During that week your airplane is in for its annual, you have to dread the phone ringing. Or worse, the message that starts out, Yeah, this is Joe at AeroMech, we found three soft cylinders on your engine and need to know what you want us to do.
At that point, you have two options, neither pretty. Return the call and get the details behind the headlines or drive out to the airport, allowing a few quiet moments for 600 milligrams of denial to kick back the pain of a $6000 maintenance invoice. Yet most owners will face the cylinder replacement dilemma, either at a mid-time top or an overhaul. And then the recriminations start. Where did I go wrong? Should I have picked another cylinder type? What should I do now? Should I nurse the engine along or do a mid-time top overhaul?
Boiling down the choices, it sorts like this:
Do limited or spot repairs to specific problem cylinders.
A local complete top using your own reconditioned cylinders or remans.
Install new OEM or PMA manufactured cylinders.
The Road to a Top
Often, one answer lies in your logbook. Inactivity continues to be the bane of engine longevity, something we see everyday as airplanes flown 20 hours since the last annual come into the shop with corroded jugs. A frequently flown airplane may break now and again, but rusted cylinders wont be a problem.
Yet in many cases, an airplane flown regularly will still encounter unexpected cylinder problems or premature failure. By the term premature failure, we dont necessarily mean a jug that tanks in the first few hundred hours or within the warranty period.
For the scope of this article, we refer to a total-time-in-service failure to occur somewhere about mid-time, or within about three-quarters of the way to recommended TBO. By failure, what do we mean?
Some definitions of terms, first: Outside of clear-cut mechanical failures such as cracked cylinder heads or stuck valves, the most common cause of having to do premature top-end work is low compression or high oil consumption, sometimes resulting in fouled lower plugs.
Both Continental and Lycoming have specifications for their engines on both counts and you should check the specs for your engine, not rely on airport folklore. TCM even goes one step further and calls for a special orifice/restrictor assembly to be used when measuring compression differentials with compression bleed-down gauges. Dont let an A&P talk you out of doing the manufacturer-specified compression check before you resort to drastic action. And get a second opinion, even if it means calling the factory direct.
Also, prior to putting wrench to metal and if the situation warrants it, run your engine around the patch a time or two and check compression again. Often, rings can be lined up with the gaps all in a row or there can be carbon stuck under a valve, skewing the compression readings. High oil consumption is another matter. Sometimes cylinder barrel glazing-lack of proper ring seating-can be the culprit and a hard run at full power with mineral oil might cure it without having to disassemble the cylinders. In desperation, it may be worth a try before topping.
Stop at a Top?
If you have to top, at what point does it make more sense to major it instead? This is a tricky decision to make when the numbers reach some point near or beyond mid-time for your engine. Obviously, at 600 hours into the run with soft cylinders and a solid bottom end, a top makes sense. At 1400 hours, it might not.
If your engine is near its recommended TBO, say beyond the two-thirds or three-quarters benchmark or 12 to 15 years since its innards have seen daylight, its nearly always economic to major it and often uneconomic to top it. The top-only strategy at high-time often leads to a sad why-did-I-do-that? sentiment later on.
A halfway measure-economically speaking-is to do a power-section- only overhaul on the engine core, sans the accessories, mounts, hoses and such, rather than the typical complete overhaul. Remember, major overhauls add measurably to Bluebook value while tops rarely do.
One or All?
If you have to top, the next decision is to do it surgically on one jug or the whole lot, or something in between. Logic suggests that since all the cylinders have been living and breathing off the same block, subject to the same air/fuel mixture, when one goes kaput, the others arent far behind. Sometimes thats true, often its not. It depends on why the suspect cylinder failed. The problem in doing low-compression/high oil consumption patch jobs is that youre buying the labor to remove nearly all of one bank of cylinders but you end up replacing or repairing only one jug per side.
Most of the baffling and the exhaust and intake manifolds have to be removed and so on. For another hour or two of labor, you can yank and inspect the other cylinders.
