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Engine Mounts Explained

Engine Mounts Explained

The engine mount represents a crucial link between your engine and airframe, and it should be treated as a mission-critical accessory. STEVE ELLS visited Loree Air, an FAA-certified repair station, for insight into the engine mount repair process.

I have found no evidence that my engine mount—that web of steel tubes that supports the engine and nosegear on my 1960 Piper PA-24 Comanche airframe—had ever been overhauled or recertified.

It seems a bit hard to believe. After all, it’s been bolted onto my airplane for 57 years. You’d think one mechanic or owner along the way would question whether the mount had suffered the ravages of time or had any issues. But like I said, when I started digging in the logs, I found no maintenance record entry that showed me it had ever received specific attention.

I recently discovered a cracked tube, and when I scrubbed it with a wire brush, I found a gaping hole—the tube had rusted through from the inside. I removed the welded steel mount in order to send it in for repair and recertification. 

As it turned out, the tube with the rusted spot was only one of seven tubes that had to be replaced. I had no idea the mount was in such bad shape!

What engine mounts are made of

SAE grade 4130 steel, also known as chrome-moly, is a through-hardened chromium-molybdenum steel alloy that is used in the light airplane industry where light, strong tubing is needed. It’s strong for its weight, easy to work, easy to weld and provides a good cost-to-strength ratio. 

Chrome-moly steel is available from aviation parts suppliers such as PFA supporters Acorn Welding, Aircraft Spruce and Airparts Inc. Wicks Aircraft also supplies this tubing. (Another PFA supporter, Wilco Inc., carries SAE 4130 in sheets. —Ed.)

The seven tubes that were replaced on my engine mount consisted of one 1/2-inch diameter tube, two 5/8-inch diameter tubes and four 3/4-inch diameter tubes. 

Chrome-moly tubing is purchased by specifying the outside diameter (OD) in 1/16-inch steps and the wall thickness. The wall thickness of the 5/8-inch OD tubes in my engine mount is 0.035 inch, which is close to the thickness of a credit card. The wall thickness of the 1/2-inch OD tubes is 0.049 inch, which is approximately the thickness of a CD. 

The 1/2-inch and 5/8-inch tubes sell for $4.35 per foot at Aircraft Spruce; the 3/4-inch tube is $3.35 per foot. 

I needed 4 feet of 5/8-inch tube and 68 inches of 3/4-inch tube to repair my mount before it could be recertified as airworthy. The materials cost was less than $50 at retail prices. 

A chrome-moly steel mount is a sweet piece of engineering. My refurbished engine mount (as delivered to me) weighs 15 pounds, 11 ounces; yet it is strong enough to support the Comanche’s Lycoming O-360 engine (258 pounds), a Hartzell two-bladed propeller (51 pounds) and support and endure the shocks suffered by my retractable nosegear.

The refurbished engine mount of the author’s 1960 PA-24 Comanche weighs 15 pounds, 11 ounces. It is strong enough to support a 258-pound Lycoming O-360 engine and a 51-pound Hartzell two-bladed propeller, and will also endure the shocks suffered by the retractable nosegear.
Removing and sending the mount out for repairs

After I found the hole in the lower right tube, I removed the engine and nose landing gear assembly. Removing parts, like the demolition phase of a room remodel, always goes quickly. In this case, I knew I needed to label and sort the parts and engine accessories because it was going to be almost two months before I was going to be reinstalling the engine and nosegear. 

One trick I’ve used for years when removing an engine or other assembly is to take photos of everything before picking up the wrenches. When I first heard of this photo trick, shops were using Polaroid cameras. Today, a cell phone and/or tablet is more than sufficient. 

One of the decisions that I pored over was where to send the mount for repair and recertification. I wanted an FAA-certified repair station that had the capabilities to repair and recertify my mount. My favorite internet search engine turned up four options. They were, in alphabetical order: Acorn Welding Ltd., Aero Fabricators (a division of Wag-Aero), Aerospace Welding Minneapolis and Loree Air Inc. and I have no doubt that there are others. 

I also searched for a used, serviceable mount. I found one on the East Coast and negotiated what I thought was a good price—but after learning that it would take more than $500 to ship it to me on the West Coast, the deal fell through.

