14.2 Cylinder Compression Testing: Direct vs. Differential
Key Takeaways
- Differential compression testing under FAA AC 43.13-1B and manufacturer standards is the primary aviation method for evaluating cylinder sealing integrity, utilizing a regulated 80 psi air supply metered through a calibrated 0.040-inch restriction orifice for cylinder bores of less than 5.00 inches (0.060-inch for bores of 5.00 inches and over).
- Proper differential compression test protocol mandates warming the engine to operating temperature, safing ignition circuits, positioning the piston at Top Dead Center (TDC) on the compression stroke with both valves seated, and firmly restraining the propeller against up to 1,500+ pounds of pneumatic force.
- Acoustic diagnosis of air leakage during differential testing precisely isolates component degradation: hissing at the oil filler/breather indicates piston ring or cylinder wall wear, hissing at the induction scoop indicates an intake valve leak, hissing at the exhaust tailpipe indicates an exhaust valve leak, and bubbles/hissing at the cylinder head-barrel joint indicates a structural crack or blown head gasket.
- AC 43.13-1B paragraph 8-14 sets the FAA acceptance criterion at a loss exceeding 25 percent of the input air pressure, which at an 80 psi input is the familiar 60/80 reading, and it requires re-checking after at least 3 minutes of engine operation before the cylinder is suspected.
- Continental Service Bulletin SB03-3's calibrated master orifice is a manufacturer standard rather than an AC 43.13-1B requirement; it sets a day-specific minimum reading that is normally lower than 60/80 and must be paired with a borescope inspection of the exhaust valve and cylinder wall.
14.2 Cylinder Compression Testing: Direct vs. Differential
Quick Answer: The differential compression tester is the regulatory standard for evaluating aviation reciprocating engine cylinders under FAA AC 43.13-1B. Unlike automotive direct compression testing—which measures cranking pressure via a check valve and cannot isolate leak sources—differential compression testing supplies regulated air at 80 psi through a calibrated restriction orifice (0.040-inch diameter for cylinder bores of less than 5.00 inches; 0.060-inch for bores of 5.00 inches and over). The test is conducted with the engine warm, ignition grounded, and the piston held firmly at Top Dead Center (TDC) on the compression stroke. Up to 1,500+ pounds of pneumatic force acts on the piston face, requiring a firm, secure hold on the propeller. Leakage paths are acoustically diagnosed: hissing at the oil filler neck indicates ring/cylinder leakage; hissing at the carburetor/intake scoop indicates intake valve leakage; hissing at the exhaust tailpipe indicates exhaust valve leakage; and bubbles at the barrel-to-head joint indicate a cracked head or blown gasket. Suspect low readings warrant valve staking and borescope inspection before cylinder condemnation.
Direct vs. Differential Compression Testing: Fundamental Principles
Maintaining combustion pressure within the cylinder barrel is critical to engine power production, fuel efficiency, and flight safety. Technicians evaluate internal cylinder health using two distinct testing methods:
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| DIRECT VS. DIFFERENTIAL COMPRESSION COMPARISON |
| |
| FEATURE | DIRECT CRANKING TEST | DIFFERENTIAL COMPRESSION TEST |
| --------------------- | ------------------------------ | ----------------------------- |
| Test Operating State | Dynamic (Engine motored) | Static (Held at TDC) |
| Measurement Type | Peak trapped pressure (psi) | Pressure drop across orifice |
| Supply Pressure | Atmospheric (ambient) | Regulated 80 psi shop air |
| Calibrated Orifice | None | 0.040" (< 5") or 0.060" (>= 5") |
| Isolates Leak Source? | NO (Cannot determine path) | YES (Acoustic localization) |
| FAA Regulatory Status | NOT accepted for certification | Standard per AC 43.13-1B |
+-----------------------------------------------------------------------------------------+
Why Direct Compression Testing Is Unsuitable in Aviation
Direct compression testers employ a pressure gauge fitted with a Schrader one-way check valve screwed into a spark plug hole while the starter cranks the engine for several revolutions.
- Uncontrollable Variables: Direct test readings fluctuate wildly based on starter cranking RPM, battery state of charge, starter motor health, ambient air temperature, engine oil viscosity, and whether the throttle was opened completely during cranking.
- Failure to Isolate Leakage: A direct gauge captures only peak trapped pressure. If a cylinder registers 90 psi instead of 130 psi, the technician has zero diagnostic capability to determine whether pressure leaked past the piston rings, the intake valve, the exhaust valve, or a cracked cylinder head.
