Compression Tester Master Orifice Explanation

[RoundTwo]

I understand there are two types of differential testers, one with a master orifice (Continental spec) and one without. 
When looking at compression test results, I will frequently see results with a MO noted. Can someone explain the principal role the MO plays in performing the compression test, and how it is actually used while performing the test? At first I thought it was used as a sort of bench calibration but after more reading, I think that’s wrong.

Thanks!

[Guest]

The master office reading establishes the acceptable low value for that compression check on that day.

[47U]

1 hour ago, RoundTwo said:

and how it is actually used while performing the test?

Found this procedure on the Aviation Pros website related to TCM engines… 

I would add that you should periodically check the threaded fittings on your compression tester for air leaks.  

a) Make sure the Regulator Pressure Valve is backed all the way OUT.
b) Make sure the Cylinder Pressure Valve is OFF or Closed.
c) Remove the protective caps from both ends of the Master Orifice and install the Master Orifice onto the threaded end of the cylinder adapter and tighten hand tight.
d) Connect the cylinder adapter with Master Orifice attached to the female quick connection on the supply hose from the differential test unit.
e) Connect your air source to the tester at the male quick connection fitting.
f) Adjust pressure regulator so regulator pressure gauge reads 80 psi.
g) Turn the cylinder pressure valve to the ON or OPEN position.
h) If necessary, adjust the pressure regulator to maintain a reading of 80 psi on the regulator pressure gauge.
i) Record the pressure reading on the cylinder pressure gauge. This reading is the Minimum Acceptable Pressure Leakage Limit.
j) Turn the cylinder pressure valve to the CLOSED or OFF position.
k) Remove the cylinder adapter and Master Orifice from the female quick connection on the supply hose from the differential test unit.
l) Remove the Master Orifice from the cylinder adapter. 

[RoundTwo]

1 hour ago, M20Doc said:

The master office reading establishes the acceptable low value for that compression check on that day.

So if I look at a test that shows MO 38 and another test showing MO 45, do I “bookkeep” the MO 38 values up a certain amount so they are “numerically equal” to the results on an MO 45 day?

[EricJ]

FWIW, it's not just a Continental thing, it's basically a way to set a baseline for the tester, regardless of what you're testing.    I use it on Lycomings, too.   I think the only difference is that there's a Continental document that mentions it specifically.

 

[Guest]

17 minutes ago, RoundTwo said:

So if I look at a test that shows MO 38 and another test showing MO 45, do I “bookkeep” the MO 38 values up a certain amount so they are “numerically equal” to the results on an MO 45 day?

There is no adjusting of the values, you use the MO reading for each compression check.  In my experience the value at my shop elevation varies very little but always around 44/80.

Search Continental SB03-3.

[PT20J]

I wonder why Continental cylinders tend to have lower compression readings than Lycomings? Is there less choke or something?

[Fly Boomer]

1 hour ago, EricJ said:

FWIW, it's not just a Continental thing, it's basically a way to set a baseline for the tester, regardless of what you're testing.    I use it on Lycomings, too.   I think the only difference is that there's a Continental document that mentions it specifically.

 

Does that mean we can use the 2E on Lycoming:

 

 

 

 

but must use the 2EM on Continentals:

[FlyingDude]

If you were literally measuring the compression in a cylinder, you'd be bringing it to the dead bottom center, connect a manometer and turn the prop towards top dead center in the compression stroke and see what pressure you get. That'd be the pressure over 1 atmosphere. No need for a compressor. However, what you're measuring here is actually the leak from the cylinder at the TDC. therefore, the input pressure and the master orifice yield in a regulated airflow in terms of volume per seconds. Then the pressure you see on the other side indirectly shows the leak because if there was no leak, the cylinder would fill up with air at the same pressure as the input but since there are leaks, it's lower. So the output pressure is not a measure of anything per sé but rather an indirect indication of the leak.

Automotive cylinders have lower volume, therefore master orifices in automotive differential pressure testers are smaller. On really huge bore airplane engines, you use the 1.5mm diameter (60mil) MO as opposed to the 1mm (40mil) MO, hence you pump 2.25x the air (1.5^2) because you expect them to leak more.

There's a good website on kitplanes I think on how to build your own differential compression tester and the guy explains all this. I did the math and buying all the necessary hw would cost me 80$. Plus the time to carefully drill the MO. I bought one for $120$ and called it quits.

[RoundTwo]

28 minutes ago, FlyingDude said:

If you were literally measuring the compression in a cylinder, you'd be bringing it to the dead bottom center, connect a manometer and turn the prop towards top dead center in the compression stroke and see what pressure you get. That'd be the pressure over 1 atmosphere. No need for a compressor. However, what you're measuring here is actually the leak from the cylinder at the TDC. therefore, the input pressure and the master orifice yield in a regulated airflow in terms of volume per seconds. Then the pressure you see on the other side indirectly shows the leak because if there was no leak, the cylinder would fill up with air at the same pressure as the input but since there are leaks, it's lower. So the output pressure is not a measure of anything per sé but rather an indirect indication of the leak.

