Tips From Lycoming Technical Support

AMTs are usually the tip of the aircraft maintenance iceberg with huge support organizations lying beneath to ensure we have the knowledge, training, tools, and equipment necessary to keep our aircraft safe and serviceable. This support comes from all directions and in many forms: manuals, service bulletins, regulations, best practices, tips, conversations with peers, and OEM technical staff.

Aircraft Maintenance Technology magazine asked Michael D. Everhart, vice president of products & services delivery with Lycoming Engines, for a technical discussion with him and Lycoming Technical Support about engine maintenance. In turn Everhart requested Technical Support to compile a list of the most frequently asked questions and issues from customers and a list of field practices that Lycoming Engines would like to see addressed, eliminated, or changed.

Lycoming is a major U.S. manufacturer of horizontally opposed, air-cooled, four-, six-, and eight-cylinder aircraft engines. Headquartered in Williamsport, PA, the company has produced more than 325,000 piston aircraft engines and powers more than 65 percent of the new general aviation aircraft. It has produced aircraft engines for 84 years and as the industry leader, continues to improve the performance and value delivered to its customers. In November of 2010, Lycoming received the Shingo Prize for Operational Excellence, referred to as the “Nobel Prize for Manufacturing.”

Most frequently asked questions and issues from customers

Question: How do we interpret cylinder compression readings? Based on Lycoming Technical Publications SI-1191, what are the acceptable criteria for cylinder compression checks?

Lycoming: Start and run the engine until cylinder head (CHT) and oil temperature are stabilized then stop the engine and immediately conduct the compression test using an instrument with a specific orifice diameter of 0.040. There should not be more than a 10 to 15 psi difference between cylinders. If one cylinder tests below 65 psi, then troubleshoot to determine where the compression leak is occurring. Listen to the breather tubes or oil filler tube and if you hear leakage, the problem is the rings. Listen to the intake and if you can hear leakage then it would be an intake valve. Do the same for the exhaust.

Question: When does a propeller strike require an engine tear-down inspection?

Lycoming: The propeller OEM refers AMTs and owners and operators to Lycoming. Our Service Bulletin 533 is the defining document for the following situations:

With the engine running or not, if a strike occurs that results in propeller damage where a repair is required, an engine tear-down inspection is also required by Lycoming.

For example, if while being towed the prop hit the hangar door and had to be repaired, that would warrant an engine tear-down inspection as well.

With the engine running, any strike with a solid object that causes a sudden decrease in rpm and requires a repair to the propeller requires an engine tear-down inspection.

With the engine running, a strike with a yielding substance, like tall grass or water that results in a sudden drop in rpm will require an engine tear-down inspection.

If any of these events have occurred, Lycoming requires the tear-down inspection be done before the next flight using its approved inspection checklist.

Question: Why don’t the parts I ordered from the Lycoming Illustrated Parts Catalogue (IPC) match the parts in my engine?

Lycoming: Some of the parts catalogs in the field are out of date. Some of our engines have been in service for years and during that time parts evolved, suppliers changed, and pictures were updated resulting in revisions to our parts catalog. Gaskets are a great example. The parts manufacturer will change the design, profile, and composition material of the gasket so what used to be Part A gasket has been superseded and now may be Part C. This is confusing and a customer could think that they had received the wrong part. Lycoming’s practice is to incorporate and communicate part number updates via service bulletins, service instructions, and service letters as an interim amendment to the periodic IPC revisions. To avoid problems, ensure that Lycoming’s and all service providers’ IPCs are current by ordering their catalog revisions.

Field practices Lycoming Engines would like to see addressed, eliminated, or changed

Practice: During the course of regular oil and oil filter changes, the suction screen inspection is frequently overlooked.

Lycoming: Most likely it is overlooked because it can be located in different places and be a different shape based on the engine model. Inspecting it is vitally important because these screens capture large debris and prevent it from entering the oil pump. Debris in the suction screen is an indication that your engine has other problems that need further investigation.

Practice: An engine is running a high cylinder head temperature (CHT) and the normal suspects have been ruled out but not the engine baffling.

Lycoming: Service providers frequently undervalue the importance of “tight” baffling and its impact on proper engine cooling. We suggest an inspection of the sheet metal baffling with stapled seals that is attached by the airframe OEM to the engine that directs cooling air over the engine. If these baffles are missing, cracked, or have worn seals, the engine cooling is compromized. These baffles and associated seals conform to the cowling and must be intact and serviceable, or be replaced.

Practice: After an engine has experienced metal contamination, steps are frequently overlooked before returning an engine to service.

Lycoming: We occasionally see engines where service providers have overlooked checking propellers, governors, and oil coolers. We also suggest that you review the component or aircraft manufacturer’s requirements for continued airworthiness. Metal particles can migrate to these airframe components and contaminate the oil systems the next time the engine is operated. When metal contamination is known or suspected, those items must be checked and flushed as well. In particular AMTs need to send the oil cooler out to be flushed by a service provider authorized to certify that the cooler is clean and approved for return to service.

Practice: When refurbishing rocker arms, or rocker arm tips, the original geometry is not always maintained as it relates to the valve cap interface.

Lycoming: While the overhaul manual does allow the replacement of worn rocker arm bushings, no resurfacing of the rocker arm tip is permitted. Replacement of the rocker arm assembly is required whenever the tip exceeds the wear limits provided in the overhaul manual as this is an airworthiness issue. Unauthorized repairs may result in misalignment or side loading and can result in a broken valve stem.

Aircraft Maintenance Technology: Lycoming has been in business a long time and there are thousands of your engines in service. How important is it that AMTs and services providers have and are using the most current Lycoming Technical Publications?

Lycoming: It is required and necessary to have the recommended manuals to maintain standards and specifications and follow the latest service instructions and OEM best practices. It is extremely helpful to have the current technical publication(s) in front of you so when you call Lycoming Technical Support we are looking at the same information. This helps to troubleshoot and solve complicated problems quickly. Contact Lycoming ( or Aircraft Technical Publishers (ATP) for the most up-to-date overhaul manuals, operator’s manuals, illustrated parts catalogs, bulletin/letter/instruction, and special publications.

Aircraft Maintenance Technology: What information should AMTs and other maintenance service providers collect and have available before calling Lycoming Product Support?

Lycoming: First and most important is the engine serial number. All service data and maintenance history is tracked by the serial number.

Other important customer information:

  • Caller’s name, organization, telephone number, mail, and email address
  • Engine hours
  • TSO hours and who performed the last overhaul, Lycoming or another service provider?
  • Name of the aircraft owner and when the aircraft and engine were acquired. If the caller is not representing the original owner, then please provide the name of the previous owner.



Lycoming Technical Support hot lines are: Toll free (877) 839-7878 and local (570) 327-7222.


Charles Chandler began his aviation career as a junior mechanic for American Airlines and retired after 27 years of service.