Twenty years ago, in the heyday of piston powerplant production, things were different. Lycoming and Continental were working three shifts, cranking out three thousand engines a month, mostly brand new engines for brand new airplanes.
When powerplants got old and tired, they would almost always be overhauled in the field. Factory remans were available, but they cost a king’s ransom—in the neighborhood of twenty or thirty thou for a six-cylinder reman. (That was a chunk of change back then!) Owners would opt for a factory engine only if their old engine was a real basket case, or if they were so well heeled that money was no object.
Factory cylinders cost a small fortune, too. New power assemblies (cylinder, valves, rockers, piston and rings) could set you back $2,000 per jug, plus a $400 core deposit. As a result, virtually all overhaul shops used reconditioned cylinders—weld-repaired heads, barrels either bored oversize or chrome-plated back to new dimensions—and maintenance shops did the same thing when top overhaul was necessary.
Welcome to the ’90s, Folks!
My oh my, how things have changed in twenty years! Continental and Lycoming are mere shadows of their former selves, turning out maybe 600 engines a month between the two of them (in a good month), most of them remans destined for twenty-year-old airframes. Both companies have finally figured out that hardly anyone is buying new engines anymore, and have focussed on the aftermarket. Both have slashed the price of reman engines to the point that top-notch overhaul shops can barely compete, and lots of marginal shops (as well as some of the big-name ones like Schneck and Western Skyways) have gone belly-up. Today, factory reman engines account for better than half of the powerplant replacement market.The price of factory new cylindersis half of what it was twenty years ago—more like a third if you consider what has happened to the dollar. Logically, then, you’d expect that nobody would bother reconditioning old cylinders anymore. The compelling economic incentive just isn’t there nowadays.
The Cermicrome Phenomenon
That’s what you’d expect. But you’d be dead wrong. Today the cylinder reconditioningbusiness is alive and more vigorous than it ever was. It has transitioned from a cottage industry to a business dominated by two big firms: Engine Components Inc. (ECI) of San Antonio, and Diversified Manufacturing Company (Divco) of Tulsa.The watershed event in this transformation occurred five years ago when ECI introduced its Cermicrome(TM) particle-impregnated chrome-plating process. Cermicrome cylinders made it through the FAA-mandated 150-hour test cell run in fine shape, and seemed to work well for a couple of Part 135 fleet operators who “beta tested” the cylinders for ECI.
Then, in a tour de force of aggressive marketing, ECI managed to persuade aircraft owners and mechanics alike that its Cermicrome cylinders (which were, after all, simply old worn jugs that ECI weld-repaired, reground, chrome-plated, and impregnated with silicon carbide particles) were actually better than factory new ones!
Droves of owners started demanding that their mechanics and overhaul shops use Cermicrome cylinders. For awhile, all the big-name overhaul shops (Mattituck, Victor, RAM, et al) were recommending Cermicrome as their cylinders of choice. At one point, Continental jumped on the bandwagon and offered Cermicrome cylinders as an option on their factory remans!
Meantime, industry experts like John Frank of the Cessna Pilots Association and John Schwaner of Sacramento Sky Ranch viewed Cermicrome with some skepticism, and quietly recommended their members and customers to adopt a wait-and-see attitude until it had a chance to prove itself in the field for a few years. This recommendation turned out to be spot-on.
We now know that Cermicrome cylinders have not held up well in high-power hot-running turbocharged engines, nor in low-utilization owner-flown airplanes subject to long periods of disuse. There have been plenty of cases where Cermicrome jugs required replacement after as little as 500 hours in service. RAM no longer uses Cermicrome cylinders in its engines, and it appears that other big-name overhaullers are about to follow suit. ECI has started to gradually phase out Cermicrome in favor of a quite different plating process which they’ve dubbed CermiNil(TM). [See sidebar.]
(Coincidentally, Mobil introduced its all-synthetic AV-1(TM) oil about the same time, and marketed it very aggressively. It too became all the rage with owners. Five years later, Mobil withdrew AV-1 from the market because of severe lead sludging problems. But I digress…)
How Cylinders Wear Out
When a cylinder needs to be replaced, it is almost always for one of three reasons: metal fatigue, barrel wear, or valve problems.Metal fatigue failures are the culmination of repeated mechanical and thermal stresses. They are increasingly likely in high-time cylinders, particularly reworked cylinders that have been weld-repaired and kept in service for two or three TBOs. The aluminum head casting gradually becomes embrittled and more vulnerable to cracking.
Head cracks are the most common sort of fatigue failures. They usually emanate from a spark plug or injector hole. Fatigue can also cause catastrophic failure of the head-to-barrel joint.
Fatigue failures are more common in turbocharged and other hot-running engines, particularly if pilots are not meticulous about avoiding rapid throttle and mixture changes.
For example, RAM Aircraft Corp. in Waco, Texas, is a premier overhaul facility that specializes in high-horsepower turbocharged Continental TSIO-520 engines. They were plagued by warranty claims due to head cracks. Finally, in 1988, RAM decided to start using only factory-new cylinders on their engines. Head crack problems dropped precipitously after that.
