[Rutus]

When I got the RV painted last month I also wrapped up the annual, the 4th one to date. In the past, I had the local A&P do the leakdown check on the cylinders, but looking at the price for the test v. the price for the tools, I figured I ought to just buy it and do it myself. So, I took the plunge and bought the Eastman E2A-1000 kit, and did the test recently.

What I found a bit odd was that the results were 79, 79, 79 and 76/80 - and frankly, cylinders 1, 2, and 3 seemed to read almost 80/80. Cylinder 4 has always been a bit softer on compression and so the 76-77 reading there was no surprise.

I guess my question is this: isn't it expected that there is some leakage? An 80/80 test seemed, to me, to be too good to be true. This is a factory new O-360 with ~220 hrs on it now.

My main concern is the trend over time, and in that regard cylinders 1-3 have always been just a bit more hale and hearty than #4 (better cooling on 1-3 during break-in????), and from that standpoint things have not changed. But I am thinking that a "no leakage" result is too good to be true.

 

[n5lp]

...cylinders 1, 2, and 3 seemed to read almost 80/80...

Sounds too good to be true to me also. Verify that you have the correct orifice.

 

[asav8tor]

You did not mention if the engine was warm or had been run that day. If you go fly the airplane then do the test at 1 hour, 6 hours, 1 day, 1 week, 1 month, 3 months...... you will get different readings.

 

[Rutus]

Test Conditions

I ran the engine around PWT in taxi mode, to get it up to temperature. CHT was pretty close to normal, oil temps were lower than typical, but then you would need to taxi for about 6 hours here in the Seattle area this time of year to get oil temps up to normal.

The gauges were labeled as tested/certified within the last few months, so that should not be an issue. The orifice is for the "5 inches or greater" cylinder bore, which - as I understand the O-360, with a 5.125" bore - should be the right one.

Again, my view is that the relative numbers between cylinders, and the overall trend, are what counts. And, I could definitely hear air escaping as the tests were run - I believe from the vent tube, which would suggest leakage past the rings, which is what I would expect.

 

[tomwebster]

This brings up a question.

Until a few years ago, the
0.040 orifice was used for
engines up to 1000 C.I.
Now Eastern Technology says
to use the 0.060 orifice for a bore
over 5".

(I understand the reason for
the change and that it is based on the
4313 manual)

My question:
What would be the difference if I test
an O-360 (5.125 bore) with the
(old spec) 0.040 verses the
(new spec) 0.060 orifice?

Thanks for any info,
Tom

 

[az_gila]

Check your orifice....

This brings up a question.

Until a few years ago, the
0.040 orifice was used for
engines up to 1000 C.I.
Now Eastern Technology says
to use the 0.060 orifice for a bore
over 5".

(I understand the reason for
the change and that it is based on the
4313 manual)

My question:
What would be the difference if I test
an O-360 (5.125 bore) with the
(old spec) 0.040 verses the
(new spec) 0.060 orifice?

Thanks for any info,
Tom

The FAA generically says to use a 0.060 orifice for cylinders over 5 inches bore.

Since our O-320 and O-360s are 5.125 inch bore, that would seem to be it.

However, the Lycoming documents specify a 0.040 orifice, and this will give much lower reading.

The Lycoming document is SI 1191 - "Cylinder Compression" - Unfortuneately not in their on-line collection...

 

[asav8tor]

I ran the engine around PWT in taxi mode, to get it up to temperature. CHT was pretty close to normal, oil temps were lower than typical

That condition would tend to give the least leakage. The AL pistons reached full temp. The steel barrels did not. Less clearance with thick oil (cool) to boot reduces the leakage in your test. This assumes no valve leakage of course.

If you did not run the engine for a couple months, turning the prop one turn every week (not recommended), then did the check, you would get significantly different numbers. The AL pistons and steel barrel would be the same temp increasing clearance and the occasional prop movement would have been wiping oil increasing leakage. Again we are assuming no valve leakage on a low time jug.

 

[Rutus]

Hmm. Makes you wonder just how repeatable the tests are from one year to the next, if the assembly temps are that critical to the results. And, I now apparently have a test tool that is what the FAA says to uise but is not what Lycoming says......

I guess I'll just focus on differences between cylinders and annual trends, rather than the absolute values themselves.

