Matronics Email Lists :: View topic

Matronics Email Lists
Web Forum Interface to the Matronics Email Lists

Get Email Distribution Too! FAQ Search Memberlist Usergroups Register

Profile Log in to check your private messages Log in

Compression test

Matronics Email Lists Forum Index -> TeamGrumman-List

View previous topic :: View next topic

Author Message

GrummanDude

Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA

Posted: Mon May 03, 2010 2:14 pm Post subject: Compression test

Cliff,  

 

  Remember when we talked about doing cranking pressure tests?  I started doing cranking pressure tests as a side-by-side comparison test to the leak down test.  

  

 

  Yesterday, I was doing the tests on a Tiger as 

  (1) leak-down/cranking 

  (3) cranking/leak-down

  (2) leak-down/cranking

  (4) cranking/leak-down.

  

 

  I compared the previous years tests as I went.  When I got to the #4, the cranking pressure was about 6 psi lower.  Not a big deal.  I regularly see variations of plus or minus 4 or 5 lbs.  This one, however, seemed like it took 6 revolutions of the engine to build pressure.  (it normally takes about 4 passes to reach peak).  

  

 

  When I checked leak-down, it was 55/80.  The odd thing was, no sound of leaking air at the oil filler neck or exhaust pipe.  That much would show up somewhere.  As I was looking for a leak, I noticed a lot of air coming out of the #1 top plug hole.  A leak at the #4 intake was the only explanation.  I've never encountered a leak at the intake valve.  

  

 

  I removed the rocker cover and intake rocker arm thinking it might be a stuck lifter.  Same result.  With pressure on it, I gave the intake valve a quick 'pop' with a small rubber mallet.  Compression went right to 77/80.

  

 

  I took it apart and put it back together after cleaning the lifter and measuring the dry tappet clearance (0.048 inches).  Ran the engine, checked again, and 77/80.  

  

 

  Have you (or anyone else) seen such a thing?

  

 

  Gary

 

  

 

   [quote][b]

- The Matronics TeamGrumman-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?TeamGrumman-List

_________________
Gary
AuCountry Aviation
Home of Team Grumman

pitts_pilot(at)bellsouth.
Guest

Posted: Mon May 03, 2010 3:46 pm Post subject: Compression test

Well Gary ..... sort of.  I've had engines over the years where the 

pressures were normally close on all 4 cylinders, and all of a sudden 

there's a big change in one ....... never for the better.  However, I 

could detect hissing in the exhaust.  Pulling the prop through with air 

pressure on the cylinder might change the reading when we got back to 

firing position.  Sometimes it would take rapping the valve stem with a 

rubber mallet.  I remove the rocker arm when I do that ...... just to 

keep from an 'oops'.  Usually one of the above works.  I think some crap 

gets on the valve seat causing it to leak.  Don't know why it takes that 

opportunity to get in the way, but it seems to.  A last resort (to fix 

it) was to go run the engine again, leaning the dickens out of it. 

Borescoping the valve face was the final determinant.



Never had a fix for leaking rings though!!!



The rubber mallet thingy was told to me by an A&P/IA that mentored me 

years ago.  He was full of knowledge gained over the long years he was 

involved in aviation ..... and he never led me astray.  I miss him terribly.

Linn

teamgrumman(at)aol.com wrote:

 	  Quote: 	
		   Cliff, 

 

 Remember when we talked about doing cranking pressure tests?  I started 

 doing cranking pressure tests as a side-by-side comparison test to the 

 leak down test.  

 

 Yesterday, I was doing the tests on a Tiger as 

 (1) leak-down/cranking 

 (3) cranking/leak-down

 (2) leak-down/cranking

 (4) cranking/leak-down.

 

 I compared the previous years tests as I went.  When I got to the #4, 

 the cranking pressure was about 6 psi lower.  Not a big deal.  I 

 regularly see variations of plus or minus 4 or 5 lbs.  This one, 

 however, seemed like it took 6 revolutions of the engine to build 

 pressure.  (it normally takes about 4 passes to reach peak).  

