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I Found This in my Oil

I Found This in my Oil

What is, and what isn’t, typical.

Engine oil has several functions. Its primary purpose is to reduce friction and wear of internal parts by preventing metal-to-metal contact. Oil also helps to coat the bare steel internal surfaces and prevent corrosion inside the engine.

It performs several other functions, too. First, the oil system provides some cooling for the engine. Circulating oil distributes heat by cooling the hotter sections and warming the colder sections; it eliminates part of this heat through the oil cooler. 

The oil system also cleans the engine, as it suspends various particles of metal, silica, combustion by-products and other contaminants, then deposits them in the filter or screen.

Regular interval oil changes are one of the single most important things an airplane owner can do to help ensure lengthy and trouble free service from his or her engine. In addition to being excellent preventive maintenance, the oil change also provides a golden opportunity to get a diagnosis of the internal health of the engine. 

The examination of the removed oil, the sump screen and especially the oil filter or engine screen, can reveals a great deal about any internal engine wear or malfunctions that have occurred in the hours of operation since the last oil change.

Examining used oil

The removed filter should be treated as evidence, and drained into a small clean container, such as an empty oil container that has been cut open—so that the inner contents of the filter and the removed oil can be examined for contaminants. (See photo 01 below and photo 02, page 32.) 

The inner section of the oil filter contains a metal spool around which an accordion-style paper element is installed. The element has to be cut away from the spool with a utility knife and removed with pliers in order for it to be examined. Any tools used on the element should be clean and free of any contaminants. 

Once apart, the element should be placed on a clean layer of paper towels and allowed to drain further, around eight hours or overnight. It is important to remove as much of the excess oil as possible because the leftover, wet engine oil saturating the pleats makes inspection of the contaminants more difficult and sometimes hides small areas of trapped metal particles. 

If time is an issue, the element can be folded closed, wrapped in a clean shop towel or thick paper towel and compressed in a vise. This quickly removes the excess engine oil, allowing an accurate examination of the trapped particles. 

Care needs to be taken to ensure that all the surfaces the removed element comes into contact with are clean and free of any metal shavings or debris. Also, it is a good idea to wear disposable gloves when handling anything with used motor oil on it to prevent direct contact with your skin.

As an alternative to squeezing or draining the excess oil from the filter pleats, some mechanics recommend rinsing the element in a clean container of Varsol or mineral spirits and then straining the rinsed contents through a clean white paper towel or coffee filter. This process helps to free virtually all of the trapped residue in the element, allowing it to be clearly inspected. 

Oil filter screens are cleaned in a similar fashion by rinsing them in a container of Varsol and straining the rinsed contents through a filter. (Photo 03, below, and photo 04 on page 33 show an oil screen, and the residue removed from the screen, respectively.)

The drained element, towel, coffee filter, etc. should be taken out and carefully examined in a well-lit area, preferably in sunlight. (See photo 05, page 33.)

What you might find


Carbon is usually always present in the filter element. (See photo 06, page 34.) Some carbon is normal, but large amounts of carbon in the filter/screen are usually the result of excessive blow-by past the rings. Oil that rapidly turns black soon after an oil change is also an indication of blow-by. 

Carbon flakes are black and can appear shiny, but are easily distinguishable from metal. Carbon flakes found in an oil filter/screen can effortlessly be broken apart with a small pick or even between fingernails. Metal flakes will remain intact. 


Aluminum or bronze

Aluminum shavings or flakes are occasionally found in filters and are almost always the result of wear from a piston pin plug. (See photo 07, page 34.) The piston pins themselves are free floating, and the plugs installed on each end are made of a fairly soft metal like aluminum or bronze, so that as the piston pin plugs occasionally come into contact with the cylinder wall, they wear without damaging the cylinder barrel. The piston pin plugs are designed to wear slightly, and will sometimes wear a little then stop once they are no longer contacting the cylinder wall. (For more information about piston pin plug wear on Lycoming engines, see Resources at the end of this article. —Ed.)