Fluid Mechanics of the Differential Compression Tester
The differential compression tester evaluates internal sealing by measuring the rate of air leakage out of a static combustion chamber relative to the flow capacity of a precision-calibrated orifice.
DIFFERENTIAL COMPRESSION TESTER CIRCUIT
Shop Air Supply
(approx. 100 - 120 psi)
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[ Regulator ] === Set to 80 psi ===> [ Upstream Gauge ] (Reads 80 psi)
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[ Calibrated Orifice ] (0.040" Dia)
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[ Downstream Gauge ] (Reads Cylinder psi)
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[ Spark Plug Adapter ]
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[ Cylinder at TDC Comp ]
- Clean external shop air at approximately 100 to 120 psi enters the upstream regulator and is set to 80 psi, displayed on the primary (upstream) pressure gauge.
- The air flows through a precision-drilled metering orifice into the downstream chamber and through an adapter hose screwed into the cylinder's spark plug hole.
- The secondary (downstream) pressure gauge displays the stabilized pressure trapped within the cylinder.
- The Orifice Ratio: If the cylinder were hermetically sealed with zero leakage, downstream pressure would balance upstream pressure at 80 psi (an 80/80 reading). However, because piston ring end gaps permit normal blowby, air continuously escapes. If air escapes from the cylinder slower than the calibrated orifice supplies it, cylinder pressure remains high (e.g., 76/80 or 72/80). If severe leaks exist, air escapes faster than the orifice can feed it, resulting in a dramatic downstream pressure drop (e.g., 52/80 or 40/80).
Calibrated Orifice Specifications (FAA AC 43.13-1B)
Under FAA AC 43.13-1B Chapter 8, Section 1, the dimensions of the restriction orifice are strictly standardized:
- For an engine cylinder having less than a 5.00-inch bore: orifice diameter 0.040 inch, orifice length 0.250 inch, 60-degree approach angle.
- For an engine cylinder with a 5.00-inch bore and over: orifice diameter 0.060 inch, orifice length 0.250 inch, 60-degree approach angle.
- Watch the boundary: AC 43.13-1B splits the two groups at less than 5.00 inches versus 5.00 inches and over, so a bore measuring exactly 5.00 inches takes the larger 0.060-inch orifice.
- Caution: Utilizing a tester with the incorrect orifice size invalidates the test. An oversized orifice introduces excessive air into the cylinder, artificially inflating downstream pressure and masking defective valves. An undersized orifice chokes airflow, yielding falsely low compression readings.
Step-by-Step Differential Compression Test Protocol
To ensure accurate, repeatable results and protect maintenance personnel from severe injury, technicians must execute the test following a disciplined technical sequence:
1. Engine Operating Temperature
The engine must be run up and brought to normal operating cylinder head and oil temperatures immediately prior to testing. AC 43.13-1B directs that the test be performed as soon as possible after the engine is shut down, so the rings, cylinder walls, and other parts are still well lubricated.
- Rationale: Warm engine oil coats cylinder walls and ring faces, forming a representative hydrodynamic gas seal. Furthermore, aluminum cylinder heads and steel barrels expand thermally, establishing operating clearances. Testing a cold engine yields unreliably low readings because piston rings have not expanded, cylinder choke taper has not normalized, and dry cylinder walls allow excessive leakage.
2. Rigorous Ignition Safing
Before approaching the propeller:
- Place the cockpit ignition switch in the OFF position.
- Verify that magneto P-leads are physically connected and grounded.
- Place the mixture control in IDLE CUTOFF.
- Turn the fuel selector valve OFF.
- Disconnect external electrical power and turn the master switch OFF.
3. Spark Plug Removal
Remove one spark plug from each cylinder (typically all top spark plugs). Removing all top plugs accomplishes three vital tasks: it eliminates compression in all other cylinders so the crankshaft turns smoothly by hand; it prevents accidental engine starting; and it opens spark plug ports for subsequent borescope inspection.
4. Locating Top Dead Center (TDC) on the Compression Stroke
Thread the tester adapter into the spark plug boss of the cylinder being tested.
- Rotate the propeller by hand in the normal direction of engine rotation.
- Identify the compression stroke by placing a thumb or rubber whistle over the adapter tube. As the piston ascends, expanding air will force past the thumb or sound the whistle.
- Continue rotating the propeller slowly until the piston reaches Top Dead Center (TDC). At TDC, both the intake and exhaust valves are completely closed by their valve springs, and the connecting rod is aligned vertically with the crankshaft throw.
