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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 .

Wheel Bearing Service: Why & How

Wheel Bearing Service: Why & How

A&P Jacqueline Shipe describes how to service wheel bearings in this article, the second in a DIY series for pilots who wish to take on preventive maintenance of their aircraft.

FAR 43 Appendix A lists the preventive maintenance items owners may legally perform on their planes. This list is fairly long—and some of the items are a little involved for a person to perform the first time by themselves, while other tasks on the list are pretty straightforward. 

There are several preventive maintenance tasks pertaining to the landing gear, including tire changes, strut servicing and servicing the wheel bearings. (Last month, Shipe discussed the steps involved in changing an aircraft tire. See the June 2016 issue for more information. —Ed.) 

Bearings: small but mighty

While cleaning and greasing wheel bearings doesn’t seem like too difficult a task, there are some guidelines that need to be followed. The failure of a wheel bearing can cause major damage to the wheel and can even allow the wheel assembly to slide off the axle.

Wheel bearings are relatively small, but are incredibly strong. They have to support the weight of the plane while allowing the wheel to spin freely in all types of temperatures and conditions. In addition, wheel bearings and races on airplane wheel assemblies also have to be capable of withstanding hard landings and both vertical and horizontal loads without failing. 

Types of bearings

The bearings on most airplane wheel assemblies are the tapered roller-type. The outer part of the bearing is larger than the inner part, and the rollers are installed at an angle. 

The bearing itself rides in a metal cup called a race. The race has a “pressed in” fit in the wheel half, and is tapered on the inside to match the bearing. The biggest advantage of tapered bearings is the high load capacity that they can withstand. 

Automotive wheel bearings, on the other hand, usually use spherical rollers (i.e., balls). Ball bearings can withstand prolonged high speeds without building up too much heat, but cannot take high impact loads. 

Tapered bearings will bear up under the not-so-good landings that occur from time to time with an aircraft. In addition, proper servicing of these bearings will keep the wheels spinning freely and will last for a long time. 

Removing the clips

Once a wheel assembly is removed from the axle, the wheel bearings are easily removed by taking out the metal retaining clips that secure the bearings and grease felts. 

There is an indention in the outer part of one end of the clip to allow a screwdriver to be used to pry it out. The clips don’t have a lot of tension on them and can be easily removed. 

Once the clip is off, the bearing, metal rings and grease felts can all be lifted out together. 

Be sure to keep all the rings and clips organized so they can be reinstalled into the same wheel half and in the same place. The metal rings that retain the bearing are sometimes slightly smaller on the outer half than the double rings used on the inner half, and can be easily mixed up. 

Cleaning the parts

A small bucket with 100LL Avgas works well to clean the bearings. Swishing the bearing around and spinning it by hand while it is submerged will clean all of the old grease and gunk out. 

The metal rings and clips should also be cleaned, but the felt material needs to be set aside; it should not be submersed in anything. There is really no way to clean the felt, anyway—as long as it is still in one piece, it’s good to go. Any grease felt that is torn or missing a section needs to be replaced. 

Once all the parts are cleaned, they should be blown out with compressed air (if available) or laid out on paper towels to dry. The parts need to be thoroughly clean and dry before fresh grease is applied. Inspecting the parts

After the bearings, metal rings and clips are clean and dry, the bearing and race should be inspected for pitting or damage. If the race is smooth and has no corrosion, the bearing is generally corrosion-free as well. 

Races that have light surface corrosion can sometimes be smoothed out with a piece of light grit sandpaper (800 to start and 1200 to finish). Deep pits in a race mean replacement is needed. 

Discoloration on the bearing or race, such as a rainbow or gold color, can be a sign that these parts have generated excessive amounts of heat, in which case they should be replaced.

Preventing corrosion

Wheel bearings typically fail for two reasons: corrosion or overheating. 

The greatest threat to airplane wheel bearings is usually corrosion. Almost all bearings and races will eventually require replacement due to water getting past the grease seals and accumulating in the bearing cavity, causing rust and pitting. 

When cleaning a plane, strong degreasers should not be used on wheel assemblies and wheels should never be sprayed with a water hose. The pressurized water will get past the grease seals and ruin the bearings. 

Folks that want their wheels clean can wipe them out with a rag that is lightly moistened with a little Gojo original white cream hand cleaner (the non-pumice kind). Then the wheels can be wiped clean with a dry rag. 


Replacing the races

Wheel bearing replacement is easy, but replacement of the races is a little tough to do without the proper tools. 

Because the race has a pressed-in fit in the wheel half, it has to be driven out. This can be accomplished by using either a hammer and punch or a bearing driver tool. 

Occasionally a person encounters a wheel assembly with a race that has broken loose and is spinning in the wheel half itself. In this case, the wheel assembly has to be replaced; there is no permanent way to hold the race in place if the wheel assembly has lost enough metal that the race is no longer fitting tightly. 

The wheel is made of cast aluminum. When reinstalling the steel race, it is very important that it be driven in straight. If it gets cocked—even a little—the much softer aluminum will be gouged and damaged. 

The best tool for the job is a bearing driver, as it allows each blow of the hammer to be applied equally around the circumference of the race. 

Once the race is almost near the bottom of its recess, very light blows should be used to seat it in the wheel half. Many mechanics have driven the race in too far and cracked the fairly thin aluminum ring that retains the race. 

The wheel should always be thoroughly inspected for any sign of cracking on the front and back sides, whether or not a race is replaced.


Packing the bearings and reinstalling

Once all of the races are installed and the wheel halves are inspected, the bearings are ready to be packed and installed. A high-quality wheel bearing grease that has good water resistance should be used. 

The grease has to be pushed up through the bearing until it comes out the top between each roller. If it doesn’t squeeze through each opening, the inside of the bearing will have gaps and inadequate lubrication. 

It takes a little while to pack a bearing by hand. There are bearing packers sold in almost any automotive store that make the job a little faster and a little less messy. 

Once the bearing is packed, apply a layer of grease to the entire surface of the race to ensure it is covered as well. 

The bearing can then be reinstalled along with the correct order of retaining rings and grease felts. 

Lastly, reinstall the clip. It is a good idea to make sure the clip is pressed into place all the way around by pushing it outward with a screwdriver. 

After all the clips are in, the wheel bearing service is complete.


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 .


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