately find Top Dead Center (TDC) is a critical step no matter what engine you are working with.

There are different ways to do it, but in this article we wanted to show you a simple, quick, and practically foolproof method for finding Top Dead Center (TDC) whenever you are doing a rebuild, building a new engine or simply want to check and make sure the timing marks on your damper are still accurate.

So, If you’re relatively new to engine work and have ever pulled, or need to pull, the distributor or damper from your engine, finding Top Dead Center TDC) on piston #1 becomes an instant priority. Finding Top Dead Center (TDC) is an essential skill for assembling and tuning engines, and is even a necessity for performing some engine diagnostic tests, such as a cylinder leakdown test.

Top Dead Center (TDC)
Top Dead Center (TDC)

What Is Top Dead Center (TDC)

Simply put, Top Dead Center (TDC) is the position of an engine’s piston when it is at the very top of its stroke.

Depending on weather you need to be on the compression stroke or not there are many methods that you can use:

  • Compression tester
  • Your Finger
  • Piston stop tool
  • Plastic straw
  • TDC whistle
  • Vacuum pressure gauge
  • A balloon
  • Timing marks
  • Fingers from a latex glove
  • Coat hanger
  • Welding rod

The list goes on and on.


Depending on what you are trying accomplish some methods are not as accurate as others.

STEP ONE:

The first thing you would want to do is remove all the spark plugs. The reason for this is you are going to be turning the engine over by hand. Well most likely with a wrench on the harmonic balancer nut. This will make it a lot easier to turn.


STEP TWO:

I like to use the hose from my compression tester gauge. ( Make sure to remove the schrader valve from the hose before you start ) Start by screwing the hose into the #1 cylinder.

Compression Tester Hose
Compression Tester Hose

To determine the location of the number one cylinder:
On a V type engine, one cylinder head is slightly forward of the other, toward the front of the engine. Cylinder number one is the forward most cylinder in that bank.
On an inline engine, the number one cylinder is located at the front of the engine, closest to the timing cover.


STEP THREE:

At this point there are a few ways to confirm the compression stroke:

  • Finger on Hose
    Finger on Hose

    Most old school mechanics would just use the finger method. Place one finger over over the end of the hose, then rotate the crankshaft (clockwise is best but not essential). During transition between exhaust and intake strokes when one or both valves are open, there will be no pressure.

 

  • During approach to TDC between compression and power strokes when both valves are closed, there will be pressure that will push your thumb off of the hose to allow air to escape. When air stops blowing out it is fairly near TDC on the compression stroke. Move on to STEP FOUR.

 

  • Vacuum Pressure Gauge
    Vacuum Pressure Gauge

    Use a vacuum/pressure gauge. Hook up the gauge to your hose. Again turn the engine over by hand until you see pressure. Keep going until you see the pressure change to vacuum. Move on to STEP FOUR.

 

  • Use a compression tester. Again turn the engine over by hand until you see pressure starting to build ( it won’t be much ).  Move on to STEP FOUR.

 

  • Balloon
    Balloon

    Other things I have seen are Putting a balloon or a finger from a latex glove over the hose. The balloon or finger will fill up till it gets to TDC then it will suck back in. At that point rock back and forth a little till your zeroed in between suck and blow (it will be real sensitive) and there you have it, top dead center on the compression stroke. Move on to STEP FOUR.


STEP FOUR:

Now remove the hose and place a long screwdriver, coat hanger or straw into the sparkplug hole to feel the height of the piston.

You can turn the crankshaft back and forth (by hand ) while observing motion of the ( screwdriver, coat hanger or straw ) to get a better idea of the location of the piston. When you see it has reached its highest point of travel, it is at TDC. Its that easy !

Now there are times where you have to be dead on but this covers most cases.

Piston Stop Gauge
Piston Stop Gauge

A more accurate way to locate TDC is by using a piston stop tool. Screw the tool into the #1 spark plug hole by hand, and then slowly rotate the crankshaft by hand until the piston come up against the stop.

Mark your balancer with a marker, and then slowly rotate the crankshaft by hand in the opposite direction until you come up against the stop again.

Mark your balancer again. Measure the distance between the two marks and divide by two. As a result, This is your TDC.

All things considered, setting an engine to top dead center is a relatively straightforward procedure.

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