Failure to grasp this concept sometimes results in what we call progressive tops or eventual complete top overhauls done one or two cylinders at a time over a period of several years, with the owner paying for overlapping labor to R&R the basics and eventually doubling his costs.
Exchange, Fix or Buy?
Years ago, most shops had both the talent and equipment to do basic top overhauls in-house. No more. Now its the rare general airframe A&P who can even measure or inspect cylinders properly, grind or re-seat valves and perform quality top overhauls in-house. Most local repair shops send cylinder assemblies to specialty overhaul shops for repair.
So the choice is to send your cylinder assemblies out for spot repairs, rebuild or remanufacture them or opt for an exchange or new cylinder. Time generally rules this choice, as anxious owners opt to have exchange (or new) cylinders bolted on in order to get them back into the air within a day or two. This compares to weeks or longer to have your own cylinders re-worked by a specialty shop and re-installed by your local guys.
If you know for a fact that you have first-run or low-time cylinders, our advice is to not give them up for un-known used/reman cylinders of dubious total time which may be floating around in the exchange pool. Smart owners know these facts prior to making this decision as theres a probability that a high-time cylinder will have problems down the road.
This is one reason why Lycoming and Continental have dropped their new cylinder prices, spurred on further by competition from ECI (Engine Components, Inc.) and Superior Air Parts. The OEMs wish to lessen their liability by reducing exposure to cracking and other faults.
If youre not going to recondition your own first-run cylinders and/or you cant wait on that process, it often makes good economic sense to install new OEM or PMA cylinder assemblies rather than rebuilt used cylinders of dubious history.
Generally speaking, limited repairs means you do only what is mandated to only the cylinder in question and as economically as possible. When faced with low compression or high oil consumption, only the offending cylinders are removed, valves ground, perhaps new rings installed and the barrel honed to seat new rings.
This type of limited repair makes sense for an engine near TBO, the assumption being it will patch things together to nurse the engine along for another few hundred hours until major.
There are lots of variations on the limited repair, but generally you shouldnt expect to replace exhaust valves or guides or perform major machine work. Costs? About $700 to $1000 if only one cylinder is removed and about half again as much if another needs work.
The next step up, a top using your own cylinders, would generally mean replacing exhaust valves and guides and new piston rings. If you expect your upper-end work to last more than 300 to 500 hours, you should overhaul them back to OEM new limit specifications rather than the more liberal service limits. If the barrels dont pass new limit specs, you can have them chromed, treated with CermiNil by ECI or bored and honed .010-inch oversize with Lycoming or .015-inch oversize with Continental.
Many readers have questioned us on the difference between chroming, CermiNil and new cylinders, which is why were running the trial described on page 14 of this issue. Preliminary testing and field results indicate that if barrel re-finishing is called for, ECIs CermiNil may be the process of choice, long term.
Boring .010-inch (Lycoming) oversize mandates new oversize pistons and given the economics of CermiNil (a composite nickel/ceramic process) and the fact that many original pistons can usually be re-used, CermiNil may have the edge, economically if not mechanically.
Many shops still use channel chrome refinishing as an alternative to boring oversize and both CermiNil and channel or satin chrome offer further advantages in that these finishes arent susceptible to rust during periods of typical inactivity.
Except for approved over-sizing of the barrels, when you remanufacture cylinder assemblies, you take them back to OEM new limits with few exceptions. Its possible to do this both to your existing cylinders or to order reman assemblies from numerous vendors on an exchange basis. If you have a known commodity of first-run or low-time cylinders with no defect, dont give them up. Even though it may take six weeks to have them remanufactured and the barrels refinished (if necessary) to new limits, we would recommend you do that, unless the downtime is intolerable. Typical cost for remanufactured cylinder assemblies is $600 to $800, not including R&R labor.
The new cylinder choice is easy, albeit expensive. You simply take off your old cylinders and bolt on new ones. The hard part may be making the decision about which brand or cylinder to buy.
