The In’s and Out’s of Engine Overhauls


The In’s and Out’s of Engine Overhauls

By David H. Marion. Last time around, I gave all of you what amounts to practical “every day” advice regarding your choices regarding which types of oil to use in the engine(s) of your aircraft. Another area of particular concern to me is engine overhaul time – but in all likelihood, most of you won’t get to that point more than once or twice during your tenure of ownership of any particular aircraft.

First off, this is another area in which you should not go cheap or try to cut corners. Beyond that, you also need to consider right up front that there is so much more to the process of removing, overhauling, and then re-installing an engine than you might initially think. I suppose that many engine shops may quote them as “extras” but in addition to the basic engine overhaul itself (even if it is replaced with a factory rebuilt engine if you want to go that route) that is also the time to replace all engine compartment hoses, repair all of the cooling baffles as necessary, and replace all of the flexible baffle seals.

Some of the exhaust system problems that were found after it was taken off of the Cessna 182 engine pictured above – including buckled and distorted flame arrestor baffles and deteriorating slip-joint flanges.

It should go without saying that all of the engine isolator shock mounts also should be replaced when the engine is re-installed, but it is often a good idea as well to replace all of the associated hardware (nuts, bolts, and washers) at that same time too. If you are interested, I much prefer the quality and performance of the isolator shock mounts made by Lord over any others. I have found in my 30 years of experience that the Lord mounts are typically more durable and sag much less over time.

So too is an engine overhaul the time to consider replacing all of the engine control cables – the throttle, mixture, prop governor control if applicable) and carburetor heat or alternate air controls too. It is so much easier to get them changed out when the engine has been removed and is not in the way. Don’t risk losing control of your new engine because you cheaped out and chose to continue in service all of the old, worn out engine control cables and the ball-type rod end bearings or whatever other type of specialty attachment hardware they use!

FYI: Cessna in particular recommends changing out all of the engine control cables at each engine overhaul, but in my experience too many owners just don’t do it. As often as not, it may be out of ignorance as much as out of concern for the cost. Once upon a time, McFarlane Aviation was a principle supplier of such control cables to Cessna but now however you can buy their own new (FAA-PMA) direct-replacement control cable assemblies for almost any Cessna aircraft and usually for significantly less than they cost when they are bought directly from Cessna.

A Cessna 172N after its engine was removed for overhaul.

Other potential “extras” that you ought to keep in mind to be checked and repaired as necessary are the tubular engine mount (if applicable – some Cessna and Beech aircraft in particular use sheet-metal built-up keel-type engine cradle mounts) and the exhaust system. Typically both require specialized shops to carry out welding repairs if needed and those same shops are also usually best equipped even just to inspect them properly too. When in doubt, send them out. It is always sad to see a “new” freshly painted engine hung back on an aircraft using its old, ugly, chipped, or broken cooling baffles with limp, worn out seals, on a rusty old engine mount. It’s not like you’re going to do anything about the engine mount when your engine is still mid-time and otherwise going strong! And it’s also not like the old and worn-out parts of your airplane are going to “heal” themselves over time like a flesh wound either. They’ll get only worse over time.

The engine shop where I worked in the 1990’s has always been very good at stocking all of the little bits of miscellaneous hardware, hose clamps, Adel clamps, SCAT ducting, etc. that are typically needed or used when installing an engine – and they don’t nickel and dime the customer by charging them for each little bit of such hardware, etc; all of that is simply factored into the price quoted for the engine Removal and Re-installation or R&R. If you or your own mechanic’s shop do the R&R yourselves, keep that in mind too. The cost of all of that miscellaneous hardware, etc. can add up quickly – not just in terms of direct cost but in terms of time too if you don’t typically stock what you’ll need already and then have to order and wait for it to be shipped to you. As everyone knows, that time is money as well.

One more thing that needs to be considered or addressed is the subject of engine accessories. Different shops have different standard procedures or policies for what they include in a basic engine overhaul and what they don’t. Generally, the engine overhaul will include the applicable fuel system components (carburetor or fuel injection) and the ignition system (magnetos and ignition harnesses.) However, there can’t be any kind of hard and fast rule that covers all contingencies. Your airplane may have had certain accessories  replaced somewhat recently, so obviously that ought to be taken into account.

The lower aft cooling baffle from a TIO-540 engine on a Piper Navajo. It was sandblasted to remove all old paint and corrosion, but that revealed all of its numerous cracks and previous, shoddy patches. I fabricated new parts for about 70% of this piece.