Barrel Blues
Barrel wear usually manifests itself by increased oil consumption and deteriorating compression test scores. It doesn’t take much wear to do a cylinder in—most cylinders become unairworthy (beyond service limits) if any portion of the bore measures more than .005″ above new dimensions.Fortunately, cylinder barrels incur zero wear during normal climb-cruise-descent operation. This is because there is normally no metal-to-metal contact between the cylinder wall and the piston rings. The cylinder wall is coated by a thin oil film, and the rings hydroplane on this film. For this reason, it’s quite common to tear down a high-utilization Part 135 engine at TBO and see the original hone microfinish along the full stroke.
So why do some cylinders suffer significant barrel wear?
Hot-running high-horsepower engines (particularly turbocharged ones) tend to suffer barrel wear because the high combustion pressures and temperatures can breach the oil film under extreme conditions.
Low utilization is another major culprit. During periods of disuse, the oil film that normally adheres to the cylinder barrel has an opportunity to strip off—particularly if multigrade oil such as Aeroshell 15W-50 or Phillips 20W-50 is used. This has two adverse consequences: corrosion and dry starts.
If the cylinder walls are steel, the loss of protective oil film leaves the barrel vulnerable to corrosion. Rust pitting will eventually destroy the cylinder’s ability to hold compression. Chrome-plated barrels are relatively immune from such corrosion, which is why they are particularly popular in highly corrosive environments (e.g., near the ocean or in humid climates).
Even where corrosion is not a problem, the loss of oil film during periods of disuse results in a dry start—a brief period of metal-to-metal contact between the rings and the cylinder wall until sufficient oil splash has occurred to replenish the oil film on the cylinder walls.
Corrosion and dry starts explain why low-utilization owner-flown airplanes often fail to make TBO or require a mid-time top overhaul. On the other hand, freighters and flight-school ships that fly every day often go well past TBO without needing top-end work.
Cold Starts
Cold starts spell disaster for cylinders. A single unpreheated cold start (particularly at temperatures below 20 F) can inflict more cylinder damage than a thousand hours of cruise flight! Contrary to popular belief, cold start damage isn’t caused by lack of lubrication, but rather by loss of piston-to-cylinder clearance. This requires some explanation.When an engine is cold, there is quite a lot of clearance between the piston and the cylinder walls—usually more than .010″ of clearance. This is necessary because as the engine heats up to operating temperature, the aluminum piston will expand about twice as fast as the steel cylinder barrel will, and the piston-to-cylinder clearance will get a good deal tighter. And that’s okay. But it’s crucial that there always be at least a few thousandths clearance between the piston and the cylinder wall, so that the the oil film is not breached and metal-to-metal contact is avoided.
During a cold start, the piston heats quite quickly, but the cylinder warms up much more slowly because it has vastly greater thermal mass and is covered with cooling fins and bathed in frigid air. Consequently, there is often a period of time—where the piston is up to temperature but the cylinder hasn’t caught up yet—when the piston-to-cylinder clearance can actually go to zero and result in metal-to-metal scuffing of the piston and cylinder walls. That’s why cold starts can be so devastating to cylinders.
Ultra-Low Oil Consumption
Every time a group of aircraft owners get together, it is inevitable to hear at least one or two bragging about ultra-low oil consumption. “I’m using a quart in 40 hours!” These super-low oil consumption figures are often associated with Cermicrome cylinders, and/or with Continental engines equipped with the late-style center-vented oil control ring.The owners who are doing this bragging probably don’t realize that they probably won’t make it to TBO without a costly mid-term top overhaul! It turns out that ultra-low oil consumption is often a bad omen when it comes to cylinder longevity.
Here’s why. The maintenance of the critical oil film on the cylinder walls is acomplished by the oil control ring, a fancy spring-loaded perforated double-ridge ring that receives a supply of oil through small holes drilled through the piston wall and spreads it into a thin film as it moves up and down over the cylinder walls.
The oil control ring is installed in the third piston groove, below the two compression rings that are resonsible for maintaining the dynamic seal of the combustion chamber. Consequently, the oil control ring lubricates most of the cylinder wall, but it never reaches the topmost inch or so where the compression rings reverse direction at top-dead-center—the so-called ring-step area. Lubrication of this critical region can only take place if sufficient oil is allowed to flow past the oil control ring. A certain amount of this oil is inevitably burned up in the combustion process.
If oil consumption is reduced to an ultra-low level by means of a tight-fitting oil control ring (like the new-style center-vented Continental ring) or a super-smooth cylinder wall finish (like Cermicrome), it’s very likely that the ring-step area won’t receive adequate lubrication, and there’s a high risk of metal-to-metal contact between the compression rings and the cylinder wall. A “blued” ring-step area is a sure sign of such lubrication failure.
Experience seems to indicate that oil consumption lower than about a quart in 20 hours may not bode well for long cylinder life. Barrel wear in the ring-step area becomes likely, leading to rapidly deteriorating compression and accelerating oil consumption at 500-1000 hours. Once again, this tends to occur most often in hot-running high-horsepower turbocharged engines.