Suddenly-occurring or large amounts of aluminum (or bronze, if the bronze-style plugs are installed) can be a sign that the plugs have become excessively worn and need to be replaced. (See photo 08, page 35.) It can be tough to figure out which cylinder needs to be pulled because oftentimes the compression is still good since the piston pin is below the rings and the relatively soft plug metal usually doesn’t leave appreciable wear marks on the cylinder walls. 


Brass-colored nonferrous slivers are usually generated from a wearing rocker arm bushing, or possibly on some engines, a starter adapter bushing. A tiny sliver or two is generally not a big deal, but regular occurrences or large amounts would merit finding out where it is coming from and fixing it. 

Magnetic particles

The presence of magnetic particles can be detected with a mechanic’s magnet. Sweeping the magnet across all the pleats of the element or towel/filter, just above the surface, will generally pull and reveal most of the magnetic debris. (See photo 09, page 35.) 

Common contaminants consist of bits of carbon, silica or dust particles that have been ingested by getting through a leaky air filter intake, or a very small amount of nonferrous metal. Engines that are past the break-in period (having 50 hours or more) should not have any significant amount of visible metal. 

Magnetic particles, flakes, slivers, etc. are always a cause for concern. Magnetic deposits can occur due to excessive wear or a malfunction in some component, but are most commonly caused by corrosion. 

Engines that have sat without being run for extended periods of time are susceptible to corrosion formation on several of the internal steel surfaces. Once an area of rust forms, the part affected no longer has a uniform smooth surface, and subsequent use will generate metal in the oil system. Steel cylinder barrels or steel piston rings occasionally rust after a time of inactivity. 

Camshafts and their corresponding lifter bodies are especially vulnerable to damage caused by rust. (See photo 10, page 36.)

Rust formation on lifter surfaces causes small areas of pitting that then grind on the camshaft lobe and produce rapid wear. (See photo 11, page 36.) 

Once this begins to occur, there is no hope that the lifter and lobe will stop wearing; things will only get worse and worse with every hour of subsequent engine operation. (See photo 12, page 37.)

Eventually the worn cam lobe will cause a loss of power due to the affected valve not opening as far as it should. A problem with the camshaft and/or lifter body is a very expensive repair because the engine must be pulled and the case split to gain access to the camshaft. 

The importance of engine temperatures

One of the best things aircraft owners can do to ensure long engine life is to run the engine(s) often, and to be sure that the oil temperature during operation is at least 180 degrees. Engine oil systems must reach at least 180 degrees to effectively evaporate all condensation from the oil system. 

During cold winter months, some owners restrict part or all of their oil cooler openings to help temperatures reach 180. Care should be taken not to allow oil temperatures to get too high, and it is also advisable to double-check the oil temperature gauge against a known source to be sure it is accurate. Lycoming recommends placing the oil temp probe in a container of water along with a thermometer and heating the water to 180 degrees, then confirming the oil temperature reading on the gauge, or even placing a reference mark on the gauge.

Also, for owners that use an electric engine oil preheater, it is best to not leave it plugged in continuously, but only for a period of time before the engine is to be operated. Most folks plug them in the night before the plane is to be flown. Leaving a preheater plugged in continuously can cause condensation to form inside the engine, especially in humid climates.

Oil analysis

If metal (especially magnetic metal) is present in the filter, the first step in deciding what corrective action to take is to determine which part of the engine is actually producing the metal. This is where an oil filter analysis comes in handy. The filter element can be bagged and sent to an oil analysis lab to have the metal contents diagnosed to determine their source. 

Generally, the results are in the form of aerospace material specification (AMS) numbers. The lab result containing all the AMS numbered metals and quantities can then be sent (generally via email) to the engine manufacturer to determine which engine sections are represented by the applicable AMS numbers. This is a great help in determining what, if any, corrective action should be taken. 

Tiny amounts of engine wear occur over time as an inevitable result of the engine running. Over time, all moving components in the engine are subjected to wear. The metal generated from normal everyday wear and tear tends to be microscopic and occurs in small amounts, with little or no visible portions of it showing up in the oil filter. These microscopic particles can be detected and catalogued in an oil analysis report. 

