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Engine Overhaul Fundamentals, Part One: Understanding the Process

Once you’ve made the big decision to overhaul your engine, you’ll still need to figure out where and how the overhaul will happen. In order to make the best choices for your engine and budget, you’ll need to understand the overhaul process. 
In the first of a four-part series, Dennis Wolter walks you through the basics of what happens in a typical overhaul.

My mentor flew Martin B-26 Marauders in World War II. He told me a story back in 1960 when I was just beginning to learn to fly that really resonated with me. When his bomber group first arrived in England, the base commander addressed the new flight crews at their first pre-mission briefing. 

The commander began that briefing with a very good piece of advice, stating, “Remember the seven Ps: proper prior planning prevents p--- poor performance.”

The key word in that statement is definitely planning! Planning starts with accessing information and choosing the best option. By now, most all of you folks can see that proper research and planning is a central theme of my articles.

Of the many stages involved in renovating an airplane, good research and planning is most important when you’re deciding how and where to have your engine overhauled.

Due to the complexity of engine overhauls, I will cover the total scope of the topic in four articles. In this first article, I will review the step-by-step procedure of overhauling an engine. 

An aircraft engine is complex; so is an engine overhaul. 

The second article will discuss overhaul options, including a local individual A&P overhaul; having a facility specializing in major field overhauls do the job; and having an overhaul or rebuild performed at the factory. 

The third article will cover support and installation details that need to be considered to ensure that your fresh engine has a good home. 

The fourth and final article will address upgrade options, such as converting to higher horsepower, turbocharging, propeller upgrades, etc.

Overhaul process: first steps 

In order to help break down all this information, let’s take a tour through a major overhaul facility. 

The first step of teardown and cleaning begins with an organized disassembly and layout of the components by type. The parts are then chemically degreased and cleaned in a hot solution of solvent. With the gross amount of oil, dirt and carbon removed, some of the parts are also detail cleaned with media blasting to get them thoroughly cleaned.

After disassembly, parts are thoroughly cleaned.

The technicians then put every component through an alignment and a precision dimensional check to ensure that no parts are bent, worn or damaged to a degree that they cannot be reconditioned and placed back in service. 

Reusable components are then either turned over to highly-skilled in-house technicians or shipped to an off-site facility where each piece is reconditioned to meet minimum service limits or new limits depending on the quality standards the customer has chosen.

Inspecting the components

Crankshaft, connecting rods, bearings 

The heart of a piston engine is the crankshaft, so let’s start there. The technician begins by placing the crankshaft in a fixture that supports the shaft at both ends. The probe of a precision dial indicator is positioned to press against various positions on the crankshaft. 

Precision measurement of the crankshaft.

As the crank is rotated in this fixture, the dial indicator will show little to no movement if the crankshaft is straight. If too much movement is seen on the dial indicator, the crankshaft must be replaced. 

If the crankshaft is not bent, it is put through a crack-finding process known as magnafluxing. It is mounted in a machine that runs a strong electric current through the full length of the steel crankshaft, causing the crank to become magnetized. A solution of solvent and iron filings is poured over the crankshaft.

The business end of a magnaflux machine that magnetizes steel parts. 

If there is a crack in the metal, the disturbed magnetism at the point of the crack will cause the magnetically-sensitive iron filings to align themselves along the crack and clearly show a visible irregularity. 

This magnaflux inspection process will be performed on all steel parts. A cracked component must not be put back in service. 

If the crank passes these inspections, it is potentially eligible to be reconditioned and reused.

Next, the technician will inspect the round surfaces that support the crank and the four or six connecting rods and bearings that are attached to the crankshaft. These journals, as they are called, must be perfectly round, smooth and machined to a very precise dimension. If any scoring or excessive wear is identified, these conditions must be corrected by re-machining and polishing. 

The connecting rods that attach the piston to the crankshaft are precisely measured for length and straightness. After passing that test, they are magnaflux tested for cracks. 

Finally, the bushing that serves as the bearing where the piston is attached to the connecting rod is inspected for condition and wear. If the bushing is out of tolerance, a new one will be required.

Camshaft, valve lifters, cam lobes, gears and bearings

Another high-wear area in the valve drive mechanism is where the camshaft and lifters open and close the valves. The camshaft and valve lifters are inspected using the same magnafluxing methods as used on the crankshaft. 

A magnetized camshaft being doused with iron particles to identify a crack. 

