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.

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 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.

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.

The engine installation process takes longer to complete and is much more detailed than the engine removal process. 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.