Regular interval oil analysis checks are a helpful trend-monitoring tool if they are sent in regularly. The sample needs to be taken from the old oil midstream during the draining process. Oil sample reports are useful for detecting a change. Oil sample reports should be kept so that a baseline of normal wear patterns exists. Any abnormalities can be detected as a change from the normal quantities of each type of detected material present in the oil. 

As a general rule, the owner should use the same lab and operate the engine with the same type of oil. If one or the other has changed, the reports may differ a little at first, but they should become consistent again as time goes on. 

Regular oil changes, along with consistent operation of the engine—at least one hour per week—will help ensure long engine life. Avoid allowing an aircraft to sit dormant for extended periods. 

In addition to these consistent operating practices, regular oil analysis and oil filter inspections serve as two good opportunities to catch any problems that do manage to occur before they become catastrophic. 

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting any maintenance tasks.

Jacqueline Shipe soloed at age 16 and went on to get her CFII and ATP certificate. She obtained an airframe and powerplant license and has worked as a mechanic for the airlines and on a variety of General Aviation planes. Send question or comments to .


Oil analysis
AOA by ALS Labs 
Aviation Laboratories (AvLab)
Blackstone Laboratories
Lab One, Inc.
Engine operator’s manuals and service information
Continental Motors Group

Further reading

“No Appointment Necessary: DIY Oil Change” by Jacqueline Shipe
Piper Flyer, October 2016

Lycoming Service
Instruction No. 1492D
“Piston Pin Plug Wear Inspection” 

PiperFlyer.org/forum under “Magazine Extras”

Hone in the Range: Lycoming Oil Pressure

Hone in the Range: Lycoming Oil Pressure


Engine oil provides lubrication and cooling for an aircraft’s engine. Ensuring your oil pressure remains “in the green” is one of the most important things you can do for your engine’s health and longevity. Oil pressure in an engine is like blood pressure in a human. Both are important indicators of internal health, and both should be kept within proper parameters to ensure longevity.

Operating pressure

The normal oil pressure range for most Lycoming engines is between 60 to 90 pounds per square inch (psi). This range is indicated by the green arc on the oil pressure gauge. The maximum oil pressure allowed for short durations is 115 psi on most models. The maximum allowable pressure has increased over the years from 100 to 115 psi. The top red line on most oil pressure gauges is 100 psi. The lowest allowable limit for oil pressure with the engine operating at idle with hot oil is 25 psi, which is indicated by the lower red line on most oil pressure gauges.

Lycoming generally sets the operating pressures for cruise rpm on their factory-rebuilt engines to between 75 to 85 psi. Most new, rebuilt or overhauled engines require a slight adjustment of the oil pressure to finalize the setting once the engine break-in process is complete. 

Oil flow through a typical Lycoming engine

Lycoming engines use a “wet sump” oil system. This simply means that the oil sump is mounted under the engine and oil flows by means of gravity back to the sump after it has been pumped through the engine. The sump is completely open on the top so that all areas of the engine can drain back into it, and it functions like a large drain pan. “Dry sump” systems have a separate dedicated oil tank. Oil is routed to the tank once it has completed its course through the engine. 

The Lycoming oil pump is located in the accessory housing. It consists of an aluminum outer body and two steel impellers, one of which is gear-driven off the crankshaft. (See photos 01, 02 and 03 on this page.) It produces oil pressure in direct proportion to how fast the gears spin. At higher engine rpm, the pump produces more oil pressure than at low engine rpm. 

Oil is drawn up through the suction screen in the sump and through the oil pump impellers. The oil is then routed to the thermostatic bypass valve (also called a vernatherm valve). 

Oil continues to flow to the oil filter adapter on the accessory case and through the oil filter (or screen if the engine is not equipped with an oil filter). From the filter, oil is routed to the oil pressure relief valve. The oil pressure relief valve is located on the top right side of the crankcase. It relieves excessive oil pressure by opening a drain port to the sump to bypass some of the oil flow if oil pressure gets too high. 

