Determining how healthy an engine is can be tricky business. I’ve previously covered compression testing (here and here), but now I want to discuss what a leak down test is and how to perform one on a four-stroke dirt bike engine. This will be a two part series, with the first part emphasising the details of a leakdown test, and the second part explaining in detail how to correctly perform a leak down test. Let’s get started! Leak down testing is a more definitive way to assess the health of an engine compared to compression testing because a leak down test allows the mechanic to pinpoint the problematic area within the engine. Whether the valves are no longer sealing, the rings are worn, or the head gasket is leaking - a leak down test can be used to find and diagnose all these potential issues.
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.
It is possible to pinpoint where leaks are coming from once the cylinder is pressurized. When performing the test, the throttle should be fully opened and the radiator cap should be removed. Air can exit the combustion chamber at four points: past the intake valves, past the exhaust valves, past the rings, or past the head gasket. Each of these four points will exhibit a unique tell-tale sign if air is leaking.
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.
I want to address one concern you may have at this point. You have probably heard before that the piston should be at TDC when performing a leak down test. In reality there are a couple issues with this that I am going to cover. First, with the short stroke engines of today, keeping the piston precisely at TDC with 100 psi of pressure pushing down on it is next to impossible. The piston and crank will want to rock to either side of TDC. Controlling which side of TDC the piston rests is important. Depending on which side of the engine is used to lock the crankshaft in place and the direction of rotation of the engine, the nut or bolt used to lock the crank in place may either try to tighten or loosen itself from the air pressure pushing against the piston. Even though these are highly torqued fasteners, the air pressure can still occasionally loosen the nut or bolt. This creates a serious problem, because now you’ve got to figure out how to lock out the crankshaft and retorque the nut or bolt.
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 eBook, 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 build four-stroke engines.
If you have questions or thoughts, as always, I enjoy hearing them! Leave your comments below. Don't forget to follow my blog by clicking the button in the upper right hand corner of this page! Thanks everyone.
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