While low oil consumption has always been acknowledged as a sign of a tight, well-broken-in engine, there is strong evidence that a quart in 30 or 40 may well be too much of a good thing.
Cermicrome cylinders are particularly vulnerable to such ring-step wear. This is because the ceramic-impregnated layer of a Cermicrome barrel is extremely thin—a thousandth of an inch (.001″) at best. Once this very thin ceramic-impregnated layer has been worn through, what’s left is mirror-shiny chrome which is not oil wettable. Once this happens, there’s no oil film in the ring-step area, so there’s nothing to prevent metal-to-metal contact between the compression rings and the cylinder wall. Naturally, things go to hell rather quickly after that.
Stuck Rings
Bad things also happen if too much oil is allowed to reach the compression rings due to a loose oil control ring or an excessively rough cylinder barrel. The compression ring grooves may fill up with oil, the oil may be cooked into varnish by the heat of combustion, and ultimately the compression rings may become stuck and unable to flex or rotate.Stuck rings are usually revealed as a sharp and sudden increase in oil consumption, and often accompanied by oily top spark plugs. If caught early, rings can sometimes be unstuck without cylinder removal by means of a penetrant soak. Sometimes pulling the cylinder is unavoidable.
Exhaust Valve Leakage
If you are fortunate enough to avoid metal fatigue and barrel wear problems, your cylinders will ultimately be done in by exhaust valve leakage. This is unavoidable. Exhaust valves are the most thermally stressed components of the engine. They operate at rediculously high temperatures, so they have to be manufactured from the most exotic and expensive high-temperature alloys (such as Inconel and Nimonic-80).What’s worse, exhaust valve stems must slide back and forth in their guides with extremely close tolerances and virtually no lubrication. Any oil introduced into the guide would quickly be fried into varnish by the extreme heat. Lubricant would also interfere with the critical stem-to-guide heat path through which the exhaust valve sheds its heat. Consequently, metal-on-metal contact between the valve stem and guide can’t be avoided, and guide wear is simply a fact of life.
As the guide wears, it can no longer hold the valve in perfect alignment with the seat. The valve starts to wobble and no longer seals perfectly against the seat every time it closes. Hot exhaust gas leaks between the valve and the seat, causing both to overheat and warp. The warpage allows even more exhaust to leak, which results in even more overheating and warpage. This condition is commonly referred to as a burned valve. Once leakage starts, compression deteriorates rapidly. If not detected in time, the valve may fracture and a catastrophic engine failure may result.
Continental and Lycoming have made numerous changes to exhaust valve and guides in order to increase TBOs. In the 1960s, valve guildes were usually made of bronze which was relatively soft and didn’t wear well. Both manufacturers have switched to harder aluminum-bronze alloy and cast iron “ni-resist” guides, and Continental even tried super-hard nitrided steel “nitralloy” guides for awhile. Harder valve guides demanded harder valve stems, so exhaust valve stems are now often chrome plated.
These valves and guides are capable of making it to TBO and beyond if everything goes just right. But if it doesn’t, they won’t.
A common cause of premature valve problems is failure to lean sufficiently, particularly during ground operations. Rich mixtures and low combustion temperatures will cause a build-up of lead salts and other combustion byproducts on the valve stem. This buildup tends to be crusty and abrasive, and it can quicly abrade the lower portion of the valve guide into a bell-bottom shape, allowing valve wobble, leakage, and burning.
If an overhaul or cylinder shop isn’t meticulous about guide-to-seat concentricity or rocker arm geometry, the valve is sure not to make TBO. This seems to be a disturbingly common problem. We’ve even seen quite a few reports of Continental factory-new power assemblieswith serious valve misalignment problems right out of the box. We’ve talked to several top-rated overhaulshops who tell us that they don’t dare install a factory-new cylinder without first checking valve alignment. (Really makes you wonder about TCM factory remans, doesn’t it?)
Cylinder Longevity Tips
Here’s our advice about how to increase your odds of nursing those jugs to TBO without intermediate top-end work.Be careful about what cylinders you have installed on your engine. Don’t try to recondition a cylinder too many times. The likelihood of head cracks and separations increases after about two TBOs time in service. Avoid exchange cylinders like the plague—you have no way of knowing where those heads have been.
Be sure your overhaul or cylinder shop reams the valve guides to be precisely concentric with the seats. Concentricity needs to be checked even with factory new cylinders.
When you fly, become obsessive-compulsive about thermal cycles. Avoid rapid throttle and mixture changes. Throttle-up slowly and smoothly during takeoffs and go-arounds. Avoid high-speed low-power descents. Avoid going full-rich on final approach if your engine is fuel injected.
Fly often. Avoid extended periods of disuse. If you can’t, hangar your airplane and consider using single-weight oil such as Aeroshell W100 for corrosion protection. There’s nothing wrong with using multi-grade oil during the coldest months and switching to single-weight oil for the rest of the year. In fact, that’s an excellent procedure.
