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  #11  
Old 10-18-2019, 09:54 AM
akabud akabud is offline
 
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Amazing. Thanks Dan.
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  #12  
Old 10-18-2019, 12:27 PM
Bicyclops Bicyclops is offline
 
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Quote:
Originally Posted by DanH View Post
The incorrect reading would only be during the calibrated orifice test conducted before the actual differential pressure tests on the cylinders. This test is to determine the lowest acceptable reading for today's compression test and won't affect the cylinder test. You might be surprised how low a compression reading this test will show as being acceptable.

Ed Holyoke
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  #13  
Old 10-18-2019, 01:05 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by Bicyclops View Post
The incorrect reading would only be during the calibrated orifice test conducted before the actual differential pressure tests on the cylinders. This test is to determine the lowest acceptable reading for today's compression test and won't affect the cylinder test. You might be surprised how low a compression reading this test will show as being acceptable.

Ed Holyoke
Comparison with the "calibrated" leakage orifice is a Continental standard, not Lycoming. And no, I am not surprised.

Perhaps it would be best to read the appropriate manufacturer's guidance?

Postscript:

Let's make this easy. Per the OP's question, choose either below, the difference being the T-handle extension, which offers hand tightening. Both are 0.040" testers for flat Lycoming and Continental. Compare the cylinder reading with the test orifice reading for Continental. Ignore the test orifice for Lycoming. In either case, interpret results per Continental SB 03-3 or Lycoming SI 1191A.

https://aircraft-tool.com/shop/detail.aspx?id=2EM

https://aircraft-tool.com/shop/detail.aspx?id=2EM-KIT

These are ATS brand tools. Precise accord with Continental SB 03-3 would require purchasing the equivalent tool made by Eastern
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Last edited by DanH : 10-18-2019 at 01:30 PM.
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  #14  
Old 10-18-2019, 01:36 PM
gasman gasman is offline
 
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Quote:
Originally Posted by Bicyclops View Post
The incorrect reading would only be during the calibrated orifice test conducted before the actual differential pressure tests on the cylinders. This test is to determine the lowest acceptable reading for today's compression test and won't affect the cylinder test. You might be surprised how low a compression reading this test will show as being acceptable.

Ed Holyoke
Is this really a compression test?

A proper compression test would be accomplished with an automotive type compression gauge screwed into the spark plug hole and measures the compressed volume of air that is pulled through the intake valve based on cam lobe shape, and held by the rings and valves.

A leak down test will not show you a failed cam. But it will show you where the leak is. Both are worth doing.
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  #15  
Old 10-18-2019, 02:02 PM
Bicyclops Bicyclops is offline
 
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Originally Posted by gasman View Post
Is this really a compression test?

A proper compression test would be accomplished with an automotive type compression gauge screwed into the spark plug hole and measures the compressed volume of air that is pulled through the intake valve based on cam lobe shape, and held by the rings and valves.

A leak down test will not show you a failed cam. But does show you where the leak is. Both are worth doing.
You're correct, this is a differential pressure test and not a test of how much pressure you can build in the cylinder by piston movement. We're measuring leakage. The source of leakage could be rings or valves. The Continental Service Bulletin Dan referenced gives all the info you need to conduct and interpret the test. Just because it's the standard test specified by the engine manufacturer and by AC 43.13 doesn't mean that it is definitive or tells you every thing there is to know about the condition of the cylinder. The SB does require you to do a borescope inspection and shows pictures of poor conditions. This guidance only applies to Continental engines and, some would argue, only on a certificated airplane. It's still pretty good advice for other engines. Continental tested engines with very low compressions and found them capable of making rated power, though oil consumption was pretty high, and that's what they based the calibrated orifice data and test procedure on. It always used to be, that if you saw less than 60/80 on compression test, the cylinder was coming off. Even Lycoming no longer requires that, though they do still suggest the 75% of input pressure standard.

Ed Holyoke
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