5. Propeller Restraint and Critical Crew Safety
[!CAUTION] DEADLY PROPELLER HAZARD: Applying 80 psi of shop air against a 5.00-inch diameter piston exerts approximately 1,570 pounds of force (Force = Pressure x Area = 80 psi x 3.1416 x 2.5²). If the piston is even slightly off TDC, this immense pneumatic force will drive the connecting rod downward, spinning the propeller like an uncoiled spring with bone-crushing force!
- The technician assigned to hold the propeller must stand completely clear of the blade rotational arc, maintain a firm two-handed grip on the blade near the hub, and brace their body securely.
- The propeller must be held firmly before opening the tester shutoff valve.
- Maintain clear verbal communication between the technician holding the prop and the technician operating the tester regulator.
6. Pressurization and Piston Rocking
- Slowly open the tester shutoff valve to admit shop air into the cylinder, bringing the upstream gauge to exactly 80 psi.
- Gently rock the propeller back and forth slightly across TDC (no more than 1 to 2 degrees of crankshaft rotation). This minor rocking action centralizes the piston rings in their ring lands, allowing cylinder pressure to seat the rings firmly against the cylinder wall.
- Record the stabilized pressure indicated on the downstream gauge.
Acoustic Leakage Diagnosis & Fault Isolation
When a cylinder registers low downstream pressure, the technician does not need to guess which internal component has failed. By listening to escaping air at specific engine orifices, the leak source can be identified with pinpoint accuracy:
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| ACOUSTIC LEAKAGE LOCALIZATION DIAGNOSTIC GUIDE |
| |
| LISTENING LOCATION | SUSPECT COMPONENT | ROOT CAUSE |
| ---------------------------- | -------------------- | -------------------------------- |
| Oil Filler Neck / Dipstick | Piston Rings or | Normal end gap blowby; ring wear;|
| Breather Tube | Cylinder Barrel Wall | stuck rings; scored/glazed bore |
| ---------------------------- | -------------------- | -------------------------------- |
| Carburetor Throat / Fuel | Intake Valve | Carbon on seat; burned valve face|
| Injection Air Scoop | | warped head; zero lash/clearance |
| ---------------------------- | -------------------- | -------------------------------- |
| Exhaust Pipe Tailpipe Exit | Exhaust Valve | Burned/guttered valve edge; |
| | | asymmetric seat wear; valve stem |
| ---------------------------- | -------------------- | -------------------------------- |
| Cylinder Head-to-Barrel Fins | Head-to-Barrel Joint | Blown joint; cracked head casting|
| Spark Plug Boss Threads | or Head Structure | (Catastrophic structural failure)|
+-----------------------------------------------------------------------------------------+
- Oil Filler Neck or Crankcase Breather: A steady hissing sound escaping from the oil filler tube or crankcase breather indicates air leaking past the piston compression rings into the crankcase. A slight hiss is normal due to piston ring end gaps (staggered around the piston). However, a loud roar accompanied by a low pressure reading indicates worn rings, lost ring tension, stuck rings, or severe vertical scoring on the cylinder wall.
- Carburetor Air Intake or Fuel Injection Scoop: Hissing heard through the induction air filter or carburetor throat indicates air escaping past the intake valve. The intake valve is failing to seal against its seat, caused by carbon contamination, valve face burning, a bent valve stem, or a collapsed/tight hydraulic lifter holding the valve slightly off its seat.
- Exhaust Pipe Tailpipe: A hissing or whistling sound escaping from the exhaust stack indicates air blowing past the exhaust valve. Because exhaust valves endure extreme combustion heat (1,400°F+), valve face burning, seat pitting, or thermal warpage are frequent causes of exhaust valve leakage.
- Cylinder Head Fins or Spark Plug Boss: Hissing escaping externally from between the cylinder head cooling fins, or soapy water bubbling around the barrel-to-head junction, reveals a blown head-to-barrel seal or a cracked cylinder head. On aircraft cylinders, the aluminum head is heated and screwed onto the chilled steel barrel during manufacturing. A leak at this interface represents non-repairable structural failure requiring immediate cylinder replacement.
Acceptance Criteria: the 25 Percent Rule and the Manufacturer's Master Orifice
What AC 43.13-1B Actually Says
The FAA's own acceptance criterion is a percentage of the input air pressure, not a master-orifice reading. AC 43.13-1B paragraph 8-14 states that a loss in excess of 25 percent of the input air pressure is cause to suspect the cylinder of being defective. With the regulator set to 80 psi, 25 percent of 80 is 20, so the familiar 60/80 psi figure is simply that rule expressed in gauge numbers, and it is current FAA guidance rather than a retired rule of thumb.
