Piston Plug Mystery

Spotty failures continue to plague Lycoming piston plugs


The following information is derived from the FAAs Service Difficulty Reports and Aviation Maintenance Alerts. Click here to view “Airworthiness Directives.”


The FAA has issued a Special Airworthiness Information Bulletin addressing Textron Lycoming piston plug wear. The bulletin aims to help identify abnormally worn piston pin plugs and describes appropriate inspection/maintenance actions.

Lycoming engines have plugs that center the piston pin in the cylinder, which ensures that the piston is centered within the cylinder barrel. The plugs, by design, experience some wear on the plug face, which contacts the cylinder barrel. This wear is normal and part of the engine break-in process.

Recently, a number of engines have experienced an abnormal amount of wear on the piloted version of the plug. The root cause of this abnormal plug wear has not yet been determined.

Beginning with engines and cylinder head kits shipped in 1994, piston pin plugs have been experiencing an unusually high wear rate. However, the wear rate skyrocketed in engines and kits shipped since October 1997. Evidence of excessive wear was discovered as soon as 22 hours time in service. Two engine failures have been blamed on the wear.

The face of the plug in contact with the cylinder barrel may wear excessively. Normal break-in wear for this plug is considered to be 1/8 inch or less. Excessive face wear can be identified by filter inspection and oil analysis.

The FAA recommends that non-piloted plugs be used. Although Lycoming has switched over at its facility, there are numerous Lycoming cylinder kits in the field that provide both piloted and non-piloted plugs.

At the recommended oil change intervals (10/25/50 hours based on the break-in schedule), operators should inspect the filter and have a spectroscopic oil analysis performed.

Although non-piloted plugs appear to be more durable, there have been instances of excessive wear on non-piloted installations. Because the root cause of the wear problem has not yet been determined, the FAA recommends that frequent inspections be observed for non-piloted plugs as well.

Beech E-33 Bonanza
Engine Oil Loss

Ground observers noticed a smoke trail coming from the aircraft during takeoff and notified the pilot via radio. The pilot made a safe landing at the departure airport.Approximately 6 quarts of oil were expelled during the short flight. The pilot stated the engines oil pressure remained in the green during this incident. Further investigation disclosed the engines oil pressure indicating supply line was broken at the engine end. The line was made of 1/8-inch copper tubing and was flared at each end. The flare at the engine fitting was broken around the entire circumference. The line did not completely separate from the B nut which explains the supply of oil to the pressure indicator. However, oil sprayed from the fitting onto the engine muffler, causing smoke and the potential for a catastrophic fire. Part total time – 4,450 hours.

Beech A-36 Bonanza
Landing Gear Malfunction

When the pilot selected the up position on the landing gear after takeoff, the transit light illuminated and the landing gear circuit breaker opened. The pilot manually extended the landing gear and made a safe landing.The electrical contact points in the landing gears dynamic brake relay, were fused together, supplying continuous power to the landing gear motor.The aircraft was used for training and had accumulated about 15,000 landing gear cycles. Other aircraft in the operators fleet had experienced landing gear relay failure, but the contact points were not fused together.Part total time – 3,131 hours.

Beech B100 King Air
Tail Cone Structural Damage

The pilot reported the aircraft vibrated and fluttered during flight.An investigation revealed the tail cone was cracked around approximately 60 percent of its circumference about 18 inches from the aft end of the tail cone. There were several smaller cracks originating at the stringer fasteners. The stringer was broken at two locations. There was evidence of chafing wear on the broken ends of the stringer which appeared to be the starting point of the cracks.

Cessna 172R Skyhawk
Seat Lock Failure

During a scheduled inspection, the technician discovered the pilots seat would not function properly.The seat lock control assembly cable was broken. Mechanics checked the operators fleet of 15 like aircraft and found two other broken seat lock cables.Part total time – 583 hours.

Lake LA250
Nose Gear Actuator Rod Crack

During an inspection, the technician discovered the nose landing gear actuator rod was cracked.The crack was located at the bottom of the thread groove below the jamnut fitting. Similar cracks were found on actuators of other aircraft. In some of those cases, the actuator separated at the bottom of the thread groove. Also, cracks have been found below the threaded area on some actuators.This area should be examined visually by using a 10-power magnifying glass. To properly inspect the thread groove, it is necessary to remove the actuator from the aircraft, and remove the fitting and jamnut from the actuator.Part total time – 892 hours

Piper PA-28R-201 Arrow III
Broken Fuel Pump Actuator Arm

While in flight, the engine failed and the pilot made a safe landing.The technician discovered the forward engine-driven fuel pumps actuating arm was broken at the pivot point. Part total time – 11 hours.

Piper PA-46-350P Malibu Mirage
Broken Alternator Mount

While inspecting the aircraft before flight, the pilot noticed the left alternator belt displayed very little tension, and the mount ear was broken. The technician performed a more extensive inspection, and discovered the link between the alternator and the freon compressor was excessively worn. The submitter stated this area is prone to wear even when the hardware is securely tightened.Lycoming Service Bulletin 511 was issued to fix a similar problem on the alternators right side. The submitter stated, Until the hardware is redesigned to alleviate this problem, the torque of the hardware should be checked at each oil change. Part total time – 258 hours.

Piper PA-46-350P Malibu Mirage
Separated Clip

While performing an annual inspection, the technician noted the left intercooler clip was separated from its attach point.The submitter suggested the manufacturer should consider bonding the clip to the side of the intercooler with a high-temperature adhesive which would prevent the device from separating.

Slick 6320 Magnetos

This report involved a twin-engine Cessna Model 340 aircraft with Teledyne Continental TSIO-520 engines.The left engine began running very rough, the pilot made a precautionary landing, and shut down the engines on the parking ramp. Maintenance personnel could not get either engine to start. An investigation revealed the left magneto on the left engine was severely corroded internally. The right magneto on the left engine and both magnetos on the right engine were in a similar condition; however, they had not yet failed.

Teledyne Continental IO-520
Engine Case Failure

The pilot of a Beech 58 Baron reported that the right engine began running rough and made a precautionary landing. Mechanics discovered that approximately 9.5 quarts of oil were lost. The source of the oil leak was a multiple fracture in the engine case at the base of the number 4 cylinder. The technician removed and disassembled the engine, and discovered cracks running through the case stud boss area. The cracks allowed the cylinder base stud nut to loosen, and the stud migrated inward until it contacted a crankshaft throw. Then it was propelled outward, causing a star-burst shaped series of cracks. The opposite case half showed signs of hairline cracks at several of the stud bosses. Hairline cracks at these locations, which are not visible if the cylinders are installed, may indicate pending failure.Part time since overhaul – 870 hours.


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