Obviously, the cost of shipping a mount, as well as how to ship a mount, must be considered. Companies told me that the most common method is to bolt the mount to a piece of stout plywood, then either build a wooden or cardboard box around it for shipping by UPS or FedEx; or to bolt the mount to a pallet and ship it as truck freight. Since the repair facility has no control over handling after it leaves their possession, it’s critical to create a shipping container that protects the mount during shipping. 

PFA supporter Aero Fabricators quoted me $1,400, which included changing up to 10 tubes, and told me the turnaround time was two to three weeks. Aerospace Welding quoted a price of more than $2,500. 

Another PFA supporter, Acorn Welding, was unable to estimate their cost over the phone, but Paul Gyrko, head of sales, took the time to answer my questions and explain the full process when I called for information. 

Steve Loree Jr. at Loree Air told me that the cost to inspect, repair, normalize, paint and certify my mount would be $1,700 if it only required cleaning, inspecting, repainting and certification; and a maximum of $2,100 if work was needed. Loree also warned me the company had a five-week backlog. 

Given that Loree Air was only 278 road miles away from my home base—while the other three were all over 1,800 road miles away—and that I had good reports from friends that had used them, I decided to use the five-week window for other tasks and took my mount to Loree.

After another PA-24 owner offered to fly me up to Placerville to drop off the mount, I packed my sad old mount in the back of my buddy’s Comanche and flew it up to the Placerville, California airport (KPVF) where I left it with Nicole, who runs the office. 

Ready for pickup

Steve Jr. called on a Tuesday in late June to tell me that after cleaning and sandblasting all the paint off my mount, a thorough inspection revealed some surface damage to the exterior of a couple of tubes; bends in two tubes; and more tubes that showed evidence of internal rust. 

I asked him if it was OK if I drove to the shop once my mount was finished; I wanted to hang around and ask a lot of questions about mount damage and repairs. I figured this was an opportunity to pick up some hints and tips that a mechanic in the field could use to determine if a welded steel tube engine mount or landing gear support structure was airworthy. He said that would be fine.

Five weeks later I got the call; the repaired mount was ready. 

I arrived at Loree Air at 10:30 Monday morning. I met the entire staff: Steve Sr., Steve Jr. and Nicole (who is married to Steve Jr.). I was also sniffed up and down by Layla, the small four-legged office assistant and guard dog.

 

 Left to Right, Top to Bottom: Steve Sr.; Steve Jr.; Nicole; Layla (the hairy one).

Steve Sr. attained his welding certification at the San Diego shipyards and went to Sacramento City College for his A&P education at the suggestion of his flight instructor. He gained a wide range of reciprocating engine skills at the Sacramento Sky Ranch before spending 15 years working at the Sacramento Citation Center and at Aircraft Conversion Technology in Lincoln, California, with owner Bill Piper. 

Seeing the need for a certified aircraft welding shop in California and wishing to steer his own path, Steve Sr. opened Loree Air in 1992 in a small shop in the Swansboro Country neighborhood in the foothills east of Sacramento, near Placerville.

In 2011, Steve Jr. joined his father in the business. They decided that since the shop needed to grow in order to support two families, it was time to expand. To do so, Steve Jr. said, “I think we need a website,” but Steve Sr. wondered if it was necessary. Word-of-mouth advertising had been effective and the company had all the work it could handle. But Steve Sr. yielded, and today you can visit Loree Air online at LoreeAir.com. 

After consistent growth—thanks to the website—the Steves decided to move the company to a small warehouse and shop in Diamond Springs, another community near Placerville. 

With the help of many friends and family members, they planned and built a shop to fit the company’s needs. 

There had to be a large sandblast booth to clean mounts. There had to be a paint booth. There had to be an area for grinding and smoothing metal. The shop needed an area where mounts were put into jigs for alignment and buildup. A screened area was required for welding. A separate office and customer reception area were part of the plan as well.

There are also two lofts for storing parts and ready-to-ship mounts and nose strut welded tube support structures. 

While I had to take my mount to Loree Air to get in line due to the five-week backlog, the company does stock repaired and certified mounts for some popular aircraft. 

Problem areas

The Steves spent some time describing why my engine mount rusted out and passed on tips for determining if a welded steel engine mount is airworthy.

According to Steve Sr., “Piper mounts were not corrosion-proofed in the 1960s and early ‘70s.” He is referring to a practice of coating the inside of welded steel tube assemblies with a corrosion inhibitor. 