Never cold-start without a preheat. Don’t even consider it. An unpreheated start below 32 F is harmful. Below 20 F it’s a capital offense. A night in a heated hangar is the best preheat. Sleeping late in the morning is also a useful technique.
If you need to reposition your airplane on the tarmac, don’t taxi it if you can have it towed. Remember that most barrel wear occurs at engine start. So try not to start your engine unless you’re going flying.
Lean aggressively. Particularly avoid full-rich mixture during ground operations. Rich mixtures and low combustion temperatures often result in accelerated exhaust valve guide wear.
Beware of ultra-low oil consumption. An engine needs to burn some oil in order to achieve needed lubrication of the critical ring-step area. 10 to 15 hours per quart is great. It’s perfectly normal for oil consumption to increase toward the end of the oil change interval.
Shun new cylinder coatings, rings, valves, guides, rockers, and other wonderful-sounding innovations until you’re sure that they’ve been in the field long enough to prove their ability to make TBO in your type of operation.
When to Replace a Jug
Sometimes it’s necessary to pull a cylinder and rework or replace it. But such top-end work is often done unnecessarily. Top overhauls (replacing some or all cylinders at mid-TBO) is one of the most over-sold maintenance procedures in general aviation.It is common practice at many shops to pull any cylinder that measures less than 60/80 on a differential compression check, and to recommend replacement of all cylinders if two or more cylinders measure that low. Some IAs simply refuse to sign off an annual if any compression reading is less than 60/80. Such procedures are simply unfounded and erroneous.
Never allow a cylinder to be pulled on the basis of a single compression test. For one thing, the standard differential compression test is notorious for giving non-repeatable results. A cylinder that tests 55/80 today might easily test 68/80 after two more hours in service. Mechanics should treat compression readings the way doctors treat blood pressure readings: no conclusions should be drawn until at least three successive measurements have been taken to establish a baseline. In the case of aircraft engines, the measurements should be separated by at least a few hours of operation.
Furthermore, there’s nothing magic about 60/80. It’s quite common for some engines to operate quite happily with compression readings in the 50s. Anytime a questionable compression reading is observed, it’s important to determine where the compression is being lost. If air can be heard escaping from the exhaust pipe, then the exhaust valve is leaking…a potentially serious problem, and one likely to deteriorate fairly quickly. On the other hand, if air is heard coming from the breather line or oil filler cap, the leakage is coming past the rings…a much less worrisome situation. In fact, low compression readings due to leakage past the rings can probably be disregarded unless it is accompanied by an alarming increase in oil consumption.
If a cylinder exhibits a deteriorating compression trend over several readings, or if low compression is confirmed by at least one additional symptom (elevated oil consumption, rough running, anomalous EGT readings, metal in the filter, etc.), go ahead and pull it. But don’t let the mechanic talk you into “topping” all the cylinders just because one has gone soft. A complete top overhaul is seldom justified (unless part of a carefully planned TBO-extension program).
Top or Major?
Suppose you have a relatively high-time engine—let’s say a few hundred hours from published TBO—and a cylinder goes soft. Should you repair the cylinder, or simply bite the bullet and pull the engine for major?One approach to answering this question involves simple arithmetic. If your airplane is a Skyhawk, your engine probably has a 2000-hour TBO and an overhaul cost of $8,000, which works out to $4/hour. To justify an $800 cylinder replacement, you’d need to be reasonably confident that it would buy you at least another few hundred hours. If, on the other hand, you’re flying a P210 or Malibu with a 1600-hour TBO and an overhaul cost of $27,000 (which works out to $17/hour), then a $1,200 cylinder replacement could make sense if it delays the major overhaul by even 100 hours.
New or Reconditioned?
If you do decide to change out a weak cylinder, should you grind it oversize, have it plated, exchange it for a chrome or Cermicrome cylinder, or buy a factory new jug? The answer depends on exactly what you’re trying to accomplish by the cylinder change.If you’re simply trying to buy another couple of hundred hours before springing for a factory reman engine, then it makes sense to do something cheap. Maybe a new exhaust valve and guide, some new rings, and a light hone is all you need. If your jug is beyond service limits, consider exchanging it for a cheap reworked jug. After all, that cylinder is going on the scrap heap in a few hundred hours.
On the other hand, if a cylinder problem arises at low- or mid-time, then you want to make sure the replacement cylinder will take you the rest of the way to TBO. Consider buying a new cylinder (factory or PMA), or having your first-run cylinder rebarrelled or chromed by a top-notch cylinder house.
The Bottom Line
As we pointed out at the outset of this article, Continental and Lycominghave become downright aggressive about pricing and marketing of their jugs.A factory-new complete power assembly for a big-bore engine costs about$1,100 or $1,200 these days. At these prices, it’s hard to resist.A new Millennium jug from Superiorcosts about $100 more than an OEMcylinder. If field experience proves Millenniums to be as durable as theylook, they’ll be worth the slight premium. Mattituck already offersMillenniums with a warranty pro-rated to TBO, something not offered with OEMcylinders. Other shops may offer similar extended warranties.
