Testing your engines compression is a great way to measure the wear of an engines rings and valves. It can also be useful in diagnosing head gasket leaks. Compression is measured in pounds per square inch or PSI. In order to get an accurate reading, you'll need to purchase or borrow a compression gage. This will cost anywhere from $5 to a couple hundred. I got mine from NAPA for $60. It has a hose with several different adapters and a removable gage with a pressure release on the other end.
Unless you have an enormous hole in the top of your piston or have bent valves, you will always feel compression with your finger. The compression gage is necessary to get an accurate reading of the engines compression in PSI. Start with a clean bike. We don't want any dirt making its way into the cylinder. Engine compression readings are most accurate with a warm engine. Remove the spark plug from the cylinder head and insert it back into the coil wire. Failure to do this may burn out electrical components. Next, screw the gage into the spark plug hole. Some gages have a rubber end instead of threads. You'll need a buddy to hold the gage if this is the case. Once the gage is connected, hold the throttle wide open and kick the engine over several times.
CAUTION! Be careful not to touch the spark plug while turning the engine over or you'll get a nasty shock!
Take a look at the reading on the compression gage and you're done. All bikes and ATV's are different but any compression reading lower than 100 PSI indicates that some engine wear has occurred. Compressing readings in this area usually indicate worn piston rings. You can test your rings by shooting a little oil into the cylinder and performing the test again. If the compression PSI reading comes up, you could have worn rings. Remember that 100PSI is only a guide line for most two and four stroke motorcycle and ATV engines.
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