In the early days of aviation, linseed oil was used to inhibit corrosion. When I asked what else works, he replied that either LPS 2 or 3 heavy-duty lubricant works well and is readily available. 

The other Piper mount problem was the build sequence, which left small gaps at each firewall fitting around the bolt bushing boss. The gaps are small, but can allow moisture to get to the inside of the tubes. Loree has developed a build process that seals the mounts. 

Steve Sr. also pointed out that many Piper PA-28 Cherokee engine mount assemblies allow moisture to get into the tubes at the four engine support reinforcements, where the rubber vibration isolators—often called Lord mounts—are installed, because the two halves of the reinforcements are not sealed. This is also addressed when Loree repairs a PA-28 engine mount. 

Inspection tips and tricks

I asked the Steves for tips to help field mechanics determine if the welded steel mounts they inspect are airworthy. They said one test is to use an automatic center punch to put a small dent in the end of a tube that is believed to be unaffected by internal corrosion and compare that to the dent when the punch is used on the part of the tube that is suspected to be corroded. Usually this means comparing the dent at the highest part of the tube near a weld cluster to a dent in the lowest part of the tube. 

Any difference in the depths of the two dents is clear evidence the lower end of the tube has been weakened by internal corrosion.

Dents are repaired during the Loree Air rework. According to Steve Loree, the circular slot around the bolt hole is how moisture—a cornerstone of the rust process—enters the tubing in the mount. Loree seals this slot during rework.

While at the Loree shop, I also saw tubes that were dented during installation and removal by sloppy tool handling, and tubes that had been scratched or scored by abrasion.

Since these tubes are so thin, what may at first appear to be negligible damage usually needs attention. “Our standard for repair is 10 percent of the tube thickness,” said Loree.

One thing Loree was adamant about is avoiding the use of plastic tie-wraps (i.e., zip ties) to secure anything to a welded steel mount. He has seen it again and again: plastic tie-wraps will wear a welded steel mount tube faster than a pilot heads to a restroom after a cross-country flight. It takes longer to install properly-sized Adel clamps, but they are the only clamping device Loree wants used on an engine mount. 

You and your mount

I was surprised to hear Steve Sr. say that in all his years repairing mounts he had seen very few engine mounts pass through his shop that needed no repairs. 

I was also surprised when my mount needed seven tubes replaced. 

Then I saw pictures of the inside of those tubes. They were all rusted to one degree or another. I believe good fortune was smiling on me when I found the crack that lead me to remove my mount to send it for repair. 

Rust was clearly present in seven of the author’s engine mount tubes. They were all replaced by Loree Air.

Based on what I learned and saw, I recommend that owners send their engine mounts to a certified mount repair shop to get inspected, repaired-as-necessary and recertified whenever their engine is removed for overhaul.

Steve Ells has been an A&P/IA for 44 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Templeton, California, with his wife Audrey. Send questions and comments to .


RESOURCES >>>>>

SAE 4130 STEEL TUBING – PFA SUPPORTERS
Acorn Welding

Aircraft Spruce and Specialty Co.
aircraftspruce.com/catalog/mepages/4130tubing_un1.php

Airparts Inc.

SAE 4130 STEEL TUBING – OTHER SUPPLIERS

Wicks Aircraft and Motorsports
wicksaircraft.com/tubing.html

SAE 4130 STEEL SHEETS – PFA SUPPORTER

Wilco Inc.

ENGINE MOUNT REPAIR – PFA SUPPORTERS
Acorn Welding Ltd.
acornwelding.com/products/engine-mounts.html

Aero Fabricators
(a division of Wag-Aero)
wagaero.com/repair-station/engine-mount-repair.html

OTHER ENGINE MOUNT REPAIR FACILITIES
Aerospace Welding Minneapolis
awi-ami.com

Loree Air Inc.
loreeair.com

ANTI-CORROSION SPRAY
ITW Pro Brands
lpslabs.com 

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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.

 


Propellers

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.

 

Baffling

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. 

 


Hoses

 

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 .

 

 

 

MENTIONED IN
THE ARTICLE >>>>>

 

 

ENGINE MOUNT WELDING


Acorn Welding Ltd. – PFA supporter

acornwelding.com

 


Aerospace Welding Minneapolis, Inc.

awi-ami.com

 


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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