Another factor which many pilots or owners don’t consider is that some of the things nominally installed actually on an engine are still technically “airframe” items. The basic test is the question – does the engine require it to be able to run or not? Also, is it in some way a customized fit or configuration that is applicable only to a specific type of aircraft? In that regard, exhaust systems, vacuum or pressure systems (dry air pumps, etc.)  hydraulic pumps, etc. are usually not part of an engine overhaul unless explicitly requested by the owner or recommended by the shop or mechanic doing the work and based on recognition of a specific problem or issue. Otherwise, it may be possible to determine which parts and components are “airframe” and which are actually “engine” only by consulting their respective parts catalogs.

Alternator belts are a good example of the seeming “limbo” that can exist in distinguishing between “airframe” and “engine” parts. Different part numbers may be specified for the same belts in respective airframe and engine parts catalogs – or they may be specified in only one or the other. There is no hard and fast rule to figure it out ahead of time; sometimes you just have to look in one place and then move on to the other when you don’t find it in the first place.

And for Lycoming engines in particular, which typically use the starter ring gear between the crankshaft and the prop as a pulley to drive the alternator too, changing out or replacing an alternator belt is no simple matter – it usually requires removal of the whole propeller, which itself is often more problematic than on a Continental engine. It can be a real pain-in-the-ass to install new safety wire on the mounting hardware of any constant-speed prop designed to be installed on a Lycoming engine, especially if certain types of spinner back bulkheads are in the way too.

The stages of fabricating new front, right cooling baffles for the Navajo’s TIO-540; the old parts drilled apart and flattened for use as patterns, the new pieces formed and cleco’d together, and finally all new parts Alodined and riveted together.

That brings up another consideration on which you can judge the need to replace a particular accessory or not – how easy or difficult will it be to replace it later if the old one is continued in service for now but fails later? Typically, starters and alternators are easy to access and replace “down the road” but vacuum pumps for a different example can be either difficult or easy to access and replace. Often with vacuum pumps the access issue is dependent more on the orientation of their hose fittings or the degree to which they are surrounded or obscured by other components, accessories, turbo mounts, or even exhaust systems.

By comparison, a turbocharged (Lycoming TIO series or Continental TSIO series) engine is a whole different animal. In most cases, a good part of the exhaust system will be custom-designed and built by the engine manufacturer specifically for that engine. The number of accessories that have to be overhauled and re-certified is almost always significantly higher for a turbocharged engine too. Besides the turbocharger itself, there will always be some sort of wastegate and typically too some sort of wastegate controller. The wastegate is an exhaust system by-pass that controls how much of the engine’s exhaust is routed through and used to spin up the turbocharger and how much is dumped overboard without going through the turbocharger.

Wastegates can be “fixed” – i.e. basically just permanent, calibrated leaks in the exhaust system (such as on Piper PA-34-200T Seneca II’s and -220T Seneca III’s) or they can be manually controlled such as by the vernier control cables used on many old, aftermarket Rajay turbo kits on aircraft such as Piper PA-30 Twin Comanches and some old Riley-modified Cessna 310 series aircraft.

More modern wastegate controllers are typically known as “slope” or “ratio” controllers and they are essentially automatic. Once properly adjusted or set, they use a diaphragm valve and an internal spring to balance the ratio between manifold and “upper deck” air pressures and to regulate the flow of engine oil to a “hydraulic” cylinder or actuator that opens and closes the wastegate exhaust valve itself. The “hydraulic” fluid is usually just the engine’s own oil circulated from its normal lubrication supply.

The stages of progress made in removing, overhauling, and re-installing a Lycoming O-360 engine on a Cessna 177B Cardinal.

Upper deck” pressure is distinguished from basic manifold pressure in that it is the raw output of the turbocharger “upstream” of the throttle plate inside the carburetor or fuel injection servo metering unit. Actual manifold pressure by comparison is the regulated air downstream of the throttle plate. Upper Deck pressure is then in turn typically regulated on such systems and overboost protection provided by an overboost (i.e. relief) valve or absolute pressure controller too. And each and every one of those separate components has to be overhauled, flow-checked, and re-certified in conjunction with the rest of the turbocharged engine.

All in all, each of those separate components is an integral part of the overall “powerplant” system and it won’t be completely healthy until each of its component parts are made healthy too. It can be a lot to think about and pay for, but as I have said a time or two before, an airplane is a life-support system that is designed to keep you alive in an environment that you probably would not survive without it – so take care of it accordingly!

Dave Marion is the Technical Content Editor at As A&P and IA with 30 years of experience in aircraft maintenance, he is also a Commercial Pilot with Airplane, Single & Multi-Engine, and Instrument ratings. He has a BA from Colgate University in 1984 and also graduated cum laude from Embry-Riddle Aeronautical University (DAB) with a BS in Aviation Technology in 1990. He can be reached along with all of the editors via E-Mail: [email protected]

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