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Author Topic:   Leakdown Test Procedure and Equipment

normo posted 11-28-2009 11:49 AM ET (US)
I'm a back yard mechanic looking for a used 50-HP motor. Could someone explain how a leak down test is accomplished? [Tell about the] piston position, equipment needed, securing the crank so it doesn't rotate, [and so on]. Is there an affordable alternative to the $80 tool for assuring a proper seal at the plug port?
How is the test done? How are the results interpreted? How [is percentage of] blowby determined? What does it mean?
jimh posted 11-30-2009 09:41 AM ET (US)
[Separated from another discussion.]
Stevebaz posted 11-30-2009 12:30 PM ET (US)
Try This,
http://www.motorcycleproject.com/motorcycle/text/leakdown.html
seahorse posted 11-30-2009 10:34 PM ET (US)
http://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/ index.html
http://www.lafishmag.com/LeakDownTester.html
http://www.650motorcycles.com/LeakDownTester.html
number9 posted 12-01-2009 07:47 AM ET (US)
Looked for some info on the tests and testers for light aircraft since I have one in my box and never used it.
The link below to the Continental Service Bulletin provides the best information. It also explains the difference between the static and dynamic seal check, even provides a tip for correcting a leaky valve.
Most of the the other links don't mention the static check or that no static leakage is allowed. The aircraft testers vary the orifice size depending upon displacement rather than allowing different dynamic leakage rates.
Explanation of test:
http://www.swaircraftappraisals.com/MeyersForum/Maintenance/ Continental%20SBs/M8415.pdf
Explains allowable differences between cylinders. http://home.comcast.net/~r123rs/Documents/ SI%201191%20Cylinder%20Compression.pdf
Page 84 of this indicates up to 20% leakage allowable for the dynamic portion of test. http://www.rotax-aircraft-engines.com/pdf/dokus/d04157.pdf
Testers:
http://www.aircraftspruce.com/menus/to/test_cylinders.html
http://www.skygeek.com/engine-compression-testers.html

Bill
HarlemSgt posted 12-02-2009 07:42 PM ET (US)
A leak down tester measures how much air will leak past the rings. You must place the piston at TDC (top dead center). The tool will have two gauges, one for incoming pressure and one for how much pressure is in the cyl. You compare the two gauges and the difference. If you have imput pressure @ 100 psi and cyl pressure is 95 psi, that would be 5% leakage, which is very good. Anything over 15% is considered poor. It is more accurate way of testing the internal components. You can also test the valves on a 4-stroke engine and see if your problem is rings, intake or exhaust valve or blown head gasket.
modenacart posted 12-02-2009 08:22 PM ET (US)
Depending on how much money you want to spend, you can get static leak testers that have mass flowmeters and don't require a delta pressure. This is the type we use at work.

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Author	Topic: Leakdown Test Procedure and Equipment
normo	posted 11-28-2009 11:49 AM ET (US) I'm a back yard mechanic looking for a used 50-HP motor. Could someone explain how a leak down test is accomplished? [Tell about the] piston position, equipment needed, securing the crank so it doesn't rotate, [and so on]. Is there an affordable alternative to the $80 tool for assuring a proper seal at the plug port? How is the test done? How are the results interpreted? How [is percentage of] blowby determined? What does it mean?
jimh	posted 11-30-2009 09:41 AM ET (US) [Separated from another discussion.]
Stevebaz	posted 11-30-2009 12:30 PM ET (US) Try This, http://www.motorcycleproject.com/motorcycle/text/leakdown.html
seahorse	posted 11-30-2009 10:34 PM ET (US) http://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/ index.html http://www.lafishmag.com/LeakDownTester.html http://www.650motorcycles.com/LeakDownTester.html
number9	posted 12-01-2009 07:47 AM ET (US) Looked for some info on the tests and testers for light aircraft since I have one in my box and never used it. The link below to the Continental Service Bulletin provides the best information. It also explains the difference between the static and dynamic seal check, even provides a tip for correcting a leaky valve. Most of the the other links don't mention the static check or that no static leakage is allowed. The aircraft testers vary the orifice size depending upon displacement rather than allowing different dynamic leakage rates. Explanation of test: http://www.swaircraftappraisals.com/MeyersForum/Maintenance/ Continental%20SBs/M8415.pdf Explains allowable differences between cylinders. http://home.comcast.net/~r123rs/Documents/ SI%201191%20Cylinder%20Compression.pdf Page 84 of this indicates up to 20% leakage allowable for the dynamic portion of test. http://www.rotax-aircraft-engines.com/pdf/dokus/d04157.pdf Testers: http://www.aircraftspruce.com/menus/to/test_cylinders.html http://www.skygeek.com/engine-compression-testers.html Bill
HarlemSgt	posted 12-02-2009 07:42 PM ET (US) A leak down tester measures how much air will leak past the rings. You must place the piston at TDC (top dead center). The tool will have two gauges, one for incoming pressure and one for how much pressure is in the cyl. You compare the two gauges and the difference. If you have imput pressure @ 100 psi and cyl pressure is 95 psi, that would be 5% leakage, which is very good. Anything over 15% is considered poor. It is more accurate way of testing the internal components. You can also test the valves on a 4-stroke engine and see if your problem is rings, intake or exhaust valve or blown head gasket.
modenacart	posted 12-02-2009 08:22 PM ET (US) Depending on how much money you want to spend, you can get static leak testers that have mass flowmeters and don't require a delta pressure. This is the type we use at work.