How to Locate a TDC Compression Stroke

Top dead center, or TDC, is the highest point each piston reaches as it travels up and down in the cylinder when the crankshaft turns in a combustible engine. Locating TDC is important for many engine repair and maintenance procedures such as replacing the timing chain or camshaft. Unless specified, instructions for locating TDC compression stroke usually refer to cylinder number one. The compression stroke is the up stroke of the piston, so the piston is traveling up in the cylinder.

Things You'll Need

  • Combination wrench
  • Socket wrench
  • Spark plug socket
  • Plastic rod
  • Helper
  • Engage the parking brake. Disconnect the negative battery cable with a combination wrench.

  • Remove the spark plugs with a socket wrench and spark plug socket. Locate cylinder number one according to your vehicle manual.

  • Hold your finger over the spark plug hole on cylinder one while a helper rotates the crankshaft clockwise slowly using a socket wrench.

  • Insert a plastic rod into the spark plug hole when you feel air pressure on your finger. Have the helper continue turning the crankshaft very slowly until the piston pushes the plastic rod out of the hole while it is rising. When the rod stops moving out, the piston is at top dead center on the compression stroke.