Oil then travels to the crankshaft bearings and through predrilled passageways in the case to lubricate the internal engine parts through either pressure or splash lubrication. After completing its course, the oil drains back to the sump.

Thermostatic bypass valve

The thermostatic bypass valve is similar to a thermostat in an automotive engine cooling system. (See photo 04, far left.) The valve remains open when the oil is below 180 F, allowing the oil to bypass the passage to the oil cooler. As the oil heats up past 180 F, the vernatherm expands and eventually contacts its seat, forcing oil to pass through the oil cooler.

An engine that has abnormally high oil temperature may have a thermostatic bypass valve that is not expanding as it should with increased temperature, or that is not seating properly due to a worn seat. The valve seat wears over time and typically gets a worn groove that gets slightly worse every time it closes. If the valve gets excessively worn it allows some oil to bypass the oil cooler even when the oil is hot. (See photo 05, left.)

Some of the older bypass valves had retaining nuts that were improperly crimped during manufacture. Lycoming issued Mandatory Service Bulletin 518C that contained instructions for performing a heat treatment using a special Loctite to permanently secure the nuts in place. Valves that have had the Loctite treatment are typically inscribed with an “L” near the part number to indicate they have been repaired. 

As of August 2016, Lycoming no longer recommends this repair. Mandatory Service Bulletin 518D supersedes 518C and states that valve repair/rework is no longer allowed. Older-style valves with loose crimp nuts should be replaced.

Engines that suddenly develop an oil temperature problem may have one of the older-style valves with an improperly crimped nut that has come completely loose. Lycoming Service Instruction 1565 provides the procedure for replacement.

Oil pressure relief valve

The oil pump is a direct drive pump. This means that the pump impellers spin in direct relation to engine speed and produce oil pressure that also varies directly with engine speed. At high engine rpm, the pump produces far more pressure than the engine is designed to handle. Therefore, a pressure regulator must be incorporated into the system to keep pressures high enough at low engine speeds to protect the bearings and low enough at high engine speeds to prevent rupturing or damaging any of the engine components. 

The oil pressure relief valve (or oil pressure regulator) is located on the top right side of the crankcase; behind the number three or the number five cylinder, depending on whether it’s a four- or six-cylinder engine. (See photo 06, page 34.)

The oil pressure relief valve is very basic in its method of relieving excessive oil pressure. It consists of an aluminum housing with a strong spring, which presses against a steel ball. The spring keeps the ball seated. As oil pressure builds beyond the amount the spring is adjusted to maintain, the ball is forced off its seat by the excessive pressure. This exposes a passageway (bypass) that directs excess oil back to the sump, relieving some of the oil pressure. 

There are three types of housings. The latest style has an adjustable spring seat that can be cranked in or out as needed by means of an attached castellated nut on the end of the shaft. The older styles were adjusted by removing the housing and spring and adding or subtracting washers behind the spring to increase or decrease pressure. (See photos 07, 08 and 09 on page 34.)

The oldest style housing was short and had an adjustment of zero to three washers maximum. (See photo 10, page 36.) The longer housing allowed up to nine washers maximum to increase spring tension. (See photo 11, page 36.) Each added washer increases oil pressure approximately 5 psi. On the externally adjustable models, one turn in (clockwise) increases oil pressure approximately 5 psi. 

There are also springs of varying tensions and lengths which can be interchanged if the above adjustments do not yield the desired results. Some of the springs are color-coded to help differentiate them from one another. The most commonly used ones are the white LW-11713 springs (thick, heavy springs that are used to increase oil pressure at all settings), the 68668 (purple springs that are short and have much less tension than the others), and the 61084 non-color-coded spring that is standard equipment on most regulators. (See photo 12, page 36.)