It looks like CermiCrome is gradually being phased out by ECI in favor ofCermiNil. Costwise, a reworked CermiNil cylinder can save you around $400apiece compared with factory-new, a savings of about 35% on a complete powerassembly. (The same applies to NuChrome.) If you include the $300 or so inlabor that it takes to remove and replace a cylinder, the cost savingsbecomes more like 25%.
If it’s major overhaul time and you’re replacing six big-bore jugs, CermiNilor NuChrome could save you $2,400 over new cylinders. That seems like a lot.But don’t forget that a reworked cylinder is a good deal more likely to failprior to TBO than a new one. Some failures can get downright exciting, whileothers are just costly. If you have to top two jugs on the way to TBO, your$2,400 savings just vanished (and then some if you include R&R labor). Andalthough the early reviews of CermiNil jugs have been positive, we’d like tosee some more real-world experience with them before we jump on thebandwagon.
If you do decide to go with CermiNil or NuChrome because of its lower costand/or corrosion resistance, we think it makes sense to have your ownfirst-run cylinder core reworked. This will involve some downtime, but atleast you’ll know what you’re getting. We’d steer clear of exchangecylinders—there’s no telling how much time, abuse, or repair work they’veaccumulated before you get them.
Be particularly wary of the bargain basement cylinder kits that you can findadvertised in any issue of Trade-A-Plane. The quality of weld repairs,chroming, oversizing, rebarrelling, and other cylinder reconditioningtechniques varies all over the map. As with most things, you usually getwhat you pay for. A low-ball reworked jug might make sense if you’re justtrying to buy another 100-200 hours until major.
Otherwise, it’s hard to go wrong with a brand new Continental, Lycoming, orSuperior cylinder assembly. New cylinders have never been a better bargainthan they are right now.
Anatomy of a Factory Cylinder
Piston aircraft engine cylinders are more complicated than you might expect, both in their construction and their metallurgy. In evaluating cylinder reconditioning options, it’s helpful to have a good understanding of how a factory-new cylinder is created.
Each cylinder starts out with the cylinder head, a very elaborate finned sand-casting of aluminum alloy. The raw casting undergoes a complex series of precise numerically-controlled milling, boring and tapping operations that create holes for the spark plugs, injector or primer nozzle, CHT probe, valve guides, valve seats, rocker shafts, pushrod tubes, intake and exhaust port studs, plus threads for the head-to-barrel joint.
The cylinder barrel is machined from a large forged steel billet. The billet is bored to the required inside diameter and turned to create the base flange, barrel fins, and head-to-barrel threads.
The inside of the barrel is then hardened using a nitriding process in which the barrel is placed in an oven, heated to about 1000 F, and exposed to ammonia gas. The ammonia liberates nitrogen which impregnates the surface of steel barrel bore and hardens it. Nitriding creates a very hard and durable wear surface five to fifteen thousandths of an inch thick.
Nitrided steel barrels were adopted by both Continental and Lycoming in the early 1970s. They were instrumental in the extension of TBOs from 1200-1500 hours in the `60s to 1600-2000 hours today. (Improved valves, guides, and cams helped, too.)
After nitriding, the steel barrel is mated to the cast aluminum head. This is accomplished by heating the head in an oven, chilling the barrel in a refrigerator, and then quickly screwing them together. As they return to equal temperatures, the head contracts and the barrel expands to create a tight interference fit. After the head and barrel have been mated, the cylinder base flange mounting holes are drilled.
The next step is to grind and hone the cylinder bore to its final fit and finish. The bore must be exactly the proper diameter and perfectly round, within less than a thousandth of an inch of new specifications.
The bore is not precisely cylindrical—it is tapered in the top two inches of piston ring travel, usually by .003″ to .007″. This choke is used to compensate for the fact that when the cylinder is at operating temperature, the top of the barrel is considerably hotter than its base, and therefore it expands more. If the cylinder were not adequately choked at room temperature, the piston-to-cylinder clearance at top-dead-center would become loose and sloppy as the cylinder heats up, and the rings would flex excessively.
Finishing Touches
Once the cylinder dimensions are precisely correct, the cylinder bore is honed to create a very fine crosshatch pattern of scratches, typically 25 microinches deep and at a 35 angle. This microfinish is crucial to ensure proper break-in, cylinder lubrication, and ring rotation. A perfectly smooth cylinder wall won’t hold an oil film, while an overly rough one will result in accelerated wear and poor ring sealing.At this point, the cylinder is complete except for valves. This is known in the trade as a stud assembly.
Valve guides and seats are press-fit into the cylinder head (after heating it). The guides are normally reamed and honed in place to ensure perfect concentricity with the seats as well as a close-tolerance fit with the valve stems. Then the valves, valve springs, valve retainers and rotators, rocker arms and rocker shafts are installed. At this point, we have what is known as a valve assembly.
Add a piston, piston pin, and a set of rings (two compression rings, an oil control ring, and sometimes a scraper ring) and you have a complete power assembly. Replacement cylinders are most often ordered in this form.
