Several factors to consider
By David J. Pryor
In the complex world of aircraft engines, the debate continues to rage over cylinders at the time of overhaul. Do you repair them if they are deemed repairable by the overhauler, or simply replace them with all-new assemblies? Next, if you choose the new cylinders do you opt for nitrided barrels, or through-hardened?
Here we look at both situations to analyze the subject from a perspective of value, particularly as it affects the safety parameters of your decision, as well as the ultimate cost.
When your engine reaches its TBO — Time Between Overhaul — as recommended by the manufacturer (OEM), it must be torn down, inspected and reassembled before it receives a further green light for operation in a certificated aircraft. All major components and parts must display dimensions and tolerances which fall within the OEM's specifications. Those which don't must be replaced or reworked to meet FAA compliance.
What happens at teardown is the subject of much discussion, and first, we look at how to make an educated decision about cylinder condition.
Many shops believe that cylinders with 2,000 hours on them are repairable — usually by crack welding and barrel honing. What on the surface appears to be a simple decision is actually quite complex. Few people realize that individual engine components do not carry with them a log showing total time since new, even though the engine on which they are installed, does. This means cylinders which may or may not have been repaired could have three or four 'lives' on them — 8,000 hours or more — without the owner or the overhaul shop knowing about it.
Your 'freshly overhauled' engine's main workings — the cylinders — might not be new at all. And metallurgists all agree: metal fatigue does happen in cylinders. Cylinder heads go from standard temperature to 500 degrees Fahrenheit and back each time they run; the pistons within the barrels are cycling more than 4,000 times a minute; and hot gaseous explosions — the stuff that delivers the power — are cooking the metal at more than 1,500 degrees Fahrenheit.
This scenario presumes proper oil lubrication is continually taking place. Below-freezing or desert temperatures put added strains on engines when starting and operating. It's no wonder cylinders develop cracks and brittleness.
Dissimilar metals in play
Remember too that cylinder assemblies are comprised of a lot of dissimilar metals — aluminum alloy, steel, cast iron. Each has a different coefficient of expansion and is, therefore, always 'working' against another. Sooner or later cracks and fatigue will overtake their ability to get along.
Complete cylinder assemblies include barrels, pistons, rings, wrist pins, clips, springs, valves, guides, gaskets, seals, and heads. Essentially, it's everything from the crankcase up.
Benefits of repair?
So why repair the barrels and heads? Many shops want to save the operator money, and believe they are doing so by Tungsten Inert Gas (TIG) welding cracked barrels, and boring to first or second oversize to reduce the cost of the total overhaul. Face it, engine overhaul is not cheap. A typical fuel-injected 200-hp four-cylinder carries an overhaul cost between $15,000 and $20,000, depending on the condition of the part and the degree to which the shop uses new parts. A factory overhaul or rebuild carries a premium of 15 to 20 percent over field, or third-party overhauls. But is it worth the extra cost? This depends on your point of view.
For example, both Lycoming and Continental maintain a policy that they believe proves the adage, 'a chain is as strong as its weakest link.' As an engine comes to the factory for an overhaul (all specifications and tolerances are returned to within factory acceptable limits) or rebuild (in a 'rebuild', the engine is returned to factory-new specifications and tolerances, and returned 'as new' with a zero-time logbook as though it had never been in service), its old cylinder assemblies are scrapped.
Since neither the factory nor anyone in the field can track how many overhauls the cylinders have survived they feel only new assemblies can qualify for continued service. Many fleet operators are of the same opinion, and want only new cylinders installed on their overhauls. If a repaired cylinder fails a newly overhauled, yellow-tagged, freshly painted engine will now attract a whole new invoice. Since the engine must be removed and taken down once again, the cost might now exceed the cost of an engine with new cylinders installed at the initial time of overhaul.
Essentially, the OEM argument is you get what you pay for. While there are many highly qualified overhaul companies vying for your business, only you can decide if the money you might save today needs to be spent within the next 50 or 100 hours when a cylinder develops cracks due to old age, which no one could possibly predict.
Look for cracks
Fresh paint looks great. Fresh metal feels better, according to the OEMs. Again, you might find your engine, if you bought it new, can keep its cylinders if they meet tolerances at overhaul. The important red flag is if cracks are found. Metallurgists strongly suggest that these will never be as strong again even if the cracks are welded.
Places to look for cylinder cracking include the line between the spark plug and the fuel injector aperture. Cooling fins also become brittle with age. Welding and polishing look good on the surface, but the casting never returns to its original contiguity, and therefore remains weak along the weld. Indeed, cracks can now develop and propagate along both sides of the weld, effectively doubling the original problem.
As well, only the most outstanding welders can pass the annual FAA qualifying exam, and even then, he or she can leave tungsten in the weld — an unacceptable fix.
A welded seam also induces thermochemical attack. Corrosion. No matter how good the final weld is the heat-affected zone adjacent to the weldment is prone to poor fusion, and will corrode. The next crack usually starts in the heat-affected zone.
Effect of poorly operated engines
The engine you get back from a field-overhaul could have cylinders from a poorly operated engine — shock cooling being the most damaging. Improperly treated engines - some use the term 'abused' — can develop cracks at an alarming rate. Fast descents from altitude with low power settings are the main culprit of shock cooling.