One of the more common problems with the oil pressure regulators is with the seat that the steel ball contacts every time it closes. The seat is simply a machined aluminum section of the crankcase itself on most engine models, and over time it can become worn, especially if the ball is not contacting the seat dead in the center. If oil pressure varies excessively with engine rpm, especially at lower engine speeds, the regulator ball and seat may not be closing properly. Poor contact allows some of the oil to bypass back to the sump when it shouldn’t. (See photos 13, 14 and 15 on pages 36 and 38.)

If the cast aluminum seat has an irregular wear pattern in it, Lycoming recommends rigging up a makeshift tool out of an old ball welded to a steel rod that is thick enough to be struck with a hammer, then inserting the newly made tool squarely against the seat and giving it a couple of sharp hammer strikes to reform the seat, allowing a tighter fit between a new ball and the seat. 

The field method of repairing a worn or non-concentric seat that most mechanics employ is to use the same tool mentioned above, but instead of striking it with a hammer, they use a tiny bit of valve grinding compound on the ball to re-lap the seat. Care must be taken to prevent the compound from getting into any of the oil passageways during the process, but overall this method tends to work well to reform the seat and regain a good seal between the ball and seat. (See photo 16, page 38.)

Some of the earlier engines did have a replaceable seat insert that could be changed out and replaced if it was worn, but the most common seat is the cast aluminum type mentioned above.


Oil pressure gauge

The oil pressure gauge on many airplane models consists of a mechanically-actuated “Bourdon tube.” The Bourdon tube is a somewhat rigid, coiled, hollow tube. The tube is connected to a small oil pressure line and as oil pressure increases, the tube is stretched to a straighter, uncoiled position. The amount that it stretches varies directly with the pressure. An attached needle and gear mechanism allows the varying pressure to be read on the oil pressure gauge. These mechanisms can get dirty and stick, or the gearing mechanism can get worn and not indicate correctly. A shaky needle is often caused by a worn gear mechanism in the gauge.

Some aircraft use an oil pressure transducer or sending unit that looks similar to the oil pressure switch used for Hobbs meter installations. It is a unit that has an oil pressure line piped into one side, and electrical wires connected to the other side. Pressure is converted to an electrical signal and wires are run to a gauge that displays the oil pressure reading.

The oil pressure in most Lycoming engines is taken off the top rear accessory case. The oil pressure fitting has a reduced orifice in the outlet to the gauge. This helps prevent catastrophic oil loss if the oil pressure line or gauge begins to leak. Carbon or dirt can sometimes clog the orifice and cause an abnormally low oil pressure reading. 

Troubleshooting oil pressure problems

Most oil pressure problems can be adjusted back to normal with the regulator or traced to a malfunctioning regulator or gauge. Sometimes, the trouble is a little more difficult to repair. 

The first step in correcting abnormally high or low oil pressure should be to double-check the pressure reading with a separate pressure gauge to confirm that the oil pressure really is too high or low. Check the oil temperature, too. Low oil pressures will produce increased oil temperatures, and vice versa; overly high temperatures thin the oil and can cause a lower-than-normal oil pressure reading.

Excessive internal engine clearances due to excessive wear or a bearing failure can become so great that the output of the pump is insufficient to fully pressurize the oil system. This is typically a worst-case scenario and lower oil pressure readings occur gradually over time. 

Excessive oil pump clearance between the impellers and the housing can also cause degraded oil pressure output.

Oil viscosity plays a role in oil pressure as well. A slightly lower than normal oil pressure may be caused by using too thin an oil depending on where the plane is operated. 

A clogged suction screen or partially blocked passage between the screen and pump can also cause low oil pressure.

A higher-than-normal oil pressure reading, especially one that occurs suddenly, can be indicative of a blockage somewhere in the system, usually downstream of the pump.



Oil pressure readings should be consistently monitored so that any deviation from normal operation can be detected and remedied quickly. Consistent, normal oil pressure from startup to shutdown helps assure that an engine will run reliably for a long time.

Know your FAR/AIM and check with your mechanic before starting any work. 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Lycoming Mandatory Service Bulletin 518D

Lycoming Service Instruction No. 1565A


No Harm, No Foul: Spark Plug Maintenance

No Harm, No Foul: Spark Plug Maintenance

In the last of Piper Flyer’s series on owner-performed preventive maintenance, A&P Jacqueline Shipe looks at the servicing and replacement of aviation spark plugs.         