 

[bhester]

Good info!

Here is some good info:
http://www.tcmlink.com/pdf2/SB03-3.pdf

 

[az_gila]

Should be no confusion...

Hmm. Makes you wonder just how repeatable the tests are from one year to the next, if the assembly temps are that critical to the results. And, I now apparently have a test tool that is what the FAA says to uise but is not what Lycoming says......
...

Check the first paragraph of the Continental document above - that is what the FAA says...

GENERAL
F.A.A. Advisory Circular (AC) 43.13-1. Chapter 8, Section 1, paragraph 8-1 states; ? Consult the manufacturer?s manuals, service bulletins and instruction books regarding the repair and overhaul, inspection, installation, and maintenance of aircraft engines, for that particular make, model and type of engine. This section lists acceptable inspection and repair procedures that may be used in the absence of an engine manufacturer?s maintenance information.? The following procedures are to be used as the standard for performing a cylinder differential pressure test on all TCM engines. Reference 14 CFR Part 43.13.

..sort of a polite way of saying "RTM"....

A 0.060 orifice is over 2 times the area of the 0.040 orifice, so any readings with the wrong orifice will indicate less than half the leakdown rate.

A Lycoming 0.040 reading of 74/80 will show as a 77+/80 if you use the Continenal 0.060 orifice. Since you are just reading numbers on a small round gauge, resolution will suffer, and an accurate trend will be hard to determine.

The Lycoming Service Instruction SI 1191 document is briefer than the Continental document, and calls for the test to be carried out as soon as possible after the engine has reached operating temperatures.... get out those heat resistant gloves...

 

[Mel]

You guys are putting way too much emphasis on differential compression testing.

The differential compression test is looking for specific problems and is relative at best. You are looking for broken rings, burned valves, etc. The test is done at top dead center, above where most wear occurs. What's important is that all cylinders measure close to the same.

 

[RV7AV8R]

Here is some good info:
http://www.tcmlink.com/pdf2/SB03-3.pdf

Thanks for this link. It explains for me the first time how this test is done. It looks to me like the Master orfice size makes no difference in the readings you get off your engine. It only establishes the reference number which you compare to your actual readings. Am I right here?

 

[az_gila]

No...

Thanks for this link. It explains for me the first time how this test is done. It looks to me like the Master orfice size makes no difference in the readings you get off your engine. It only establishes the reference number which you compare to your actual readings. Am I right here?

...the orifice size is directly related to the reading.

You are using the pressure drop across the calibrated orifice to measure the leakage out of your cylinders. A bigger orifice lets more air through and makes the pressure drop less.

 

[RV7AV8R]

...the orifice size is directly related to the reading.

You are using the pressure drop across the calibrated orifice to measure the leakage out of your cylinders. A bigger orifice lets more air through and makes the pressure drop less.

Got it! The orfice you guys are talking about is the restrictor orfice and not the master orfice. Thanks.

 

[Allen Barrett]

TMI

Lets not get carried away with compression checks here. It's not uncommon to see high differential compressions on a warm engine. What you are after is trending the overall health of the cylinders from check to check. A healthy cylinder can give you a mid to high 60's reading at one check and a mid to high 70's readng 2 months later. What is more important than the actual reading is where is the leakage occuring? As long as the cylinder can hold compressions in the mid 60's and up and the leakage is through the rings, keep an eye on the cylinder. Chances are good that there is some ring gap aligning occuring. However, if the leakage is past a valve there is cause for alarm. occasionally a little valve staking (a little)will knock some carbon off the valve seat and compression will come back to more normal readings. If low compressions persist, you have more serious issus to deal with.

Allen

 

[chaskuss]

SI 1191 available here

The FAA generically says to use a 0.060 orifice for cylinders over 5 inches bore.

Since our O-320 and O-360s are 5.125 inch bore, that would seem to be it.

However, the Lycoming documents specify a 0.040 orifice, and this will give much lower reading.

The Lycoming document is SI 1191 - "Cylinder Compression" - Unfortuneately not in their on-line collection...

I have SI 1191 in my collection. It's a small file (209 K) Anyone who wants a copy, contact me off list via email. You can find my email address in my profile. I'll send a batch email with the Service Instruction to those who email me. You will need to be "signed in" to be able to view my email address.
Charlie Kuss