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  #11  
Old 06-29-2020, 01:02 PM
Robert Sailor Robert Sailor is offline
 
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Location: Nanaimo BC Canada
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Differential leak down tests should be done hot. Bring the prop to TDC and hang on to the prop with two hands while you add air. You can move the prop a little either side of TDC and often you'll see the increase in air pressure, sometimes that's all it takes to seat a valve better.
Listen carefully for the air leaking out. If the noise is coming from your exhaust pipe, that's valves, if it's coming from the oil add location, that's rings. The compression is not as important as the difference between cylinders as the closer they are the better. Keep one thing in mind, it's not uncommon for these pressures to change so don't read too much into a low jug..it might be low for that moment in time only and a few months later back to normal.
These tests have been the gold standard for many years but these days the bore scope is a much better test as long as you have a very experienced operator who knows exactly what to look for..doesnt hurt to do both
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  #12  
Old 06-29-2020, 09:57 PM
OKAV8r OKAV8r is offline
 
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Location: Edmond, OK
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Cold compression higher than hot? Using straight weight oil? Oil control rings are worn.
Verify by reading your sparkplugs.
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  #13  
Old 06-30-2020, 05:54 AM
kkmarshall kkmarshall is offline
 
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There is some misinformation at times in this thread. Rocking the prop at TDC on the compression stroke does NOT move the valves at all to affect their seating. That only happens at TDC of the exhaust stroke. Also,the oil control ring does not appreciably add or subtract from compression readings,just the top two rings.
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  #14  
Old 06-30-2020, 07:54 AM
Tom023 Tom023 is online now
 
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Since there is a leak path for the pressurized air through the ring end gaps, will there be a very tiny amount of air escaping into the crankcase that is slightly audible in the oil filler tube?
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  #15  
Old 06-30-2020, 08:46 AM
lr172 lr172 is online now
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by Tom023 View Post
Since there is a leak path for the pressurized air through the ring end gaps, will there be a very tiny amount of air escaping into the crankcase that is slightly audible in the oil filler tube?
Yes. the leakage is through the ring gaps and sometimes between the rings/land interface. You always have a decent amount of air leakage past the rings during a leak down test. This is why they have a standardized air orifice size; To keep everything relative to the leakage expected on an optimal engine.

The rocking at TDC is to help the air pressure to push the rings fully against the lands and mainimize that leakage during the test, which would not occur when the piston is moving, as in normal operation.

Larry
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Last edited by lr172 : 06-30-2020 at 08:55 AM.
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  #16  
Old 06-30-2020, 08:52 AM
lr172 lr172 is online now
 
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Quote:
Originally Posted by OKAV8r View Post
Cold compression higher than hot? Using straight weight oil? Oil control rings are worn.
Verify by reading your sparkplugs.
Oil rings do not affect compression readings, cold hot or otherwise.
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