 

 When I checked leak-down, it was 55/80.  The odd thing was, no sound of 

 leaking air at the oil filler neck or exhaust pipe.  That much would 

 show up somewhere.  As I was looking for a leak, I noticed a lot of air 

 coming out of the #1 top plug hole.  A leak at the #4 intake was the 

 only explanation.  I've never encountered a leak at the intake valve.  

 

 I removed the rocker cover and intake rocker arm thinking it might be a 

 stuck lifter.  Same result.  With pressure on it, I gave the intake 

 valve a quick 'pop' with a small rubber mallet.  Compression went right 

 to 77/80.

 

 I took it apart and put it back together after cleaning the lifter and 

 measuring the dry tappet clearance (0.048 inches).  Ran the engine, 

 checked again, and 77/80.  

 

 Have you (or anyone else) seen such a thing?

 

 Gary

 

 *

 

 

 *

- The Matronics TeamGrumman-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?TeamGrumman-List

flyv35b(at)minetfiber.com
Guest

Posted: Tue May 04, 2010 5:48 am Post subject: Compression test

I've only done a couple of cranking/leak-down  comparison tests.  But I have had valves that did not seat due to  carbon or some contamination.  But that has typically been the exhaust  valve as I recall.  What you described doing is called "staking" the valve  and has been used for years to try and get a leaking valve to seat.  Doing  it with pressure in the cylinder hopefully lets the escaping air to blow out the  contaminant.  There almost always is some leakage past the rings and you  will always hear some air leak into the case (heard at the dipstick fill tube  with the stick out or loose).  My Lynx has 2 cylinders that test 80/80 and  the other 2 at 79/80.  On the 80/80 ones you don't hear any air leak.   As you know the orifice in the compression tester is very small and even a small  leak will cause a drop on the gauge reading.  I don't know how well this  correlates with a cranking test where you are moving a lot of air and how well  that same small leak will affect the cranking pressure.  Obviously it does  some.

  

 Intake valves can also have problems with the guides  and possibly not seat squarely on the seat, resulting in a leak.  I had a  cylinder not long ago that had a loose intake guide in the head.  It was  moving up and down with the valve.  But almost all the time the problems  are with the exhaust valve and guide.

  

 I've also seen an engine that tested several cylinders  at 20/80 or even less after sitting around for 8 months that came back up to  normal after running the engine and flying it for a few hours.  Continental  says that compression test numbers that are as low as about 46/80 are acceptable  provided the valves are sealing and boroscope inspection shows no exhaust valve  burning, etc.  That also assume that oil consumption is reasonable and not  excessive.  And I've heard of an engine being run without the piston rings  installed and it supposedly developed normal power!  But this is just  hearsay evidence.  So I wonder how important the static compression test is  relative to leakage past the rings and what happens when the engine is  running.

  

 Cliff

 [quote]   ---

- The Matronics TeamGrumman-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?TeamGrumman-List

GrummanDude

Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA

Posted: Tue May 04, 2010 9:16 am Post subject: Compression test

I thought I said, maybe not, but there was no leakage at the oil filler or the exhaust pipe.  With 55/80, I expected to hear something at one or the other.  I, too, have never experienced an intake leak.   

 

  It was while searching for the hissing sound that I notice a lot of air coming out of #1 top plug (hole).  Since that would be the cylinder that should be in the filling cycle, that pointed me to the #4 intake.  

  

 

  I talked to Ken and he said he sees a lot of blue/black residue in the intakes.  He said it was possible there was some residue holding the valve just off the seat.  I explained what I did and he said there really wasn't anything else I can do.  These are new cylinders with 400 hours on them.  Hard hours, but, just 400 hours.

  

 

  I'll repeat the tests after the test flight.

 

  

 

 

  --

- The Matronics TeamGrumman-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?TeamGrumman-List

_________________
Gary
AuCountry Aviation
Home of Team Grumman

Display posts from previous:

	Matronics Email Lists Forum Index -> TeamGrumman-List	All times are GMT - 8 Hours
Page 1 of 1

Jump to:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum