Heavy Metal: In Filters and Screens

Other than during initial break-in, finding contaminants in filters and screens is bad. Quantities are important. Here are some identification tricks.


What if you’ve got visible metal bits in your oil? Since source identification is critical, the first question is what kind of metal is it that you’re looking at?

There are two sources of filtration; one the suction screen, which has bigger screen holes to stop big parts from being passed to the oil pump and destroying it. In Lycomings it is removable and should be checked at each oil change and clear.

The second line of defense is the oil filter or pressure screen that filters out anything else that may be in the oil before it is pumped throughout the engine. It catches contaminants down to ~40 microns.

But the really big stuff will be stopped at the suction screen. Left alone long enough, the screen can become clogged and oil starvation result. But this is not typical and debris may come out with the oil change or sit in the bottom of the pan doing no harm, and not be discovered until who knows when—such items as a screw or part dropped down the oil fill tube may sit doing nothing the entire TBO run—or it may cause trouble.

It may even be viewable with a flex borescope through the pan drain hole. Dropping the pan is a huge decision not to be made lightly unless serious and dangerous contamination is highly likely to be in the pan from strong evidence of contamination and affecting the suction screen’s ability to do its job. The Continental suction screen is attached to the pan and not removed without dropping the pan.


A large portion of an aircraft engine is aluminum, namely the crankcase, cylinder heads, pistons, etcetera. Thus, aluminum is one of the most common metals to turn up in filters and screens.

For example, if you broke a piston ring, you’d make aluminum. If a valve guide comes loose in a cylinder, you’ll make aluminum. If the valve spring breaks and it’s rubbing on the side of the cylinder head, you’ll again be making aluminum.

Aluminum is usually readily identifiable. It will appear shiny and silvery, be non-magnetic and very hard, although aluminum from cast parts can appear dull or whitish. Generally, it will either not crush or deform when squeezed with your thumbnail, or it will snap if the bit is long and thin enough.

Obviously, with so many possible sources, it’s not enough to simply identify the metal as aluminum. Many part failures tend to distinctively shaped bits.

For example, a common failure for Lycoming engines is a piston pin plug going bad. Usually when this happens, you’ll find a bunch of very thin aluminum flakes in the filter. The flakes will have substance to them (see Lyc SI 1492 D).

They have been dubbed Ivory Snow flakes by some overhaulers. A borescope test can be helpful here in seeing any marks of damaged parts on the cylinder walls or valve area.

If you find very fine aluminum filings, you may be looking at a broken piston ring that’s chewing away at the ring land. If you’re finding aluminum filings that resemble a coarse salt, it’s usually due to a failure producing a grinding situation.

Examples of this could be a loose valve guide in the head, or a broken valve spring rubbing against the guide well. Sometimes in the pickup screens you find very coarse chunks, or nuggets of aluminum from the piston skirts. Overhaulers tell us they have seen pieces big enough to identify the exact spot it came from on the piston by laying it up against another piston and matching the pieces.

If you ever find large metal chunks, say they come out with the oil change, bring them to your engine shop for identification. They have the parts and can lay your piece up against a new one to match it.

Steel and Iron

Iron and steel are the next most common metals in your engine, and thus the next most likely to be found in the filter or screens. It’s usually pretty easy to distinguish from aluminum by simply putting a (clean) magnet near it.

Iron can a quite tricky, though, especially if your first view of it comes via an oil analysis report. It’s possible to have comparatively huge increases in iron readings on oil analysis and nothing wrong with the engine. Let the engine sit over a winter gathering rust on the cylinder walls and you can bet that the next oil analysis is going to show very high iron content.

On the other hand, finding visible bits of iron or steel in the oil filter is cause for immediate concern. You’ve got to find the source before attempting further flight.

Most mechanics feel, and rightly so, that the most likely suspect when iron bits are found in the filter is the cam or the cam followers. If you get little iron filings, dark in color, sticking to the end of your magnet, usually those are from the cam or cam followers appearing as spalled-off flakes that have a “wiped-off” appearance.

A bad cam typically shows up over a long period of time as very fine steel, almost undetectable in the filter unless you go looking for it with a magnet. Then, as the failure progresses, it shows up as flakes or even nugget-sized pieces that are almost black in color.

These nuggets are the little chunks that are coming out of the tappet face that is running on the bad cam lobe. This is followed by very thin steel (magnetic) flakes, made as the cam wears rapidly and tends to mushroom over on the sides. The little flakes break off and end up in the filter.

In the beginning stages, though, you just get these fine iron filings that most people don’t see or notice. This failure is more prominent in Lycomings.

In a Continental you are more likely to find very fine ground up steel, extremely shiny, mixed in with some bronze This is from your rocker shaft grinding on the rocker bushing. The rocker shaft, when removed, looks like someone used 50 grit sandpaper on it.

You also might find small, dull-colored bits of iron in the filter that came from a badly broken-up piston ring. As a general rule, though, such ring failures do not produce solo appearances. You’ll be finding bits of iron and aluminum because the busted ring is also beating up the ring land and taking pieces out of there.

Shiny steel bits are probably coming from someplace else. They stem from a cutting action—like something cutting on the crankshaft, or maybe an idler gear shaft. But the problem is they could be coming from anywhere. Unless the stuff has a part number on it, the only way to find out the source is to tear down and inspect.

Another material you may find is real fine ground up steel in the filter from cylinder rust. The cylinders have gotten rusty and now you’ve started the engine and you’ve scraped all the rust off. This should be a one-shot deal and should not recur.


Maybe you’re finding bits of bronze in your oil. This usually shows up in the oil filter as “goldish” flakes and may be from any of the many bronze bushings throughout the engine. Bronze shavings are not usually found. Whenever bronze is found or suspected, checking the suction screen is essential.

On Continental engines, you may find some lengthy (up to two inches long) bronze “hairs” trapped in your filter or screen. A good bet is that these are from the starter adapter spring rubbing on the gear.

At the suction screen, you may find larger, semi-circular bits of bronze. These are usually from connecting rod bushings that have worked their way out of the rod end and are now breaking off in hunks.

In Lycomings, a common source of bronze nuggets is the tach drive bushing on the back end of the camshaft. The tach’s ‘T’ drive pin is supposed to ride in a groove in the bushing. Over time, though, the groove can start to wear, especially if the tach cable looses lubrication.

You’ll start out with tiny bronze flakes that increase in size and numbers. Drop the sump and check the suction screen. If you’re getting flakes and have no other obvious sources, pull the accessory case and check the tach drive bushing.

Tin (Babbitting)

This is one metal that leads to no end of confusion out in the field, simply because most people look at it and think it’s aluminum. However, finding tin babbit material is a major league danger sign.

If you get readings of high levels of lead and tin in your oil analysis, it’s coming from the babbitt on your connecting rod bearings—the only place lead and tin are used in the engine.

But babbitt and aluminum coming out of an engine look very similar, the color being extremely similar. If the component pieces are big enough, you can do a chemical test.

When babbitt is going bad it makes flakes very similar to the piston pin flakes. Sometimes you can tell the difference by examining the flakes under a magnfying glass.

If you find a copper color on one side of the flake and the other side is silver or aluminum in color, you’re looking at a hunk of babbitt. This is the inlay of the bearing itself, and is a sign of major distress.

Another way to tell is the pinch test. Babbitt is very soft and pliable compared to aluminum. If you had a chip or flake big enough where you could try to fold it over with a couple of picks, the aluminum would be more brittle and would tend to want to break. The babbitt is soft and will bend over easily.

Tin in oil analysis is one of those things that may mean nothing or everything, depending on engine age. With a new engine you may get a high reading for a little while but then it will go away. If tin levels start rising with an engine that’s past break-in, though, you’d better watch out.

If you’ve got a Continental, maybe it’s just the rocker arm thrust washers (which are tin-plated). Or maybe not. If it’s your bearing journals starting to get wiped, catastrophic failure is not far off.

Occasionally you’ll come across oil analysis scenarios that start out with a sudden increase in tin, antimony and lead. It may stay up for two oil changes, then it falls back to nearly nothing. Then copper starts going up quickly. You’ve worn completely through the babbit and are now eating into the copper undershell of the bearing.


Chrome is rare enough to be most easily isolated. If you’re finding chrome, start thinking top end, since cylinders are the primary source. Chrome can also be quite confusing.

If you have chrome in the oil filter, your mechanic may not be able to distinguish it from aluminum. A little piece of chrome and a little piece of aluminum will appear identical. After all, both are non-magnetic, both can appear whitish in the filter. But there are some distinguishing characteristics to look for.

Chrome is always flat, equal-axial, and much more decidedly flake-like. If you suspect you’ve got chrome on your hands, you can resort to some quick home chemistry to distinguish between chrome and aluminum.

Take a bit or two of the suspect metal and drop some dilute Drano (sodium hydroxide), on them. If the pieces dissolve, they are aluminum. It’ll take about five minutes and it will start bubbling. The solution will not attack any other material.

Possible sources of chrome are piston rings (the outer edges being chrome plated), exhaust valve stems (on Continentals) or cylinder barrels (when using chromed cylinders).


Magnesium is a pretty rare find in your filter or screens. It’s much more likely to show up on oil analysis. The reason for this is where magnesium is used, and the fact that so little is used to begin with. Magnesium particles will look exactly like aluminum.

Two differences distinguish them-magnesium is flammable and magnesium will not react during the Draino test. Flammability is pretty much a moot point, since if you’ve got a piece big enough to burn you’ll probably be able to tell right off where is came from. Besides, if you light the stuff, it’s almost impossible to put out.

The Drano test can be used to tell if it’s aluminum or magnesium in your filter. The magnesium will not react with the sodium hydroxide, but aluminum will. Magnesium will react with diluted hydrochloric acid and aluminum won’t. You can cross-check the samples this way.

But what if you do find bits of magnesium in your filter? One likely suspect is the Bendix 1200 series magnetos, if your engine has them. A bad mag bearing may rub the mag case enough to abrade particles which then might find their way to the filter (see how tenuous the chain can be?).

In a Lycoming, there’s generally more magnesium used, especially in Lycoming’s helicopter engines. Also, in some Lycomings there were magnesium oil sumps. In most cases, though, the only place you’ll see magnesium is on your oil analysis report. Even then, what you’re seeing is more likely due to minor corrosion products than to any actual component failure.


There are occasions where nonmetallic contaminants may be found in the filter. The two most common are carbon deposits, which look like coffeee grounds. They are caused by excessive blowby and can be identified by pressing them on a table top with a screwdriver blade. They will crush relatively easily, but are a bad sign of ring problems if abundant.

The other contaminant is silicone sealant from careless or excessive use of the stuff, or even using the wrong sealant that may break down and get into the oil. Silicone sealant is not the magic elixir when it comes to sealing up an engine.

This will block oil coolers, oil passages and cause no end of trouble. Sometimes it can be successfully flushed out, but you need to watch the filter like a hawk and change oil very frequently until it’s gone—if it is, in fact, going to go at all.


In the real world what you’re usually confronted with is two or more different metal bits in the filter. Things rarely happen in isolation, and in your engine one part may take another (or a couple of others) with it when it goes.

The trick is to discern what metals you’re looking at and consider what goes where. Above all, don’t leap to conclusions before you’ve considered all possibilities.

For example, many people when they find aluminum in the filter assume it’s from the oil pump, but usually that’s the last place it comes from. You have to think, “Did I have good oil pressure the last time I ran the engine? Is there an oil pressure problem? No? Then there’s probably nothing wrong with the oil pump.”

You’ve got to play detective and follow the clues. “Do I have fine ground up aluminum in the filter? Could I get that from the cylinders? Yes! Then let’s start by pulling the rocker covers off and taking a look.”

Why pull the rocker covers? First, it’s cheap and easy. Second, you should make sure the valve guides are not loose. If the guides are loose you typically would not get bronze in with the aluminum because the aluminum is a softer metal than the bronze guide.

It would look almost like paste packed into the pleats of the filter. Consider a few of the following combinations of metals.

For example, a possible duo could be aluminum and some copper/bronze appearing stuff, ground up to a fine powder. If this is found in the oil filter, start thinking about the possibility of a loose rocker bushing.

If a bushing is spinning in the aluminum head, it will usually produce this kind of combination of metals, in this particular shape.

Sometimes you’ll find steel with the aluminum—a very fine aluminum with very fine steel. Sometimes the steel appears in strands like steel hair.

This combination may indicate a broken ring, with the steel powder or hair coming from the ring and the aluminum from the ring land.

You probably wouldn’t notice the steel fuzz in with the aluminum when eyeballing it in the filter, but when you run a magnet over it you would get some of those steel hairs sticking to the magnet as whiskers.

If you are getting chrome in your oil filter, it’s usually because of a broken piston ring and typically it will have steel with it, along with aluminum, since those are the components that will be struggling with one another as they all self-destruct.

So you see a great deal can be learned, and don’t hesitate to call in local help.

This article originally appeared in the November 2013 issue of Light Plane Maintenancemagazine.

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