While the leak-down tester pressurizes the cylinder, the mechanic can listen to various parts to determine where any leak may originate. You might think of a compression test like a doctor taking your pulse, and a leak-down test like a full EKG stress test. The readings are taken from the second gauge, the first gauge which you use for zeroing will be lower as well. Others may not be clamped on to the hose and allow air to escape during a test, thereby causing a false reading.

I make little grooves with a triangle jewelers file and paint 1/4 white and 2/3 a different color (red for me). On top of that, racing engines are usually modified to have higher compression anyway, so you can't rely on a factory manual for the answer. To use a leak-down tester, you remove the spark plugs as above and rotate the engine until the piston is at top dead center for the cylinder you are testing. You may not know the answer just from using a compression tester, but you will know one answer: Whether or not you need to open up the engine for a closer look. If you take the valve cover off you can "POP" the valves slightly with a wooden dowel and a small hammer.It DOESN"T mean you have fixed your valve problem by this method when you achieve good readings using this trick, merely that you have for sure found your problem.

Just as a side note dont lend it to friends I am still looking for the one that I worked for and someone borrowed! After the short block is assembled I install the head WITHOUT the cams, or the followers, or the pushrods, or without whatever operates the valves. Insert the leak-down tester hose into the spark plug hole, then connect it to a compressed air source of at least 100 psi. If the timing mark is off more than 1/2 degree a free engine (no plugs and not in gear) will spin 180degrees when 100PSI is applied to the cylinder at TDC compression.

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