Top Dead Center (sometimes referred to as TDC) refers to the piston at the engine's number one cylinder position when it is at its highest point in the compression stroke. You will likely need to identify this point if you want to install a new distributor in the correct orientation, connect the spark plug wires in the proper position, or undertake any other automotive project. Running this procedure can be easy if you have common hand tools. However, if you use a piston blocker specifically designed to help you find top dead center, you will get more accurate results.
Steps
Method 1 of 3: Install a Piston Blocker
Step 1. Disconnect the battery
Before beginning this project, you should use a socket or hand wrench to loosen the nut that holds the black "ground" wire over the negative battery terminal. You should pull the cable out of the terminal and insert it to the side of the battery to prevent it from coming back into contact with the terminal until you are done.
- You should disconnect the battery to avoid getting electrocuted or blowing the fuses while working.
- Also, the car cannot start if the battery is disconnected.
Step 2. Disconnect the spark plug wire from cylinder number one
You should consult your vehicle's service manual so that you can identify the spark plug that matches cylinder number one. Once you have found it, you must connect the wire at the base where it connects to the spark plug and pull it back to remove it.
- Be sure to review your service manual for information specific to your vehicle (year of manufacture, make, and model).
- When you have to remove the wires from the spark plug, you should not pull on the wire, but on the base of the spark plug.
Step 3. Remove the spark plug from cylinder number one
You should use a socket wrench with a long extension to remove the spark plugs at the number one cylinder position on the vehicle. You must turn the spark plug counterclockwise to unscrew it before removing it.
- The spark plug can be inserted into the key thanks to a rubber ring inside.
- You should check the spark plug for signs of damage. Then you should put it in a safe place.
Step 4. Install the piston lock into cylinder number one
Insert the blocker into the socket wrench that you used to remove the spark plug and place it in the same hole that you removed it from. Turn it clockwise to screw it into place.
- You must be very careful not to let any debris fall into the spark plug hole as you disconnect it to set the piston lock.
- You can find this blocker at most auto parts stores.
- You don't need to adjust the blocker too much. You just have to adjust it by hand.
Method 2 of 3: Finding Top Dead Center
Step 1. Use a wrench to slowly turn the engine
Locate the crankshaft pulley near the bottom of the engine. This circular pulley provides electrical power to accessories (eg power steering and air conditioning) through serpentine belts. In the center of the pulley, you will find a nut. You must place a wrench of the appropriate size over this nut and turn it counterclockwise to turn the engine.
- You must make sure to use the correct size wrench. Otherwise, you could damage the pulley bolt.
- You can use an acceptable amount of force to turn the engine. Big engines are even more difficult than small ones.
Step 2. Never use the starter switch to crank the engine
As you determine top dead center, you have to rotate the motor mount. You should not turn the key in the ignition switch to accomplish this, as you can cause serious engine damage if the piston hits the jammer you inserted.
- Also, you won't be able to start the engine with the battery disconnected.
- Never try to start the engine while you have parts removed.
Step 3. Mark the pulley when the piston hits the blocker
Continue turning the pulley with the wrench until the piston in cylinder number one contacts the blocker. When it stops, you should mark the location of the pulley on the surrounding harmonic damper using a marker.
- Make sure you can clearly see the mark you drew on the shock.
- You can use a felt-tip pen or permanent marker to draw the mark.
Step 4. Turn the motor in the opposite direction
Once you've drawn the first mark, you should use the socket or hand wrench on the crankshaft pulley to turn the engine clockwise until it contacts the piston lock again.
- You should draw the second mark on the shock where the pulley stops rotating because of the blocker.
- Make sure both marks are clearly visible on the shock before proceeding to the next step.
Step 5. Find the center point between the two marks
With both marks clearly visible, you should find the center point by measuring the distance between the two and dividing it by two. You can measure from one of the two lines to identify the exact center point between the two. This constitutes top dead center.
- Make sure to remove the piston lock and return the spark plug before starting the engine.
- Reconnect the battery to restore electrical power to the vehicle once you're done.
Method 3 of 3: Locate Top Dead Center Without Using a Blocker
Step 1. Remove the spark plug from cylinder number one
Instead of removing the spark plug to replace it with a blocker, you can use your thumb to get a good approximation of top dead center (which is accurate enough that you can install the distributor or spark plug wires, but not so much for camshafts can be aligned).
- Make sure to remove the spark plug using a socket wrench. Otherwise, just unscrew it and drop it in the hole.
- You must be very careful not to let anything fall into the hole while removing the spark plug.
Step 2. Place your thumb over the spark plug hole
As the engine turns, the piston rises inside the number one cylinder and you can feel the pressure rise. Insert your thumb into the hole that you removed the spark plug from so you can feel the change in pressure inside the cylinder.
Make sure your thumb is positioned properly so that it forms a seal over the hole
Step 3. Have a friend turn the camshaft using a wrench
While keeping your thumb over the hole, you should have a friend turn the motor counterclockwise using an appropriately sized wrench. Ask him to continue cranking the engine until the increased pressure pushes your thumb out of the spark plug hole as it approaches top dead center.
- You should pay attention while your friend turns the motor so that you notice when your thumb is out of the hole.
- As soon as your thumb moves, the pressure will decrease, allowing you to put it back over the hole.
Step 4. Examine the hole using a flashlight to find the TDC
Once your thumb is out of the spark plug hole, you should use a flashlight to examine it to see how close the cylinder is to the hole. As you watch, you should have your friend turn the engine very slowly to bring it as close to top dead center as possible.
- This procedure is accurate within 15 degrees, so it should not be used to install new camshafts.
- Make sure to reconnect the battery after reinstalling the spark plug.