The choice used to be simple, since only Lycoming and Continental supplied new cylinders for their engines. Now, in addition to the OEMs, ECI and Superior manufacture and sell quality cylinder assemblies for many-but not all-popular engines.
Superior offers investment-cast Millennium cylinders that by all accounts, are high quality and have been well received in the market. Superiors barrel finish however, is plain steel and not coated or nitride-hardened as are both OEM products. Nor do they have a finish such as ECIs CermiNil, which seems to wear less than nitride-hardened or chrome barrels, according to anecdotal evidence.
Not being totally metallic however, CermiNils hardness cant be accurately measured on a typical Rockwell C-scale, as can plain steel, nitrided steel or chrome. The CermiNil process does, however, require separation of the aluminum head from the steel barrel and then re-assembly, a process which must be done with the utmost care.
ECI points out that the disassembly affords the opportunity to examine the heads at points where they are known to crack. Additionally, ECI performs what they call IFR heat treating on the aluminum heads. This is supposed to refresh the cylinder metallurgic ally and thus increase the tensile strength some 20 percent. ECI claims that this process has helped reduce their warranty claims some 30 to 40 percent.
Superior claims their through-hardened barrel doesnt need to be nitrided. But some shops, such as Zephyr Aircraft Engines in Zephyr Hills, Florida, say they dont expect Superior barrels to fare quite as well as nitride-hardened cylinder through multiple overhaul cycles.
Superior argues the point, of course, but the long-term evidence doesnt exist. Millenniums are too new on the market. Interestingly, through-hardened barrels allow for more inexpensive over-sizing when and if its ever necessary. Nitrided barrels are limited in this regard. Then again, with new cylinder prices so low, heroic boring and oversizing is not attractive, economically.
Finish, Re-Barrel Options
While we dont dispute Superiors quality and applaud them for the workmanship they put into their cylinders, our guess is that unless the barrel itself has some sort of hardening or treatment, it may not wear as well beyond the first overhaul as a harder nitride, chrome, or CermiNil jug. However, wear isnt the only factor. Propensity to rust is critical when picking a barrel finish. Nitride cylinders have proven to rust more readily than plain steel or especially CermiNil or chrome, which generally arent susceptible to rust at all.
When compared to nitride cylinders, Superiors through-hardened barrels should (and seem to) show less propensity to rust if left inactive for extended periods. In aircraft flown infrequently, they may actually last longer than the harder nitride finish.
Both ECI and Superior manufacture separate barrels that can be used on an owners existing cylinder heads but are typically used on remanufactured exchange cylinders, too.
However, re-barreling your existing heads is an option when considering barrel finish and although ECI does offer CermiNil barrels, most separate barrels seem to be through-hardened.
For engines flown frequently, it appears that nitride resists premature wear nicely, at least on most Lycomings. However, certain model OEM Continental nitride cylinders seem to wear out dimensionally before typical recommended TBO, the IO-360 series being among them, which is why we chose this engine for our cylinder flyoff.
Certain engine/airframe combinations tend not to do well with chrome barrels and may have higher-than-acceptable oil consumption, such as older Mooneys, which tend to be overcooled.
With the oil consumption problems channel chrome has had in select model engines, we would put our money on CermiNil if we were the typical owner not flying every week and smearing oil on the cylinder walls. Weve heard good reports about CermiNil, none bad thus far.
Our side-by-side test of CermiNil against new Continental cylinders may give us a better idea of whose durability claims have the ring of truth.
Also With This Article
Click here to view the Top Overhaul Checklist.
Click here to view “The Great American Cylinder Flyoff.”
Click here to view the Overhaul Checklist.
Click here to view “The Issue of Barrel Hardness.”
-by Coy Jacob
Coy Jacob is a Mooney broker/dealer who owns who operates The Mod Squad (formerly Mooney Mart) in Venice, Florida. Hes a regular Aviation Consumer contributor.