Both the cam lobes and lifter faces where the cam rubs the lifter are heat-treated and polished to a very smooth and hard finish when manufactured. These hard surfaces are very thin. 

Camshafts can be reconditioned. However, if a significant amount of this thin surface material is removed during the re-grinding process, the life expectancy of the reconditioned part is limited. 

I believe that re-grinding a camshaft lobe or mating surfaces of the valve lifters may not always be the best choice. Think seriously about installing new cams and lifters. 

In the back of the engine are several steel gears and bronze bearings that need to be magnafluxed and inspected for cracks, condition and wear.

Oil pump

Certainly, let’s not forget the oil pump. All three basic parts of this important component must be assessed. Personally, I would not reinstall used oil pump gears in an engine that’s being overhauled. New gears come with new bushings, so the only “old” part remaining would be the oil pump housing. The oil pump housing can be measured to confirm that it is within limits and if it is, the pump is good to go until the next overhaul.


The next big component to be inspected (and possibly repaired) is the crankcase. This is the big aluminum casting that holds together the lower end rotating crankshaft timing gears, camshaft, magnetos and cylinders. 

This complex and massive aluminum casting must first be checked for cracks by using a non-destructive fluorescent dye penetrant process, often known as Zyglo testing. 

With the case thoroughly cleaned and dry, the dye (a penetrating fluorescent oil solution) is applied to all the surfaces of the case and allowed to soak into any potential cracks. The surfaces of the case are then thoroughly cleaned. Existing cracks will retain some of the fluorescent material. 

When the case is inspected with a black light, the fluorescent material remaining in a crack will glow in a yellow-green color revealing cracks or porosity in the metal. If problems are found, the case can be sent to a company that specializes in welding and machining engine cases to new limits.

Using a black light to check for cracks in the crankcase.

If there is no evidence of cracks, the case is checked to ensure that all mating surfaces and areas that support rotating parts, such as crankshafts, camshafts, etc., are straight and not distorted.

Cylinders, valves, valve guides and other mechanisms

Next, it’s on to the cylinders, the most heat-stressed components in an internal combustion engine. Once thoroughly cleaned, all areas of the aluminum cylinder heads are checked with the Zyglo test I mentioned earlier. 

If no cracks are detected, the valves and valve guides are inspected and machined. Excessive wear in valves or valve guides will require replacement. The steel valve seats must meet minimum dimensional standards. If not too worn, valve seats and valves can be precisely re-ground to recreate factory specifications. 

Next, the valve drive mechanisms and their supporting components, rocker arms, bushings and supporting bosses are inspected using the previous techniques.

Within limits, steel cylinder barrels can be re-machined back to serviceable or new limits. The area where the aluminum head and the steel cylinder barrel are connected is closely checked for leakage. A leak at this juncture means the cylinder is not repairable. 

The next step is to measure the bore of the cylinder for wear and condition and, for some cylinders, choke. Choke is a difference in diameter between the hot top end of the cylinder barrel and the cooler lower base of the cylinder. Cylinders can be re-bored to a permissible oversize limit or chrome plated back to new limits by a company that specializes in cylinder work.

Reconditioned cylinders, with new pistons and piston rings, ready for installation. 
Assembling the engine

After days and days of preparing all the engine components for reinstallation, it’s finally time for the fun part of assembling the engine. 

All the new and reconditioned parts ready for assembly.

The process begins with mounting the crankshaft to an engine stand vertically by securing the propeller flange to a mating surface located at the top of the engine stand. Then, an assembly lubricant is applied to the rod bearings. The connecting rods are bolted to their crank journals with new high-tech rod bolts and nuts. 

Crankshaft and connecting rods mounted on an engine stand.

The bolts are carefully tightened to a specific tightness torque with a special calibrated torque wrench and double-checked by a second technician. This two-step verification system will be used throughout the entire buildup process for any critical mounting hardware—smart! 

Next, the engine case, with the pre-lubed camshaft, camshaft bearings, valve lifters and main bearings, is mated to the crankshaft and secured by properly-torqued case bolts. 

It’s time to install and properly index the magneto, cam timing gears and oil pump, and mount the accessory case cover at the back of the engine. The oil pickup is installed and the oil sump case is bolted on.

Next, the cylinder and pistons are installed, and all cylinder base bolts are torqued to the correct values. The pushrod tubes, pushrods and rocker arms that actuate the valves are installed. Then, it’s on to installing the intake manifolds, magnetos and fuel system, including the engine-driven fuel pump (if required). 

