The way a leak down test works is fairly simple. With the piston just shy of TDC and the valves closed (compression stroke), air pressurizes the cylinder to a defined pressure which is recorded by a pressure gauge. A second pressure gauge is used to monitor the amount of air escaping the combustion chamber. A comparison is made between the air going into the cylinder and the air escaping. The percentage of air escaping is used to determine the overall health of the engine.
The amount of air escaping can roughly be quantified to assess the condition of the engine. When race engines are built, the accuracy and precision that goes into the build results in the lowest leakage values. Most race engines will have a pressure loss of between 0% and 5%. Standard builds resulting in good running engines typically lose up to 15%. Any engine that is close to or past being ready for service will leak from 16% to 30%. These engines will most likely be running poorly, if at all. Engines beyond 30% leakage more often than not are broken and will not run. The more the engine leaks, the worse the engine’s health. Keep in mind these values are provided as a reference point and each engine can be a little different.
Intake valve leaks can be diagnosed by listening for air escaping out the carburetor or throttle body. Exhaust valve leaks can be found by listening for air escaping out the exhaust system. A leaking head gasket will result in air bubbles showing up at the radiator fill cap neck. Excessive leakage past the piston rings will result in pressurizing the crankcase and the resulting air can be traced out the crankcase or cylinder head breather hose. Air escaping past the rings may also be heard or felt passing through the access hole in the engine side cover, where the wrench has been inserted to position the crankshaft. The location at which the air exits when it leaks past the piston rings will depend if the engine has a separate crankcase cavity or a joint cavity. Usually on engines with a separate cavity, the air will be routed through a one way valve, which then directs the air up into the cylinder head and out the cylinder head breather hose.
The second issue is not so much a problem as it is a minor detail. To best simulate ring sealing conditions the rings should sit in the bottom of the ring grooves. This is how they would sit on a running engine and how the test should be performed. By ensuring the rings always sit in the bottom of their grooves, another level of repeatability is added to the test. Simply make sure when setting piston position that the piston is always traveling up just before you hold it in position. If you are working from the left side on a forward rotating engine, it will be necessary to rotate the piston past TDC then reverse direction so the rings sit in the bottom of their grooves and the flywheel nut will not try to loosen itself from the air pressure.
I hope you enjoyed my write up detailing leak down testing. Stay tuned for my next post where I’ll show you exactly how to perform a leak down test yourself. In my book, The Four Stroke Dirt Bike Engine Building Handbook, I cover leak down testing in further detail and invite you to pick up a copy if you want to learn more about how to diagnose engine troubles and four stroke engine building.
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