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top overhaul explained

top overhaul is a term used by the general aviation industry when all the cylinders on the engine are overhauled or replaced with new. time since top overhaul is abbreviated tstoh or stoh. for example, you might find a cessna 150 for sell with a classified ad that reads "50-stoh". the seller is telling you that the aircraft has flown 50 hours since the engine had a top overhaul. a common mistake a new pilot or owner makes is to assume a top overhaul is like a complete engine overhaul. a major engine overhaul requires that the engine be completely disassembled, inspected, certain parts of the engine to be replaced, and other parts brought back to serviceable limits or better. during a top overhaul all the cylinders were removed, inspected, restored to at least serviceable limits, reassembled with various required new parts, and reinstalled at the same time. the work done on the cylinders may vary. a top overhaul has no faa regulatory status as there are no regulations or time limitations to define the overhaul. the term cylinder overhaul implies that the cylinder assembly meets the overhaul criteria specified by the manufacturer of the cylinder.

the prospective owner or current owner needs to be aware of the many reasons for an engine to require a top overhaul since some of the reasons may be deceptive. a top overhaul is a normal part of owning an aircraft with a turbocharged engine. the cylinders on turbocharged engines work harder due to additional manifold and exhaust pressures from the turbocharging; the valves, valve seats, guides, pistons, rings, and cylinder walls all wear faster than those on a normally aspirated engine. in general, high altitude flights accelerate engine wear. to continue the turbocharged engine in service, it is often necessary to top overhaul or install new cylinder assemblies about mid life or later to make it to it�s scheduled major overhaul. there is the option of repairing the cylinder assemblies, overhauling them or installing new. see january 2007 aviation consumer article concerning repairing, reboring, or replacing the engine's cylinders. the top overhaul is typically done at mid-life or later on the turbocharged engine when a majority of the cylinders begin to approach minimum compression readings and / or excessive  oil consumption. those who wish to fly high generally pay the price. a prospective owner should be prepared for the costs of a top overhaul, if they are planning to own an aircraft with a turbocharged engine.

here are the other reasons for a top overhaul regardless of type. buyer beware:

1. inactive aircraft may require a top overhaul due to corrosion factors, providing the engine passes a cam / cam follower inspection. in other words, the engine may have had a top overhaul in order to address corrosion or rust in the cylinders. the cam / cam followers may also have been corroded, but were not inspected while the cylinders were removed.  in a short period of time, the engine may need a complete disassembly to address the cam and followers. engines with internal corrosion generally have external clues such as rust inside the oil cap (tcm engines).

2. a poor quality major overhaul can result in a top overhaul. often, the owner did not save up sufficient fund to pay for quality overhauled or new cylinders. a poor quality overhaul is best found in the logbook, usually evident by more than one cylinder repair since the major overhaul even with few hours or years intervening.

3. overheating and cooling problems can cause an early top overhaul. the owner needs to ensure the baffles are repaired and maintained properly. most piston engine aircraft are air-cooled and are dependent upon the baffles for cooling air. oil coolers and thermostatic / veratherm valves also may contribute to higher than normal engine temperatures. overheating and cooling problems can be found by flying the aircraft in the summer and reading the indicated oil and cylinder head temperatures.

4. dust and dirt causes increased wear on the cylinders and engine components. maintenance of engine air filters and associated ducting can substantially reduce digestion of dirt. often a poor fitting / sealing carburetor heat or alternate air valve negate the effectiveness of the air filter. plug the induction air inlet(s) if the aircraft is stored in a dusty location.

5. operation on a lower grade of fuel can cause detonation resulting in the need to scrap the cylinder assemblies. over advanced ignition timing contributes to the problem. usually the rings break as a result scoring the cylinder walls.

6. overly aggressive leaning of the engine causes the engine to run hotter especially during take off and climb out.  during these operations there is no extra fuel to help cool the cylinders if aggressively leaned. most fuel systems are designed to supply additional fuel when the throttle is fully open. normal procedure is to take off with full throttle (except on turbocharged engines without automatic wastegate controllers). additionally, an over aggressive leaning technique results in an elevated air/fuel ratio resulting in excess oxidization both in cylinders and exhaust system. expect earlier than normal replacement of cylinders and exhaust system components when aggressively leaning. you either buy the extra fuel or replacement engine parts. a restricted fuel injector will cause lean operation occasionally; the flow divider valve will malfunction and restrict fuel to all the cylinders due to the single restriction.

7. improper assignment of rocker arms on parallel shaft cylinders will cause excess valve stem to guide wear. the exhaust rocker lubricates the exhaust valve with engine oil; an exhaust valve without lubrication wears rapidly, seizes or sticks. the offending cylinder may need to be removed and repaired.

8. cold weather starts, thermal stress, heat soak, touch & go, and shock cooling are all hard on the engine cylinders and pistons. cool or heat the engine at a consistent rate.