Cylinders, pistons and valve-actuating rocker arms.

As these components are installed, the technician is constantly rotating the engine on the stand, checking for any excessive resistance, proper running clearances and timing of critical components such as valves and magnetos. Lots of stuff, huh? 

With all this completed and double-checked, the engine is painted. Now the engine is ready to run, either in a test cell or installed in the aircraft.

Overhauled engine ready for test run before shipping to customer. 
The paperwork

A reputable overhauler will supply their customer with the following documents and services with the newly-overhauled engine:

1. A teardown report stating the condition of all components when the engine was disassembled.

2. A thorough logbook entry specifying the limits to which the engine was overhauled (such as service limits, new limits, etc.), including a description of all work performed, a complete list of all new parts installed, and supporting certification paperwork for each new part.

3. Yellow tags verifying the identity and airworthiness of all reconditioned components installed in the engine.

4. Statements related to test flight or test run.

5. Any supporting warranties for components not repaired or rebuilt by the overhauler, such as starters, alternators or fuel system components.

6. A clear warranty policy stating what is covered, when the warranty begins and expires, and a payment policy should the warranty need to be enforced.

ADs and Service Bulletins

If defects are discovered over the years a particular model of engine is in service, ADs and Service Bulletins are issued. Some require immediate attention and others must be completed at overhaul. It is important to ensure that all ADs and Service Bulletins are complied with during the overhaul process. 

I think we’ve gone over enough for now. With general overhaul procedures covered, next time I’ll explain the three choices for where this work can be done: a local A&P, an overhaul specialist and the engine manufacturer’s factory. Until then, fly safe!

Industrial designer and aviation enthusiast Dennis Wolter is well-known for giving countless seminars and contributing his expertise about all phases of aircraft renovation in various publications. Wolter founded Air Mod in 1973 in order to offer private aircraft owners the same professional, high-quality work then only offered to corporate jet operators. Send questions or comments to .

Pre- and Post-overhaul:  Engine Removal & Installation

Pre- and Post-overhaul: Engine Removal & Installation


Wise owners (and mechanics) know that a successful overhaul starts with careful engine removal. The overhaul process isn’t finished until after the engine has been reinstalled and the break-in period completed. A&P Jacqueline Shipe walks you through best practices to ensure start-to-finish success.

An engine overhaul is a daunting repair that usually takes several weeks to complete. In addition to the engine overhaul itself, there are several maintenance tasks that are associated with pulling the engine and reinstalling it after the overhaul. (For more about what comprises an engine overhaul, see “A Step-by-Step Guide to Overhauls” in the February 2018 issue. —Ed.)



Engine removal location and airframe storage

Once the decision to overhaul the engine has been made, the next step for an owner is to decide on the location for the engine removal. Some owners have their mechanic pull the engine and ship it to an overhaul facility. Other owners fly the airplane to the overhaul location and let the overhaul specialists remove, overhaul and reinstall the engine. 


The next task is to find out where the airplane will be stored while the engine is off the airframe. Hangar space is typically at a premium for both overhaul shops and general maintenance shops. 


Some shops place the airplane outside for the duration of time that the engine is off the airframe. The airframe is unbalanced and hard to secure on a tiedown once the engine has been removed. It is also much lighter than normal, leaving the aircraft more vulnerable to windy weather. 


Make sure to have a clear understanding with whomever is doing the engine removal and installation about where the airplane will be stored while the overhaul is taking place. 


Engine removal

Removing an engine from the airplane is typically not that time-consuming. The engine can be pulled easily enough in most cases in less than a day. 


Once the cowling and propeller are removed, the next step should be to take lots of pictures from all different angles of every section of the engine. This will help to determine the routing of hoses and control cables later on during the reinstallation process. 


The exact location of clamps is not usually specified by the maintenance manual and is left up to the mechanic. Knowing where the old clamps and supports were located helps ensure that everything fits properly during reinstallation.


Once all the engine components are disconnected from the airframe, the engine is stripped of everything that is not sent with the engine for the overhaul. The exhaust system, alternator, starter, vacuum pump and engine baffling typically don’t get sent in with the engine for overhaul. These components are either replaced or refurbished as needed by specialty shops. 


After all the necessary items are removed or disconnected from the engine, the engine itself is removed from the airframe. The tail of the airplane should be secured on a support that will hold it up once the heavy engine is removed. Most engines have permanent lifting eyes installed on one or more of the upper crankcase bolts. If an engine doesn’t have a lifting eye, one will have to be temporarily installed. 


