How to Check Compression in a Big Block


Checking the compression on your big-block engine can tell you several things about its condition. A compression test can reveal leaking piston rings, leaking valves, a blown head gasket, and general overall cylinder condition. The DIY mechanic should be able to perform a compression test, including an additional procedure to test for leaking rings or valves, in about two hours. The only special tool required is a compression tester which can be purchased at any auto parts store.

Things You'll Need

  • Masking tape
  • Pen
  • Piece of cardboard
  • Ratchet and socket set
  • Compression tester
  • Piece of paper
  • Trigger-type oil can
  • Motor oil
  • Park your vehicle on a level, paved surface and set the parking brake. An automatic transmission should be placed in "Park." Place a manual transmission in "Neutral."

  • Wrap a piece of masking tape around the spark plug wire and number the wires with a pen so you can put them back in the right place. Remove the spark plug wires and all of the spark plugs.

  • Fold up a small piece of cardboard. Open the throttle lever on the carburetor to the wide-open position and wedge the piece of cardboard between the lever and its stop.

  • Screw the end of the compression tester into the first spark plug hole on either side of the engine. Crank the engine for two or three seconds. Write down the gauge reading on a piece of paper. Take a reading from the rest of the cylinders in succession on that side of the engine. Move to the other side of the engine and take readings from front to rear.

  • Compare all of the readings to each other. The lowest number should be within 10 percent of the highest number. For instance, if your highest number is 150 psi, your lowest number should be no lower than 135 psi.

  • Check a low cylinder reading to determine if it is caused by bad rings or valves by using a trigger-type oil can to shoot a couple of squirts of motor oil into the spark plug hole. Take another reading. If the compression comes up, the rings are leaking. If the compression does not change, the valves are leaking.

Tips & Warnings

  • A low reading on adjacent, or side-by-side cylinders, indicates a blown head gasket because the pressure from the cylinder being tested is leaking through the head gasket into the other cylinder. An extremely high compression reading indicates carbon build up in the combustion chamber. As a general reference, you should be looking for a compression reading of about 150 psi.

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