Monday, February 07, 2011

Camshaft Lobe Pitting Evaluation

I’ve enjoyed reading your articles very much.  They are very informative and helpful.  I have an additional question regarding cam Spalling. I have a Lycoming IO360 A1A that we removed the cylinders due to a broken ring.  While inspecting the cam we noticed minor Spalling on one of the lobes.  All the lifters and other lobes look good. The spalling is limited to a single line that runs across the one lobe.  The attached picture is not of my actual lobe, but the area circled in RED is representive of the level of damage on my lobe.  I’m trying to make an informed decision on to either place the overhauled cylinders back on or major the engine.  Can you provide any insight on how long it will be before my cam deteriorates to the point it is no longer airworthy? If this cam will last another 400 hrs I would prefer to leave it alone for now, but if it is only going to last 50 -100 hours I would go ahead and major the engine. Any advice will be very much appreciated.
Camshaft Lobe Pitting

The engine manufacturer should be consulted as to the limitations for continued airworthiness.

Assuming the cam follower face is OK? Did you reach in with your hand and rub your fingernail across the surface to detect pitting?

I want you to look at something else on the cam lobe; do you have polishing wear across the entire lobe - end-to-end? Using your exemplar picture notice how the lob surface is shinny from edge to edge. If it has then I would replace the camshaft. The reason I say this is that lobe wear leads to a reduction in power which is an airworthy condition.

Slight pitting does not hinder the proper function of the camshaft but it will progress until it does at an indeterminate rate; start budgeting. Along the way I would use the oil filter inspection technique (originally developed by Lycoming) to detect cam lobe trauma. Do not cut-out the oil filter media but place it into a can with solvent and rinse. Use a toothbrush to lightly scrub between the pleats so any debris is removed from the pleats. Next pour the solvent through a coffee filter and allow to dry. Take a small magnet under the filter paper move all of the magnetic particles from the other debris. If you have enough small metal bits to cover the end of a stick magnet then your lobes and tappets are in a state of active disintegration and the problem needs to be corrected before further operation. Hopefully, you will have none or maybe a stray bit or two indicating that the lobes and tappets have stabilized.

I don't know if this camshaft will last 50, 150, or 400 hours. I would guess that the wear (damage) rate follows roughly an exponential curve. Long duration of little damage and then as the surface starts to pit the damage rate accelerates. Therefore, inspection intervals should be progressively shortened once the onset of pitting is detected.

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