Whew! Is it any wonder that new cylinders ain’t cheap?
All About Reconditioned Cylinders
Back in the bad old days when factory-new jugs were both exorbitantly priced and in short supply, the industry developed (and the FAA approved) a variety of reconditioning techniques to permit old worn-out cylinders to be given a new lease on life. Today, factory cylinders are far less costly and far more available, but reconditioned jugs still abound and most overhaul shops still use them unless you specify that you want new cylinders.
The cheapest way to recondition a cylinder is to bore it oversize and fit it with oversize pistons and rings. Continental offers pistons and rings in .010″ and .015″ oversize versions. Lycoming offers .010″ and .020″ over.
However, the advent of nitrided steel cylinders has thrown a monkey wrench into the works. Nitriding hardens the surface of a steel barrel to a depth which varies between .005″ and .015″. If you grind a nitrided barrel to .010″ oversize, the new surface is likely to have some places that are still hardened and other places that aren’t. Such a cylinder will wear irregularly (not good). Go to .015″ or .020″ over and there probably won’t be any of the nitride-hardened layer left at all.
Lycoming does not allow oversizing of nitrided cylinders except in a handful of low-compression engines. But Lycoming’s service instruction doesn’t have the force of law, so cylinder shops still oversize these jugs all the time. Continental prohibits oversizing only in their fire-breathing 375 hp GTSIO-520 engines. But that doesn’t change the laws of physics, and doesn’t make oversizing of nitrided jugs a good idea. It isn’t.
Chrome Plating
Moving a notch up the cylinder rework food chain is chrome plating. Here, the worn barrel is ground oversize, and then a layer of chrome is deposited via electroplating to bring the cylinder back to new dimensions. (A good chrome plating job is about .015″ thick. A bargain basement one might be a lot thinner.)Standard pistons can then be used, but chrome cylinders require special cast iron rings (instead of the chrome-plated rings used with steel cylinders). An often-overlooked disadvantage of chrome cylinders is that the relatively soft cast iron rings wear out faster than ordinary chrome rings do.
Chrome is a very hard and durable wear surface—even more so than nitrided steel—and has the additional advantage of being almost immune from corrosion. However, a smooth shiny chrome surface is not oil-wettable, so something must be done to the chrome to allow an oil film to adhere to it.
The traditional solution to this dilemma, used successfully for decades, is channel chrome. In this process, when chrome has been electroplated to the desired thickness, the current flow in the plating tank is reversed for a short (and critical) period of time. This results in a chrome surface that isn’t smooth but has numerous microscopic fissures (called channels) that provide a “foothold” for oil to adhere.
There are a couple of problems with channel chrome. The channelling process is apparently more black art than precise science, and it’s difficult for even the best plating firms (such as ECI in San Antonio) to get consistent results. If the channels are too shallow, the cylinder won’t make TBO. If they are too deep, oil consumption will be high. Even the very best channel chrome cylinders tend to burn a lot more oil than steel.
Lots of folks love chrome. They believe that its durability and corrosion resistance are worth the tradeoff in oil consumption. Chrome is a particularly good choice for operators with extreme vulnerability to corrosion, such as salt water floatplanes and highly seasonal operations.
Cermicrome and Nu-Chrome
In an effort to come up with a chrome cylinder with low oil consumption, ECI secured a license for a new cylinder reconditioning process that ECI dubbed Cermicrome(TM). Cylinders are bored oversize and electroplated with chrome, just as with channel chrome. But instead of channeling, Cermicrome uses tiny silicon carbide particles that are mechanically impregnated into the chrome surface. The particles have a strong affinity for oil, and provide the necessary foothold that allows an oil film to adhere to the surface.Unlike channel chrome, the Cermicrome process yields consistent, predictable results. The relatively smooth finish supports a much thinner oil film that provides far lower oil consumption than channel chrome—even lower than steel in many cases. And the carbide particles act like a polishing compound for cast iron rings, providing extremely quick break-in.Cermicrome was an instant smash hit, and Cermicrome cylinders started selling like hotcakes. For awhile, ECI had a lock on the market. Eventually, a similar competing process called Nu-Chrome(TM) was developed by Aircraft Cylinders of America. Nu-Chrome cylinders are distributed by Diversified Manufacturing Corp. (Divco) in Tulsa. Our comments about Cermicrome apply to Nu-Chrome, too.
It took a few years before there was enough field experience with these particle-infused chrome cylinders to discover that they have one significant shortcoming: they don’t wear gracefully. The problem is that the mechanically-impregnated particles penetrate the surface layer of the chrome plating only to a depth of perhaps .001″. Chrome is very hard and silicon carbide particles are even harder, so it takes a lot to induce wear. But if dry starts, cold starts, or inadequate lubrication manages to wear a Cermicrome cylinder as little as .001″, the particle-infused layer is gone and what’s left is shiny chrome that won’t hold an oil film. Once this occurs, the cylinder is doomed. It’s been known to happen in as little as 500 hours.