Aviation spark plugs need to operate while subjected to the wide temperature ranges that are possible in an aircraft engine. A spark plug with a 0.020-inch gap must be able to handle around 14,000 volts and fire reliably during its lifespan. 

Regular cleaning, gapping, and rotation of spark plugs helps ensure that the longest and most reliable service life for each plug is obtained. Regularly pulling and inspecting the plugs also helps diagnose cylinder health. 

Under Appendix A, paragraph (c) of FAR 43, the items “spark plug cleaning, gapping and replacement” are on the list of maintenance items an owner can perform on their own aircraft. 

Anatomy of a spark plug

Aviation spark plugs have a positive center electrode that is connected to the ignition lead terminal through a resistor. This center electrode assembly is housed in a ceramic insulator, which prevents the high voltage electrical current generated by the magneto from grounding out against the metal outer shell, which contains the negative electrode(s). 

These plugs are designed to withstand severe operating conditions and typically provide a long service life if they are properly maintained.

Removing the plugs

The first step in spark plug maintenance is removal of the plugs. Once the engine cowling is removed to the extent necessary so that access to all the plugs is achieved, the ignition leads can be disconnected from the spark plugs. 

The inner part of the lead needs to be held stationary as the outer nut is removed to prevent the lead from being twisted as the outer nut is turned. The leads should be gently pulled straight out and not cocked as they are removed from the plugs. 

A good deep-well six-point 7/8-inch socket is required to remove the plugs. Aviation spark plug manufacturers, including PFA supporter Tempest, makes and sells a specialized aviation spark plug socket that works well. Be sure the socket is properly seated on the plug before attempting to break it loose.   

It is important to keep track of which position each plug is removed from. This helps for diagnosing cylinder health and for plug rotation during the reinstallation. 

Homemade spark plug trays with marked receptacles for each plug are easy to make, or plugs can be laid out on a piece of marked cardboard. Tempest highly recommends using a spark plug tray to keep plugs from rolling of the workbench and to assist with proper plug rotation.

Avoid laying a plug on top of the cylinder, or any place where it could roll off and hit the floor. Dropped plugs often have cracked insulators or damaged resistors—and even if they pass a resistance check afterward, they could still have defects that can result in malfunctions and misfiring later on. Any plug that is dropped should be discarded. 

Inspecting the spark plugs

Plugs should be inspected after removal for excessive wear and general condition. 

Oil-soaked plugs

Any bottom plugs that are wet with oil aren’t a cause for concern, but if the top and the bottom plug in a cylinder are wet with oil, it can be a sign that there is either excessive piston ring wear, the ring gaps are lined up and/or the plug is malfunctioning. It wouldn’t hurt to take a compression check on the cylinder in question. 

Plugs that are misfiring will be oil-soaked simply because they aren’t firing enough to clean off any deposits; a top oil-soaked plug could simply be the result of the plug itself malfunctioning.    

Oil-fouled plugs should also be inspected for cracks and/or chips in the core nose insulator, according to John Herman at Tempest. Cracks or chips here may indicate a broken ring, which may result in cylinder damage from the broken piece of ring scoring the cylinder wall during piston cycles.  

Cylinders with insulator plug damage should be borescope inspected to be sure the cylinder has not been damaged or there is no evidence of foreign object damage or debris (FOD).

Taking note of buildup

Normally, any removed plug has a deposit residue of some sort on it and will be a little sooty just from the normal combustion process in the cylinder. 

Plugs that have virtually no deposits on them (i.e., too clean) or that have a slight reddish-brown tint on the insulator are indicative of a cylinder that is running too hot, or too lean, or both. 

If this is noticed only in one cylinder, the intake gasket and tube should be inspected for leaks. A partially clogged fuel injector on fuel-injected engines can also cause a cylinder to run lean. 

