since 1940

Circumferential cylinder head cracks

1. Uneven temperature distribution around cylinder. Uneven heating around the circumference causes uneven expansion so that the cylinder is expanding more on the hot side than on the cold side creating a type of stress termed "Hoop Stress". You have seen a classic example of hoop stress damage - the crack in the Liberty Bell!

For IO-470 and TSIO-470 Continental engines installed in Beechcraft see Beech Service Letter No. 64-20. See also Cessna Service Letter No. 64-32 and Continental Service Bulletin M64-20.

The August 1968 General Aviation Inspection Aids Summary page 115 discusses this problem. Number 5 cylinder on these engines had excessive localized cooling of the intake (forward) side of the cylinder. A shielding baffle covering the upper forward side of #5 cylinder head greatly improves the temperature distribution.

Lycoming cylinder head after in-flight separation of barrel from head. Notice the exhaust stains to the left. The stains show that the cylinder left tale-tail signs before failure that there was a crack. The stains usually occur about 3 fins up the head from the barrel. This in-flight failure may have been prevented by looking carefully at the cylinder fins. Another method you can use is to ping the fins with a plastic pin to hear them ring. A cracked fin or crack at the base of the fin causes the fin to give a dull sound.

 Cracks around the circumference also occur higher up on the cylinder - in the head. A good example if the Continental O-200 cylinder.

The following photographs show a familiar crack  in the Continental O-200 Cylinder

	View of an O-200 cylinder. Can you see the crack? Of course not. When inspecting cylinders, orient the cylinder so you can see the base of the fins.  While were looking, notice that the O-200 cylinder has a cold side (intake) on the left and a hot side (exhaust) on the right. They are pretty well separated from one another. If we took the cylinder head's temperature by placing a thermocouple gasket at the spark plug hole what temperature would we read? Mostly the temperature of the spark plug as the gasket is in the heat path between the plug and the head. This temperature would be hotter than the head.  The temperature at the gasket is also an average of the hot side and the intake side. CHT usefulness is limited in this application.
	Now we can see down into the base of the fins. The exhaust port is a great place to start looking for cracks. Look for a stain line. The stain can be from oil, exhaust, fuel. Notice the staining between the two fins. You can do this inspection with the cylinder on the aircraft.
	Close-up showing crack and it's starting to bleed so we know it extends clear through into the exhaust port.

 

Sound of cylinder crack

Crack is between red lines. Plucking the fins reveals the crack. What you hear first sounds are clear tones (no crack) - middle thud sounds are cracked fins - end sounds are clear tones (no crack)

How Serious Are These Circumferal Cracks?

Very serious. The cylinder is a pressure vessel that contains the forces of combustion. A crack that grows around the circumference of the vessel will accelerate until the head is weakened enough that it blows off.

Tips to prevent this from happening:

1. Avoid welded repaired and high-time cylinders

2. Inspect between the fins as described above.

3. Stay current with any manufacturer issues or bulletins on the subject

4. Make sure your baffling is kept in good order so it reduces temperature differences around the cylinder's circumference.

see  Aircraft Cylinder Barrel Temperature Requirements for additional information.

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