9. oil, oil system maintenance also impacts cylinder life. an almost sure way to destroy your inactive engine is to routinely ground run it. the oil never gets up to operating temperature to drive out the moisture contained in the oil as a result of the combustion process. the best maintenance is to fly the aircraft until the oil temperature stabilizes above the boiling point of water. additionally some engine breather-oil-separator systems return water to the engine; good oil separators use warmed air from the vacuum pump discharge to separate out the water. some engine oils have better corrosion prevention characteristics than others.

is it possible for an engine to not require a top overhaul? yes!

a normally aspirated engine (no turbocharger or supercharger) with new cylinders generally should not require a top overhaul if you take care of the items mentioned above. that is providing the aircraft is flown regularly for periods longer than an hour, the oil is changed at least biannually, the air filter is changed yearly, and the engine receives proper maintenance. select new cylinders (not overhauled) when overhauling (higher quality overhaul) your engine and the cylinders will generally last to tbo on a normally aspirated engine. do not procrastinate in dealing with engine problems.

some times ring leakage due to inactivity can be alleviated by soaking each cylinder / piston / rings with a suitable solvent for an extended period. stuck, dragging valves can also be freed by using special shop techniques without removing the cylinders for repair.

what is done during a top overhaul?

the minimum done during a top overhaul is that all engine cylinders are removed, brought back to serviceable limits, and reinstalled. in some cases the cylinders may be bored oversize and fit with new oversize rings and pistons. the work on the cylinders can vary. the manufacture also list parts that must be replaced on a cylinder if it is to be overhauled. some cylinders may require new valves (usually exhaust), and some may require new valve seats or both, almost all require exhaust guides. the cylinders may be overhauled, overhauled exchanged (different cylinders), repaired, new, bored oversize, and / or plated (chrome, cermi-nil) cylinders may be used. when a top overhaul is done, it is necessary to match cylinders the same bore dimension and preferably the same bore treatment. the pistons are removed and sent with the cylinders during the top overhaul (they are numbered to the cylinder position when installed on the engine). removal and installation of the cylinders often requires removal of the cowling, intake system, exhaust system, turbocharger (if equiped), baffles, and other engine component or accessories. sometimes the exhaust is sent out for repair during the top overhaul. the baffles are often repaired or replaced at this time. when replacing the cylinders with new, the pistons and cylinder assemblies are often available in balanced packs. the balanced pack of pistons are balanced in weight reducing some of the dynamic imbalances in the engine. the cylinder shop can also weigh the pistons and assign positions with closest weights being opposite of each other. discuss with the cylinder shop the value of chrome or cermi-nil cylinders if your aircraft is located in a corrosive environment. note: some of the smaller engines do not perform well on chrome bores.

cautions:

1. it is important to keep engine crankcase torques up with the cylinders off and when turning the crank. never turn the crank without any torque on the through studs; the main bearings can shift out of position and block the oil lubricating hole.

2. after a top overhaul, the engine cylinders will require break-in oil due to new rings and fresh cylinder bores (with exception of some turbocharged engines). the owner or operator needs to follow a break-in procedure for the engine recommended by cylinder shop and / or the of the cylinder manufacture. the first take off is very critical and usually performed without the normal run-up to keep temperatures low. usually, high cruise power settings are used to break in the cylinders and rings. touch and goes, stalls, towing and other power interruptions should be avoided.

questions:

why not just remove the bad cylinder and repair it by itself? often, considerable work is involved to remove the center cylinders on a six-cylinder engine. the baffles, the exhaust, and other items are removed to access a single cylinder. the main through stud holding one cylinder on attaches to one on the opposing side of the engine. if the majority of the cylinder compressions are soft in general, the removal and replacement of all cylinders at the same time can save some money. removing all cylinders individually at different times may be a foolish maintenance philosophy.  often, special wrenches are necessary in order to remove the cylinders.  removal of one cylinder often facilitates removing the adjoining cylinder.

does the top overhaul add value to the airplane? no, the top overhaul does not necessarily add value to the aircraft (except turbocharged). the engine should be in safe operating condition if its annual is current.

how much does a top overhaul generally cost? expect the top overhaul to cost about 30-40% of engine overhaul. often there are additional engine baffling and exhaust system repairs cost incurred.  some aircraft require much more labor to remove all the cylinders than others.

how long does it take to do a top overhaul? the time it takes to get your aircraft back in service depends on available parts and assemblies. new cylinders can be installed on most engines in a couple of days if normally aspirated. if you are retaining your present cylinders expect some extensive down time for either the repair or overhaul, particularly if the heads are in need of welding or the bores need bored and / or plated. it may be in your best interest to retain your present cylinders if they are low time / first run. if you suspect your cylinders are in marginal condition you may want take your chances with overhauled exchange.

what are the benefits of a top overhaul? the removal of cylinders for top overhaul allows a cam inspection to be completed and the inspection normally takes about an hour. on many tcm engines the cam followers can be removed and the cam inspected before committing to removing the cylinders. the cam inspection can help to determine if the engine needs to be completely overhauled or replaced due to corrosion and / or spalling. if the cam fails inspection it is not wise to invest in a top overhaul.

when to do the top overhaul?

the top overhaul is typically done at mid-life or later on the turbocharged engine when a majority of the cylinders begin to approach minimum compression readings and or excess oil consumption. the time remaining before tbo should be greater than 25%. that is to say at least 25% of the operating time remains on the engine. make sure the basic engine is worthy of a complete top overhaul before committing.

when not to do the top overhaul?

when the engine is over 20 to 25 years old, the cylinders may often be the first components to fail inspection. the rest of the engine probably has a short amount of residual service life remaining. realistically, one should seriously consider a major overhaul or engine replacement. a top overhaul should not be done when the time remaining on the engine is less than 25% of the tbo; it is not financially practical.

does a top overhaul allow the engine to be operated past tbo? no, the top overhaul is not an engine overhaul and does not meet the faa or some insurance carrier�s requirements of an overhaul.

are there any other considerations or reasons for doing a top overhaul on an engine with less than 25% tbo remaining?

1. the rare case that the aircraft has a time limitation on the airframe and is approaching the serviceable time limit.

2. the aircraft has corrosion damage and may be losing its airworthiness in the short future.

3. the engine is in a teardown inspection or has had a teardown inspection at some time since major overhaul or new.

 

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last modified: march 20, 2007

 

 

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