Analysis of Engine Contaminants
Imagine you have just purchased that dream airplane. During the purchase, you did everything right. You had the airplane checked out, including a pre-purchase inspection. The engine was examined and an oil analysis sample was taken with normal engine wear reported by the lab. The airplane is in perfect condition.
Six months later, your shop tells you that the engine is contaminated with metal, the camshaft is shot, and the engine has to be disassembled. The cost of repair is no less than $3,000 and probably more.
Unfortunately, once or twice a year, I see one of these engines. What is more unfortunate in some cases is that the engine had already failed before the purchase; the normal pre-purchase inspections had failed to detect the failure.
An engine can operate normally, pass compression tests and oil analysis, and still have damage to the camshaft lobe and follower. Even when a camshaft lobe has ground down to 1/2 its original shape, the only indication is that the engine fails to produce rated power. Engines with a fixed-pitch propeller may not produce rated RPM. But on engines with a constant-speed propeller, lack of engine power may not be noticeable. Engine roughness occurs only when the camshaft lobe has been totally wiped out.
There is one specific type of inspection that detects this type of occurrence in the early stage without disassembling the engine: a thorough oil filter examination.
Proper inspection of the oil filter is the best method of detecting camshaft or cam follower spalling and failure. Cut the oil filter open and wash and rinse the filter media carefully into a bucket of clean solvent. The washing shifts particles into the bucket. Using a clean magnet, work it around in the solvent solution. Ferrous metal particles in the solution will adhere to the magnet, allowing inspection. A visual inspection of the cam followers is warranted if the end of the magnet is covered with metal slivers.
You will not detect early signs of cam follower damage by taking the filter media out of the can and spreading it out on a table. There is not enough metal at this stage of failure to be visible in the folds of the filter. Oil pressure forces the small slivers of metal into the filter media where they are not visible. A magnet concentrates the small amount of metal, making it visible. Unless the examination of the oil filter is done correctly, you will not get a reliable analysis.