A chain is most often used to attach an engine hoist to the lifting eye. Once the chain is secured, the engine hoist is raised until the chain has all the slack removed from it. Then, the bolts that secure the engine to the mount are removed from the vibration isolators and the engine can be lifted out of its mount. 


Once removed, the engine is either wheeled into the overhaul shop for disassembly or prepared for shipping if the overhaul is to take place elsewhere.


Engines that are shipped out by means of a freight company are generally bolted to a shipping pallet with a prefabricated mount. 


Owners that are having their engines sent out can save money by taking it themselves to the overhaul shop. The engine is often placed on a layer of used tires in the back of a truck and secured to four different tiedowns to keep it from shifting. 


In addition to saving money, the owner can have peace of mind knowing that he or she has overseen the engine shipment the entire time. Careless handling can damage expensive engine components and shipping companies do occasionally drop or damage items. 


If the overhaul facility is located a long distance from the aircraft location, shipping with a freight company may be the only option. In those cases, the shipment should be insured for the full replacement value of the engine. 


After the engine overhaul is underway, attention can be shifted to the repair or refurbishment of all the parts that are now easily accessible with the engine removed. 


Engine mount

Once the engine has been removed, the engine mount is easily accessible and can be thoroughly inspected for cracks and pitted areas. 

Even if the mount itself is in good shape, remove the mount from the
airframe and inspect all the attachment areas on the airframe and mount
for corrosion. 

Mounts that are free of corrosion and have good paint are often reused as-is. Mounts that are in need of repainting should be cleaned, lightly sanded and painted with a high-quality primer and then a coat of paint. 

In addition, any corroded areas on the airframe should be cleaned and treated or repaired as needed. 

Engine mounts that have pitted areas, excessive corrosion or cracks are usually sent to specialized welding shops like Acorn Welding or Kosola (now Aerospace Welding) for repair. These shops have special jigs and can cut out bad sections of tubing and weld in reinforced sections without distorting the shape of the mount. 

The firewall of the airframe is easily accessible with the engine and the mount removed. Now is an ideal time to clean and paint the firewall. Painting areas such as the firewall and the inside of the cowling with a bright color (usually white) helps to spot leaks easier. It also makes the airplane look better, and adds another layer of protection against corrosion.



Controllable-pitch propellers and propeller governors are often overhauled at the same time as the engine. This ensures that the engine will be able to develop its maximum power within the proper limits without being held back by a sluggish or malfunctioning propeller or governor.



Metal engine baffles should be repaired as needed, and any worn baffle seals should be replaced to maximize engine cooling. 


Effective engine cooling is particularly important for overhauled engines because the new cylinder rings have to wear in and seat themselves against the cylinder walls during the first few engine runs. The extra friction will generate more heat than normal, especially in the cylinder heads. 


The air that the cylinders need for cooling should flow in through the front of the cowling, through the cylinder cooling fins, then down and out the bottom of the cowling. Any air leaks in the engine compartment that aren’t sealed off will allow cooling air to escape through a gap or hole instead of being ducted through the fins where it is most needed. 


Exhaust system

Exhaust system components are sent out for repair or are replaced if they are corroded, cracked or deformed in any way. Excessively thin or leaking pipes will only cause trouble later on. Leaking exhaust gases from warped exhaust flanges at the cylinder head connection will corrode and ruin the cylinder heads over time. 


Some overhaul facilities recommend replacing the exhaust system whenever the engine is overhauled. Turbochargers and wastegate assemblies should always be sent out for overhaul or replaced whenever the engine is overhauled. 




All fluid-carrying hoses connected to the engine should be replaced at overhaul. Hoses become hardened and brittle after being heated and cooled during engine operation. A ruptured hose can cause a fire hazard or starve internal engine components of precious oil pressure. 


Also, tiny amounts of metal and debris can remain in old hoses even after they are rinsed and blown out and can contaminate the new engine. Many engine overhaul facilities will deem the engine warranty null and void if the fluid-carrying hoses aren’t replaced. 


It is also good idea to replace the SCAT hoses, but they aren’t critical like the fluid-carrying hoses are.


Oil coolers

Oil coolers should be replaced with new units or sent to an oil cooler specialty shop that can thoroughly clean the oil passageways. The oil passageways through the cooler have 180-degree turns in them that cause contaminants to precipitate out of the oil flow and build up in the turn areas.


