Compression

[thomas1142]

I have a 1997 M20J and during last years annual all the cylinder compression were in the high seventy. During this years annual three are in the high sixties and one in low sevenths.1600hrs on engine.

What could have caused this? 

[FloridaMan]

Could be the way the IA is doing the compression test. Is it the same mechanic?

[thomas1142]

Not the same guy. He took all the spark plugs out and then check the pressure via the top plugs.

[thomas1142]

Correction he only took the top plugs out, not the bottom.

[FloridaMan]

Paging @cujet

[ArtVandelay]

Did you just fly the engine or was it sitting a while, allowing it to cool before the compression test?


Tom

[Andy95W]

Lots of things could have caused this, the least likely is that your cylinders are going bad.  Cylinders rarely go bad all 4 at once; since yours are so comparable to each other, this just isn't likely.

Some possible causes, in order of likelihood, IMO:

- A&P technique (top vs. botttom spark plug holes make no difference, BTW)

- cold vs. hot (cold cylinders often register lower than hot)

- compression tester differences (or not calibrated)

The bottom line is that if your engine seems to be making the same power that it was before, there is nothing to worry about.  The limits for a compression test is 75% of pressure applied (60/80psi) and all cylinders within 10% of each other.

(FYI- I've been an A&P for more than 20 years and done a lot of compression checks).

[PT20J]

Compression checks are notoriously unreliable and a single check doesn't mean much. You want to look for trends and sudden changes in a cylinder. When they all  go the same way at once there has to be a common element - either equipment or operator. You can fly it for a few hours and then do another check with the engine warm if you are worried about it.

[Guest]

Lycoming S/I 1191A covers compression testing.

https://www.lycoming.com/sites/default/files/Cylinder Compression.pdf

Continental SB 03-3 covers Continental engines.

http://continentalmotors.aero/xpublications/xservice bulletins/pdfs/sb03-3/

Clarence

[thomas1142]

The check was done about three hours after shut down. I’ll have him do another once the annual is done and we take it around the pattern a couple of times. Thanks for putting my mind at ease, at least somewhat. 

[kpaul]

13 hours ago, thomas1142 said:

Not the same guy. He took all the spark plugs out and then check the pressure via the top plugs.

Where did you end up taking your plane for annual?

[thomas1142]

St, Clair County airport in Pell City Alabama. Ricky Gilmore, he worked at an MSC for more than thirty years. Oldguy, on MS, told me about him. There are three Monies on the field and another three within 50 miles that he takes care of. I have been doing just about everything, with his guidance/supervision. Great experience. He also was not concerned about the low compression numbers, reiterated some of the stuff that has already been said. So far so good, if I get the baffels go material that I ordered yesterday from Spruce, I should be done tomorrow. 

[carusoam]

As a go / no go....type test.  You got the go!

But, if you are trying to sell the plane this week... you want to run the test warm with the oil filling in all the tight spaces.... this will give the highest numbers possible. Indicating that the cylinders and rings aren’t worn out...

Repeating the test will...

• cost some money
• put your mind at ease
• prove the MS mechanics are really good, they know their stuff!

All good, report back what you find out...

PP thoughts only, not amechanic...

Best regards,

-a-

[Brandontwalker]

St, Clair County airport in Pell City Alabama. Ricky Gilmore, he worked at an MSC for more than thirty years. Oldguy, on MS, told me about him. There are three Monies on the field and another three within 50 miles that he takes care of. I have been doing just about everything, with his guidance/supervision. Great experience. He also was not concerned about the low compression numbers, reiterated some of the stuff that has already been said. So far so good, if I get the baffels go material that I ordered yesterday from Spruce, I should be done tomorrow. 

Ricky takes care of 4 Mooney’s at KEET, including mine. He’s a great guy.


Sent from my iPhone using Tapatalk

[Michel02]

      look to me    ,,,,,   the are 2 size orifice for tester  one for 5 inch diam .    piston and over big  ,,,,,    and a smaller one for under  5 inch(   smaller hole )  look to me your test was made  whit tester for small piston     like 0-200  ect           0-360  need big hole orifice tester  ,    small one lower your test  les air volume      , if you look at the tester number you will find orifice number ,  sorry for my English.  Michel

[Michel02]

ok  E2M    and E2MA     .040 orifice     piston under 5 inch  diam        tester  no E2M-100     orifice .060  for piston 5 inch and more like 0-320  0-360         Michel

[jaylw314]

18 hours ago, thomas1142 said:

The check was done about three hours after shut down. I’ll have him do another once the annual is done and we take it around the pattern a couple of times. Thanks for putting my mind at ease, at least somewhat. 

I'd say don't even bother with re-doing it.  That would just give you another data point that where all the data points are subject to more noise than signal.  The only reason to do it is that it is a required part of the annual, now that it's done forget about it until next year.

If you had any real reason to be worried about the cylinders, you'd be having them borescoped to look inside anyway.

[Guest]

6 hours ago, Michel02 said:

ok  E2M    and E2MA     .040 orifice     piston under 5 inch  diam        tester  no E2M-100     orifice .060  for piston 5 inch and more like 0-320  0-360         Michel

That information on orfice size conflicts with the Lycoming S/I posted above. 

NOTE
The orifice size of the differential compression measuring device is critical if consistent and meaningful cylinder analysis are to be obtained; the larger the orifice the less chance of detecting potential problems. Therefore, a specific orifice size that provides an acceptable leak rate has been selected for all Textron Lycoming engines; the instructions described herein are based on thisorificewhichis.040in.dia.(No.60drill)x.250inlong,withentranceangleof59/60 °

Clarence

[Michel02]

just to point  for Tomas 1142  what could  be the reason for is change in test        ,Some engine(0-360) are test    .060   and give reading   70/80  ,you change next year for instrument .040 and have a drop 65/80 if you no  about ,  it   ok  .      Test    serve to find  air escape , identify the leak ,intake valve ,exhaust valve """"by muffler , ring by air in  oil pan ,  , source of leaks ..      and boroscope to finish  .      Many   AP  use .060  on 5 inch  piston  and more , but  you   right about 1998    Lycoming bulletin ,      Michel

[Michel02]

Ps      larger orifice , make more air escape, more sound,  easy to hear for old men like me , 70 Michel

[cujet]

As mentioned above, perform the compression test on a hot engine. 

Consider purchasing your own $99 compression testing device with a master orifice such as the ATS brand 2EM, (master orifice to determine leakage rate prior to use, a Continental requirement but always useful) and a 0.040 orifice. Insist on the use of your device every time , and not some random, different, beat up, abused and non calibrated unit. 

You have far less assurance of repeatable results using any other method. 

 

One quick and dirty test is to simply pull the prop through a full 2 revolutions, feeling the compression of each cylinder. A low compression cylinder will be self evident as the prop will pull through that cylinder's compression cycle with relative ease. 

[INA201]

My gut says thats since all are about ten points lower that would point to the person testing it vs the engine changing that much.  That being said, if you try you can certainly burn up the engine but my bet is on the tester being different.

[Michel02]

15 hours ago, INA201 said:

My gut says thats since all are about ten points lower that would point to the person testing it vs the engine changing that much.  That being said, if you try you can certainly burn up the engine but my bet is on the tester being different.

yes sir 10/10

[Michel02]

copper washer missing   on the tester  part  to cylinder spark  hole   ,air leak  just a little   5 to 10 psi  lost