The most common deposits on spark plugs are lead and carbon. Lead buildup forms hardened balls that can eventually bridge the electrode gap and cause a plug to not fire. Carbon is jet black and sooty in appearance.

Excessive lead and carbon buildup on several plugs is a sign that an engine is being run too rich and not leaned properly. A good practice, endorsed by the folks at Tempest and others, is to lean on the ground any time the rpm is below 1,000. Always be sure to richen the mixture prior to takeoff. 

Cleaning the plugs

Once the plugs are removed and organized as to which position they came from, the next step is to clean the plugs. 

Lead deposits can be very built up and hardened, making them difficult to remove. Safety glasses, a dust mask, and chemical resistant gloves should be worn to protect eyes, lungs and hands during spark plug cleaning.

Vibration cleaning

Champion makes a machine that uses two cleaning prongs that vibrate at a high frequency to break loose the lead and pulverize it into fine particles that can be shaken out. Avoid breathing any of the dust generated from this process, as it contains lead particles.

These two-prong machines can be a little pricey, but there are handheld single-prong models that retail for a little over 20 dollars. (See Resources for a list of PFA supporters that sell the handheld spark plug vibrator cleaners. —Ed.) 

Abrasive blasting

In addition to getting the lead out of a plug, some shops clean the firing end of a spark plug in a sand or glass bead blast cabinet. 

Tempest does not recommend glass bead blasting on its plugs because some of the glass bead residue can become lodged between the center electrode and the ceramic insulator. As engine temperatures heat up, the glass beads melt into a conductive coating which can cause the plug to misfire. 

If a plug is to be blasted, Champion and Tempest both recommend using an abrasive grit that is made specifically for cleaning plugs. These companies advise lightly blasting only the tip of the plug; excessive blasting erodes the electrodes, causing premature wear. 

Some mechanics don’t recommend any kind of abrasive blasting to clean plugs due to the electrode erosion it can cause, especially on fine wire plugs. Tempest doesn’t recommend abrasive cleaning for its fine wire spark plugs for this very reason. 

Manual cleaning 

If plugs are oily, a little solvent (e.g., Varsol or other traditional mineral spirits) works well to clean the residue out of the firing end. Note: the plug should not be fully immersed in the fluid; it should only be used on the firing end. 

For stubborn lead deposits on the firing end, a good gun cleaning solvent, such as Hoppe’s #9 Bore Cleaning Solvent, is recommended by Tempest.   

A swab soaked in Methyl ethyl ketone (MEK, or butanone) works well to clean the insulator and ignition lead contact in the opposite end of the plug. Note: never use Tetra ethyl chloride on the terminal well area of the spark plug; rubbing alcohol will work just fine, according to Tempest.  

The threads on the firing end can be cleaned using a wire brush; just be sure not to clean the electrodes with the wire brush, as this can damage them. 

Gapping the plugs

Once the plugs are cleaned and dried, they are ready to gap. There are a few different styles of gapping tools, but they all essentially work the same. 


The plug is threaded into a receptacle on the tool, and a prong is pressed or screwed against the ground electrodes to move them closer to the center electrode. The recommended gap varies according to the plug and can be located on the spark plug manufacturer’s website. 

A wire-style feeler gauge is used to measure the gap between the center and outer electrodes. Care needs to be taken to not close the gap too much, as the electrodes can’t be spread back apart. 

Do not leave the feeler gauge between the electrodes when setting the gap. This can put a load on the insulator and cause it to crack.

Fine wire plugs typically don’t require re-gapping too often. Champion makes a specialized gapping tool for use on fine wire plugs if they do need to be reset. Tempest doesn’t currently have a similar tool, but is in the process of expanding its spark plug tool product line.  

Bench testing

Bench testing the plugs helps to detect and prevent reuse of a faulty plug. 

Both Tempest and Champion recommend the use of a bomb test to check a plug’s ability to fire under pressurized air. These types of testers are expensive and are usually found only in an equipped maintenance hangar, but it should cost only a few dollars to have the shop do the checks for you. 