Automotive cylinders have lower volume, therefore master orifices in automotive differential pressure testers are smaller. On really huge bore airplane engines, you use the 1.5mm diameter (60mil) MO as opposed to the 1mm (40mil) MO, hence you pump 2.25x the air (1.5^2) because you expect them to leak more.

There's a good website on kitplanes I think on how to build your own differential compression tester and the guy explains all this. I did the math and buying all the necessary hw would cost me 80$. Plus the time to carefully drill the MO. I bought one for $120$ and called it quits.

Actually, the Master Orifice is not used for the actual test. The internal .040 or 0.060 orifice is used for the test. The Master Orifice is used as a benchmark only.

[EricJ]

2 hours ago, PT20J said:

I wonder why Continental cylinders tend to have lower compression readings than Lycomings? Is there less choke or something?

I've always thought it just means they have a greater tendency to leak.   

[FlyingDude]

1 hour ago, RoundTwo said:

Actually, the Master Orifice is not used for the actual test. The internal .040 or 0.060 orifice is used for the test. The Master Orifice is used as a benchmark only.

Thanks.  I re-read some stuff and the device works how I explained it.  The "master" orifice is used for calibration.  

[PT20J]

2 hours ago, Fly Boomer said:

Does that mean we can use the 2E on Lycoming:

 

 

 

 

but must use the 2EM on Continentals:

Yes. ATS also provides 5years of free calibration. I had mine recalibrated when I noticed I was getting 82 psi on the right gauge with the left reading 80 and the output valve shut.

[ragedracer1977]

11 hours ago, M20Doc said:

The master office reading establishes the acceptable low value for that compression check on that day.

So if you got a 38 MO reading, a cylinder at 38 psi would pass a compression check?

 The other way, if you just used a straight compression tester and didn’t use a MO would it even matter if your compressions were in the 70’s?

[Guest]

7 hours ago, ragedracer1977 said:

So if you got a 38 MO reading, a cylinder at 38 psi would pass a compression check?

 The other way, if you just used a straight compression tester and didn’t use a MO would it even matter if your compressions were in the 70’s?

Any reading above the MO value would be acceptable.  If your Continental engine has readings in the 70’s the MO value wouldn’t matter. 

[jlunseth]

16 hours ago, PT20J said:

I wonder why Continental cylinders tend to have lower compression readings than Lycomings? Is there less choke or something?

I asked the question of one of my A&Ps once. It was awhile ago so don’t quote me, and I don’t perform the tests so this is my second hand translation. I was told that the pressure test technique is different for Lyc’s than for Conti’s. The Lyc test is more of a peak pressure test and the Conti is more of a hold pressure test. I was also told that lots of mechanics don’t know the difference between the two test procedures. Maybe someone who knows can enlighten us.

Edited February 9, 2023 by jlunseth

[Guest]

Both Lycoming and Continental use the same compression testing method.  The lowest acceptable value is where the difference is.  SB03-3 (which is now included in their M-O manaul) for Continental and SI1191A for Lycoming.

[PT20J]

I was always curious about how the barrel choke was machined in and how the heads were aligned properly since they are screwed onto the barrels. I saw how Lycoming does it when I visited the factory a few months ago. The barrel bores are machined straight. The heads are heat soaked in an oven and screwed onto the barrels by a machine. When the head cools, it shrinks slightly compressing the barrel and creating the choke. The mounting holes in the barrel skirt are drilled after the head is attached ensuring correct alignment.

Skip

[Fly Boomer]

2 minutes ago, PT20J said:

I was always curious about how the barrel choke was machined in and how the heads were aligned properly since they are screwed onto the barrels. I saw how Lycoming does it when I visited the factory a few months ago. The barrel bores are machined straight. The heads are heat soaked in an oven and screwed onto the barrels by a machine. When the head cools, it shrinks slightly compressing the barrel and creating the choke. The mounting holes in the barrel skirt are drilled after the head is attached ensuring correct alignment.

Skip

Wow!  That is surprising.  I would not expect that process to create much of a choke, but it certainly circumvents the issues with trying to machine the choke.

[Guest]

It must be possible to hone choke into the cylinder walls, how do they home them to 0.010” oversized?

[Fly Boomer]

44 minutes ago, M20Doc said:

It must be possible to hone choke into the cylinder walls, how do they home them to 0.010” oversized?

I would assume that once the heads are on the cylinder, they are never coming off, so probably oversize cylinders are bored or honed without choke?

[Pinecone]

When you hone a cylinder, at least the way it is done with auto engines, you get the same shape as you started with, you remove metal evenly.

If you bore one, it would be hard to bore in the choke

[RoundTwo]

9 hours ago, Pinecone said:

When you hone a cylinder, at least the way it is done with auto engines, you get the same shape as you started with, you remove metal evenly.

If you bore one, it would be hard to bore in the choke

Aircraft cylinders are considered “blind cylinders” because the hone can’t penetrate all the way through as it does when honing a block. Special honing techniques are required to hone blind cylinders in order to prevent tapering the bore during the process.