What’s worse, there’s no easy way to rejuvinate a worn Cermicrome cylinder in the field. You can’t just hone it and put it back in service with new rings, the way you might do to clean up the ring-step of a steel cylinder. You certainly can’t grind it oversize. There’s basically no alternative but to have the clinder put through the entire Cermicrome process again, or to swap it for an exchange cylinder.
The bottom line is that Cermicrome and Nu-Crome work well for low-horsepower high-utilization aircraft where barrel wear isn’t usually a problem. But high-horsepower low-utilization aircraft may not get good longevity from these cylinders. Most owner-flown aircraft fall into the low-utilization category, and many are also high-horsepower as well.
One might well argue that cylinder longevity is irrelevant in a truly low-utilization aircraft. If the aircraft flies only 100 hours per year, who cares if the cylinders make 2000 hours TBO? The cam and lifters will most likely rust out long before the cylinders wear out.
CermiNil
In mid-1994, ECI announced yet another cylinder plating process that they call CermiNil(TM). In this process, the cylinder is plated with a particle-infused nickel-based coating instead of chrome. Like Cermicrome, CermiNil uses silicon carbide particles to increase wear resistance, speed break-in, and improve oil adhesion. But unlike Cermicrome, the silicon carbide particles in CermiNil permeate the entire thickness of the nickel coating, not just the surface. So CermiNil should wear much more gracefully than Cermicrome, and it should be possible to re-hone CermiNil in the field.Nickel is not nearly as hard as chrome. However, CermiNil contains three times the concentration of silicon carbide particles as the surface of Cermicrome does (7-10% by volume for CN, compared to 2-3% for CC), and silicon carbide is incredibly hard (think of it as man-made diamond dust). Consequently, ECI believes that the CermiNil composite should resist wear and corrosion just as well as Cermicrome. And when it does wear, CermiNil won’t lose its oil wettability.
Chrome plating (including Cermicrome) is accomplished with the cylinder assembly intact, but the CermiNil process requires that the barrel be de-mated from the head before it is plated. ECI actually touts this as an advantage, since the head-to-barrel joint can be inspected with dye penetrant, the threads cleaned up, and an epoxy sealant applied during reassembly.
Another advantage is that nickel is much more eco-friendly than chrome. The EPA is making life more and more difficult for chrome plating firms, and that situation can only be expected to get worse.
All in all, CermiNil looks very promising to us…on paper, anyway, But frankly, Cermicrome looked awfully good five years ago, too. The fact is that there’s simply no substitute for field experience, and CermiNil hasn’t yet had enough to be meaningful. Will CermiNil make TBO in high-horsepower low-utilization aircraft? Nobody really knows. We’re inclined to take a wait-and-see attitude about CermiNil until it has been in the field for a few years.
Rebarrelling
One more cylinder reconditioning technique is simply to de-mate the head from the barrel, discard the old barrel, and screw a new steel barrel into the old head. For many years, ECI has been the predominant cylinder rebarreller. ECI’s replacement barrels are made of through-hardened steel and are not nitrided.Lately, ECI has started offering replacement steel barrels with silicon carbide particles impregnated into the surface. As always, they have a catchy name for this: CermiSteel(TM).
Heat Treating
Regardless of how a cylinder is reconditioned—bored oversize, plated to new dimensions, or rebarrelled—an old cylinder head is being reused. As it accumulates thousands of hours in service, the aluminum head casting becomes more brittle and prone to cracks. Most of the time, cylinder reconditioning requires weld-repairing of minor cracks that commonly develop in the exhaust port area of the head. Even the most careful welding can heat-stress the head casting and make it more susceptible to future cracking.A few years ago, ECI introduced a heat treating process for cylinder heads that they claim will restore the original crystalline structure and metallurgical properties of a cylinder head, and thus reduce the likelihood of head cracks. They call this process IFR(TM) (Improved Fatigue Resistance), and charge an extra $120 per cylinder for it. ECI has an impressive collection of engineering data and electron microscope pictures to support their claims for this process. Frankly, it hasn’t been in the field long enough for anyone to know how effective it is in preventing head cracks. At this point, all we can say is that it probably couldn’t hurt.
Superior’s Millennium Cylinders
One of the most exciting cylinder developments is the advent of a second source for new cylinders other than Continental and Lycoming. The new source is Superior Air Parts of Dallas, Texas.Superior is the largest manufacturer of replacement parts for piston aircraft engines, and holds FAA parts manufacturing authority (PMA) for some 1,700 different parts for Continental and Lycoming engines. Superior parts are widely used by top overhaul shops like Mattituck and RAM, and by all accounts are every bit as good as OEM parts.
Two years ago, Superior introduced its Millennium(TM) brand cylinder for Continental O-200 engines (also approved for O-300, GO-300, and various older A- and C-series engines). Six months later, they received approval for a Lycoming O-235 cylinder. Recently, Superior started shipping cylinders for parallel-valve Lycoming 320, 360, and 540 engines. Millenniums for big-bore Continental 470 and 520 engines are anticipated by spring of 1995, and a cylinder for the angle-valve Lycoming TIO-540J2BD is also in the works.