A resistance test can be performed in addition to the bomb test, but it’s not a replacement for the bomb test. 

Tempest recommends using an electrical multimeter to check the resistance value between the ignition lead terminal in the upper part of the plug and the center electrode. The electrical resistance should not exceed 5,000 ohms on Tempest plugs. Any plugs with readings higher than 5,000 ohms should be discarded. 

Reinstalling the plugs

After the plugs are gapped, they are ready for reinstallation. 

Replacing gaskets

The copper gasket that seals the plug against the cylinder head hardens as engine temperatures heat and cools the gasket over a period of time. 

A hardened gasket does not seal as well as a soft gasket does, and can also keep the plug from properly seating against the cylinder head. Therefore, copper gaskets should be replaced before reinstalling the plugs. Spark plug manufacturers recommend that the gaskets are replaced each time the plug is removed and cleaned. 

Plugs that have thermocouple gaskets attached to CHT monitors do not require a copper washer in addition to the thermocouple washer.

Anti-seize lubricant 

Before the plug is threaded into the cylinder, a thin coat of a high-quality anti-seize material should be brushed on the threads. 

The first two threads closest to the electrodes should not be coated to prevent the conductive anti-seize compound from getting on the electrode and causing a misfire. 

Champion and Tempest make specialized anti-seize lubricants that they recommend for use on their plugs. 

A high-quality graphite- or copper-based anti-seize works well. Nickel-based anti-seize has always worked well for me. Aluminum-based anti-seize lubricants typically don’t work well because they don’t hold up under the severe heat. (Per Lycoming Service Instruction No. 1042AH "Use a copper-based anti-seize compound or engine oil on spark plug threads starting two full threads from the electrode, but DO NOT use a graphite-based compound.")

Never use a general or all-purpose graphite-based lubricant; use only lubricants that are designed for spark plugs.

Rearranging the plugs

Aviation spark plugs should not be reinstalled in the same location they were removed from. 

Ignition leads are polarity-sensitive on all magnetos (other than some of the dual magneto models); this means that the north and south poles of the spinning magnet in the magneto generate a negatively-charged spark that is sent down one lead, alternately followed by a positively-charged spark sent down the next lead. 

Plug electrodes wear in predictable ways. The plugs connected to the positively-charged leads always fire from the positive center electrode to the negative electrodes, eroding the center electrode. The plugs on the negatively charged leads always fire from the negative electrodes back to the center electrode, eroding the outer electrodes. 

Keeping the plugs rotated so the positive and ground electrodes wear evenly will double spark plug life. They should also be rotated from top to bottom, as the bottom plugs usually incur more deposit material. A rotation that a lot of mechanics use is top-to-bottom, and next in firing order. 

Torque values

Proper torque values should be used when reinstalling the plugs. Lycoming recommends 30 to 35 foot-pounds (420 inch-pounds); Continental recommends 25 to 30 foot-pounds (300 to 360-inch pounds). 

The ignition leads should be installed with care, and the leads should not be allowed to twist as the outer nut is tightened. 

Mag check and troubleshooting

An engine runup and magneto check should always be performed to ensure that all of the plugs are firing properly. A smooth runup and magneto check indicate a job well done.

A rough-running engine during the magneto check is most likely indicative of a little debris or excess anti-seize on the electrodes of one of the plugs causing it to misfire or not fire at all. Take note of which magneto the engine is running rough on. 

Once the engine is shut down and cooled off, check to see which plugs are fired by the magneto in question by visually following each ignition lead from the rough magneto all the way out to each plug. 

These plugs can then be removed, and any debris can be gotten out with a small pick. Any anti-seize lubricant that has gotten on the electrode can be cleaned off with a little degreaser.

Over the last seven months, I’ve given you some general tips and step-by-step ways you can work on your own aircraft according to what’s allowed in FAR 43, Appendix A, paragraph (c). 

This DIY series, along with guidance from a trusted mechanic, should give you a better understanding of preventive maintenance on your airplane—and might even save you a little money in the long run.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send questions or comments to .


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