A compression test is a simple way to ascertain the general state of the piston rings which may assist in determining any necessary modifications prior to working on the engine.
There are two fundamental forms. Both are equipped with a gauge that monitors the strain within the cylinder as the piston ascends.
Compression gauges with a rubber nipple that you insert into the spark plug hole and then crank the engine over with the starter are the quickest to use. The primary drawback of this gauge is that it requires two individuals to perform the measurement. While one individual secures the gauge, the other cranks the motor. The other drawback of this gauge is that it would not operate on the majority of current engines with heavily recessed spark plugs in the valve cover or cylinder head.
Compression gauges with a hose connected and a mounting on the end that screws into the spark plug opening and seals with a “O” ring are the second kind. This gauge does not require a second individual to operate and is compatible with almost every kind of engine. This is truly the only gauge to provide if you want to do extensive engine operation.
Notice TO OWNERS OF DIESEL ENGINES:
Diesel motors have a far higher compression ratio than gasoline engines, necessitating the use of a special gauge capable of reading the increased load. If you attempt to use a “standard” scale on a diesel engine, the gauge would almost certainly be ruined.
Utilization of a compression tester
Utilizing a Best Compression Tester is fairly straightforward, but some procedures must be followed to obtain correct readings.
It is preferable to do the tests with the engine warmed up, although this is not always practical, and cold is acceptable as well.
1. Disconnect all spark plugs from the motor. 2. Disable the ignition to avoid causing harm to the ignition mechanism or electrocution. 3. Uninstall the fuel pump while the engine is fuel pumped by cutting the fuel pump relay, disconnecting the pump connector, or some such method that is convenient. 4. Uninstall the fuel pump if the engine is equipped with a carburetor and an automatic fuel pump. If the engine is equipped with a mechanical fuel pump, you should have no difficulty keeping it linked. 5. Fully open the throttle and close it. This enables the engine to take in unrestricted air and accurately measure compression. 6. Begin with cylinder number one and screw the compression tester into the spark plug opening. If you’re using a hand-held model, you’ll need to insert it into the spark plug hole as your assistant cranks the motor. 7. Crank the engine at least four to five revolutions with the starter or before the gauge does not rise much further with each revolution. 8. Take notes on the reading. 9. Remove the tester from the chamber and depressurize it using the tester’s escape valve. 10. Repeat the exercise on all cylinders and document the findings.
Analyzing the outcome of the tests
Compression readings can differ between engines. You should match the readings to the plant specs if you have them. A new engine with a higher compression ratio or a race engine would provide much higher readings than a 1940’s tractor engine.
The critical point is that both of the readings should be reasonably similar to one another. If one cylinder is significantly lower than the others, there is almost always an issue with the piston rings, valves, or other components.
If one cylinder is lower than the others, you should detach the tester, squirt a few squirts of engine oil into the spark plug cavity, and repeat the inspection. If the reading rises, the piston rings are almost definitely the source of the issue.