We took a close look at the Millennium cylinders, and frankly we were impressed. Superior has not simply duplicated the OEM cylinders—they’ve made a bunch of nice improvements.
The most striking feature of the Millennium cylinders are their head castings. While Continental and Lycoming both use sand-cast heads, Superior employs a technique called “investment casting” that results in a far denser, less porous casting with smoother surfaces, more consistent dimensions, and fewer inclusions (bubbles). In addition, Superior has beefed up various areas of the OEM cylinder head that are historically susceptible to cracking: exhaust ports, valve guide bosses, rocker shaft supports, and injector and sparkplug holes. Intake ports and cooling fins are noticeably smoother than OEM head castings, and that should provide better airflow.
Superior mates these heads with through-hardened steel barrels. These are not as hard as the nitrided barrels that Continental and Lycoming use, so they probably won’t wear as well in problem cases. On the other hand, the Millennium barrels can be reground and honed without any of the problems associated with oversizing nitrided jugs.
Our natural skepticism leads us to suggest waiting a few years before buying Millenniums for your big-bore Continental or slant-valve Lyc. We’re particularly interested in seeing how the through-hardened barrels hold up. But these sure look like damned good jugs. As a bonus, competition should help keep the price of OEM cylinders in line. Good show, Superior!
TCM Cylinders…
Engineering or Trial-and-Error?
It’s amazing how different Lycoming and Continental are in their engineering approach.Lycoming is a strong believer in not fixing things that aren’t broken. Year after year, Lycoming turns out high-quality engines, and very seldom makes any engineering changes.
In contrast, Continental seems to be perpetually tinkering with the design of their engines. Sometimes these changes turn out to be improvements, and sometimes not. In any event, those of us who fly Continental-powered aircraft wind up being the “beta testers” for TCM’s tinkering.
Take Continential IO-520 and TSIO-520 engines, for example. In the last 20 years, these engines have received larger-diameter crankshafts, heavier cases, VAR-process crankshafts, and several camshaft changes. But the most interesting changes occurred in the cylinder assemblies of these engines.
Up until the early 1980s, TCM used forged all-aluminum dome-topped pistons, bottom-vented oil control rings, and well-choked nitrided-steel cylinders. Most of us had excellent luck with this combination. But TCM became concerned about the rate of warranty claims due to cylinders with excessive oil consumption, premature ring-step wear, and occasional compression ring breakage. So they started tinkering.
In an attempt to reduce the frequency of these warranty claims, TCM decided to take most of the choke out of their cylinders, and to redesign their piston from the ground up. The new “steel belted” piston was a flat-top piston made of cast (not forged) aluminum with a steel insert to reinforce the top compression ring groove. The purpose of the steel insert was to provide better support for the compression ring, needed beause the elimination of cylinder choke caused the ring to flex much more than before. The new piston was first introduced in the 375 hp GTSIO-520 engine, but soon propagated to all 520-series engines.
At about the same time, TCM introduced a new center-vented oil control ring that dramatically reduced oil consumption by metering less oil to the cylinder walls.
But even as the old problems were addressed by these changes, new problems emerged. Engines started developing barrel cracks of a kind that had seldom been seen before. The barrel cracks occurred on the bottom of left-bank cylinders and the top of right-bank cylinders. In some cases, catastrophic separation resulted.
The reason for these barrel cracks became pretty obvious. The new steel-belted piston was much heavier than the old all-aluminum piston. The heavier piston was creating greater stresses in the skirt area, and barrels were cracking as a result.
TCM came out with several versions of beefed-up cylinder barrels, and issued a service letter (which became an AD) calling for repetitive 33-hour inspections of old-style barrels. Unfortunately, the new barrels didn’t make the cracks go away.
Ultimately, TCM recontoured the cam-ground area of the piston skirt so that stresses would be spread over a larger area of the barrel. It’s starting to look like this change cured the cracking problem. Still, many engine experts believe the heavy steel-belted piston was a big step backwards. Until recently, old-style all-aluminum pistons were available from ECI, but they have now been discontinued. Several companies (including Superior) are seeking PMA approval for forged aluminum pistons for the 520-series engines.
In the meantime, it’s starting to look as if the center-vented oil control ring may have reduced oil consumption too much, at least in higher-horsepower and turbocharged versions of the 520. There have been a rash of premature cylinder wear problems that appear to be due to inadequate barrel lubrication. Lots of brand new Malibu engines lost compression after 400-500 hours, and exhibited the tell-tale “blued” ring-step characteristic of lubrication failure. The same thing happened with brand new IO-550-powered Bonanzas, including some flight school fleets that flew every day. In some cases, TCM quietly and verbally authorized modification of the oil control ring expander spring to permit additional oil film thickness. And they appear to have modified the expander in new production rings (although they didn’t announce this or change the part number).
It has become a favorite passtime of aviation magazine writers to bemoan the lack of innovation coming out of Continental and Lycoming. But innovation can be painful when you’ve been involuntarily appointed as a guinea pig to test it.