Reworked cylinders might or might not have been abused and you can never know. The guarantee the factory offers is that no one has ever mistreated a brand new cylinder. For some, that can be money in the bank.
Treating cylinder walls
So, let's say you have made the decision to go with new cylinders. Now you have another choice. And that's the 'hard' decision. When cylinder barrels are cast the steel liners against which the pistons move up and down must survive scuffing, friction, and wide temperature swings, all the while maintaining solid compression through the piston rings. During a typical service life of 2,000 hours, each cylinder will endure more than 550 million strokes from the movement of its piston.
For this, the cylinder walls must be treated to infuse additional hardening. There are two approaches to this requirement: Through-hardening and nitriding. How do you measure the hardness? This is done according to Rockwell "C" hardness numbers, measured on a scale of 1 (softest) to 100 (hardest).
Again, there are two sides to the debate here, each approach having its champions.
More on nitriding
Nitriding is the process of adding nitrogen to the surface of an alloy steel to produce a hard, wear-resistant surface. Commercially, the introduction of nitrogen into the surface layers of alloy steel is brought about by subjecting the practically finished parts to an atmosphere of ammonia gas. The process requires special heat treating furnaces which are air tight and capable of holding the parts at a high temperature. At this heat level (975 degrees Fahrenheit), the ammonia gas flowing into the furnace is broken down into its elements of hydrogen and nitrogen, and this is the source of the nitrogen which penetrates the surfaces of the steel. In order to produce a satisfactory nitrided surface, the process is operated for an extended period of time, generally from 25 to 80 hours.
Through-hardening describes a process of heat treating to infuse a deep consistency to the integrity of the cylinder walls. This process takes effect to a depth of about 1/100th of an inch. Proponents of through-hardening include manufacturers of aftermarket cylinders. They see through-hardening as sufficient for the rigors of aviation piston powerplants to reach TBO. This may indeed be true.
The OEMs use a process called nitriding on their cylinders. After subjecting their cylinders to a through-hardening stage, nitriding is performed. This gives extra strength to the cylinder wall surfaces, to a depth of about .025 inch. Nitriding is accomplished by an ammonia gaseous infusion process and permeates, rather than coats, the surface.
Almost-finished parts are subjected to heat (975 degrees Fahrenheit) and ammonia in an airtight state for between 25 and 80 hours. The ammonia breaks down to its components — hydrogen and nitrogen — and the nitrogen then penetrates the steel. The result is reduced cylinder wall wear, improved piston ring life, the ability to use chrome-plated rings for longer life, and better compression, and an increased fatigue strength.
Rockwell "C" hardness
What Rockwell "C" hardness numbers does each process indicate? Through-hardened cylinder walls have a Rockwell "C" hardness of between 30 and 35. Nitriding adds to this, and factory cylinders test out at 53 to 55 on the scale, to a depth of .025 inch. (To put this depth in perspective, a typical first-oversize re-bore is just .0010 inch). Nitriding has been applied to Continental and Lycoming barrels since 1960. Aftermarket parts makers contend their through-hardened cylinders will achieve TBO, and according to the FAA standards, they should. Each process has its proponents and critics.
Confusion in terminology?
Perhaps some of the confusion lies in terminology. "Through-hardening" sounds — and is — tough. But the factory nitriding process is applied after through-hardening has been done, so naturally nitriding, while not a very positive descriptive, is a much harder surface, as verified in the Rockwell "C" tests.
Engine technology continues to evolve, develop, and improve. Both factory and aftermarket component suppliers produce FAA-approved parts which theoretically are equal, or at least equal to the FAA standards. In the two instances discussed here, the reader must make decisions. Is it acceptable to you to inherit reworked cylinders which may have gone through several TBO cycles? If so, then your choice is easy.
If you want assurances that no one has abused or fatigued your cylinders, then the only guarantee is factory new cylinder assemblies. Statistically, the long-term costs are about the same.
When it comes to choosing between normal through-hardening or hardening plus nitriding for your cylinder barrels, again, it is a matter of choice. It is fallacy to suggest, as some do, that aftermarket parts manufacturers are alone in R&D, while the OEMs twiddle their thumbs. Competition driven by customer demand has driven both OEMs and PMA manufacturers to improve their products.
The main beneficiary of this continued development is the owner of the engine. Over the past 10 years quality and strength have improved, and prices have gone down. Competition is the best prodder of product improvement and price containment, and the engine overhaul market is a perfect example of this theory in action. It's your money, and it's also your choice of overhaul and manufacturing procedures. The more questions you ask, the better educated you will be on this important subject.
There are many opinions when it comes to treatment of cylinders. With so much information on the pro's and con's of the different processes, you need to make the decision that best fits you. Below are companies that manufacture engine cylinders, both OEM and PMA. Contact them for more information on their products and services. As the author suggests, ask lots of questions in order to be better educated on this subject.
San Antonio, TX 78217
652 Oliver St.
Williamsport, PA 17701
Air Parts, Inc.
621 South Royal Lane, Suite 100
Coppell, TX 75019-3805
Teledyne Continental Motors
P.O. Box 90, 2039 Broad St.
Mobile, AL 36615