The AC pairs that criterion with two mandatory follow-up steps that candidates routinely forget:
- Re-check after running the engine. A low reading must be re-taken after operating the engine for at least 3 minutes, so the rings have a chance to seal with oil. A cylinder condemned on a single cold reading has not been tested to the AC.
- Attempt the correction before condemning. If leakage persists, the AC directs the technician to stake the valve with a fiber drift before removing the cylinder, then rotate the engine to reseat the valves normally before re-checking.
AC 43.13-1B PARAGRAPH 8-14 ACCEPTANCE LOGIC
Regulator set to 80 psi --> Cylinder gauge reading
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Loss = 80 - (cylinder gauge reading)
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Loss > 25% of 80 psi (i.e., reading below 60/80)
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+--> Run engine 3 minutes minimum, RE-CHECK
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+--> Still low --> Stake valve, rotate engine, RE-CHECK
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+--> Still low --> SUSPECT CYLINDER
The Manufacturer's Master Orifice Is a Separate, Narrower Standard
The master orifice tool is a manufacturer procedure, not an AC 43.13-1B requirement, and the two must not be confused on the exam. Continental Aerospace Technologies Service Bulletin SB03-3 directs that the differential tester first be checked against a calibrated master orifice, which reproduces the leakage of a known-bad cylinder under that day's shop temperature, hose length, and supply conditions. Whatever the master orifice reads becomes the minimum acceptable cylinder reading for that tester on that day, and it is normally lower than 60/80.
- Which one applies? Follow the engine manufacturer's current service document when one exists, because 14 CFR § 43.13(a) requires the use of the manufacturer's maintenance manual or other methods acceptable to the Administrator. Where the manufacturer publishes nothing, AC 43.13-1B's 25 percent criterion is the accepted method.
- Why the difference matters: a Continental cylinder reading 52/80 may be airworthy under SB03-3 if it exceeds that day's master-orifice reading and a borescope confirms a sound exhaust valve and cylinder wall, while the same reading fails the AC's flat 25 percent test. Reporting one standard's result under the other's authority is a recordkeeping error, not just a technical one.
- Neither standard stands alone. Both direct the technician to correlate a low reading with a borescope inspection of the exhaust valve face and the cylinder wall before removing the cylinder.
The "Valve Staking" Procedure
If an initial differential test yields an abnormally low reading (e.g., 48/80 psi) with heavy hissing heard at the exhaust pipe or intake throttle body, the cause is frequently a small particle of loose carbon trapped between the valve face and seat, preventing full valve seating.
VALVE STAKING PROCEDURE
Mallet Strike
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[ Fiber/Brass Drift ]
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( Rocker Arm Tip ) === Momentarily Unseats Valve ===> Carbon Crushed/Freed
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[ Valve Spring ] === Snaps Valve Closed Firmly ===> Clean Metal Contact
- Staking Sequence:
- Relieve air pressure from the cylinder.
- Remove the rocker box cover.
- Place a non-marring fiber or brass drift against the rocker arm directly over the valve stem tip.
- Strike the drift sharply but lightly with a small mallet.
- This mechanical shock momentarily unseats the valve, allowing the powerful valve springs to snap the valve closed, pulverizing or dislodging the trapped carbon flake.
- Re-pressurize the cylinder to 80 psi and observe the downstream gauge.
- If pressure rises back to normal (e.g., from 48/80 to 74/80), the low reading was caused entirely by transient carbon fouling. If pressure remains low, the valve seat is physically damaged or warped, requiring cylinder repair.
Independent Prep Note
Independent FAA AMT Powerplant prep by OpenExamPrep. Not sponsored by or affiliated with the Federal Aviation Administration (FAA). Technical data compiled from FAA-H-8083-32B, FAA AC 43.13-1B, and 14 CFR Parts 33, 43, and 65.
In accordance with FAA Advisory Circular AC 43.13-1B, what are the standardized dimensions for the calibrated restriction orifice in a differential compression tester utilized on aircraft reciprocating engine cylinders having less than a 5.00-inch bore?
While conducting a differential compression test at 80 psi on an aircraft cylinder held at TDC on the compression stroke, a loud hissing noise is clearly heard escaping from the carburetor air intake throat. What internal engine defect does this indicate?
What primary safety hazard exists when introducing regulated 80 psi shop air into an aircraft engine cylinder during a differential compression test if the piston is not precisely restrained at Top Dead Center (TDC)?
During a differential compression check on an aircraft engine cylinder, a reading of 52/80 psi is obtained with air hissing at the exhaust pipe. What approved mechanical procedure should the technician perform prior to condemning and removing the cylinder?