It is impossible to get all the sludge, metal particles and dirt out of the old cooler by rinsing it in a parts cleaning vat. It’s not worth risking contaminating a freshly-overhauled engine with debris from the old engine in order to save a few dollars on the oil cooler. Clean oil coolers also have better oil flow through them and cool the oil more efficiently. 


Rubber vibration isolators

Most engines are mounted with the four attachments for securing the engine to the mount located on the rear of the engine. The rubber vibration isolators (often called “rubber engine mounts”) that are installed between the engine mounting pads and the engine mount should always be replaced whenever the engine is removed.


Vibration isolators lose elasticity over time and will begin to sag under the weight of the engine. Once the isolators start to age, they allow the front of the engine and the propeller to not only sag, but also to tilt down. 


The cowling is secured to the airframe and the propeller is connected directly to the engine, so as the engine mount isolators droop, the clearance between the bottom of the spinner bulkhead and the cowling becomes smaller while the gap between the top of the spinner bulkhead and the top cowling gets larger.


Isolators that are severely aged and distorted on these types of engine mounts can cause the engine to droop so much that the bottom of the spinner bulkhead actually starts rubbing on the lower engine cowling. 


In addition, rubber engine mounts are easily damaged and prematurely age if they are exposed to leaking oil or hot exhaust leaks. Constant oil leaks soften the rubber, causing it to swell and bulge. Exhaust leaks overheat the rubber, making it brittle and prone to cracking.


The isolators play a critical role in helping to secure the engine to the engine mount. They are typically not that expensive in comparison to other parts, and are easily accessible any time the engine is removed from the airframe—but difficult or impossible to replace without pulling the engine. 


Engine installation

The engine installation process takes longer to complete and is much more detailed than the engine removal process. Installing the engine mount on the airframe and then hanging the engine on the mount can be done quickly in most cases because there are usually only four bolts and nuts that secure the engine mount to the airframe, and an additional four bolts and nuts that secure the engine to the mount. 


Sometimes it is difficult to get the engine hoist adjusted just right so that the engine lines up correctly when attaching it to the mount. It can take a few attempts to get the bolts inserted through the mount and isolators. Components like the magnetos, fuel servo or carburetor may have to be removed to provide enough clearance to get the engine into the proper position on the mount. 


Engine mount bolts should always be torqued to the specified setting listed in the airframe maintenance manual and any specified torque sequence should be adhered to.


Once the engine has been hung, the baffling, accessories, hoses, oil coolers and all remaining parts can be installed. Clamping and securing hoses, wires and ignition leads is one of the most time-consuming tasks in this phase of the project. 


The exhaust system and propeller are usually two of the last items that are installed because once they are installed, they block access to other parts of the engine. 


Many overhaul shops run an engine on a test cell for an hour or so before sending the engine out. Some shops send the engine out with no run time on it at all. 


After reinstallation on the airplane, the engine should be started and run on the ground for the minimum time needed to ensure that there are no leaks; that the magnetos have the proper rpm drop when checked; and, if a controllable-pitch propeller is installed, that the propeller changes pitch as it should. 


Idle mixture and idle speeds should be checked and adjusted if necessary—but ground runs should be kept to a minimum, especially if the engine has not been on a test cell. 


After an overhaul, the rings are not seated. In order for the rings to seat properly, they must be blown out against the cylinder walls. The rings need high manifold pressures to force them to have metal-to-metal contact with the cylinder walls so they seat properly. 


Running an overhauled engine at too low of a throttle setting for any length of time (on the ground or in the air) increases the likelihood of glazing the cylinder walls. Glazing results from the oil oxidizing on the cylinder walls and creating a hardened surface that prevents the rings from ever seating properly. 


After the first flight, the cowling should be completely removed and the entire engine looked over for leaks and to make sure nothing has vibrated loose. Some shops will change the oil at this time if the test flight was the first run on the engine. 


The recommended break-in oil is generally used for the first 50 hours. After the 50-hour mark, there should be no metal in the oil filter when it is inspected. Metal found in the oil filter after this time may be indicative of an internal problem with the engine. 


Most overhauled engines perform well and provide many hours of trouble-free flight time and it is generally a relief for owners to have this major expense behind them.


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 question or comments to .








Acorn Welding Ltd. – PFA supporter



Aerospace Welding Minneapolis, Inc.



To find resources for other components and services for engine overhauls, please go to the Piper Flyer Yellow Pages at piperflyer.org/piper-yellow-pages.html, or contact Kent Dellenbusch at Email or phone 626-844-0215. 


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