Savvy Aviator Newsletter 2004-10

The use of reconditioned or continued-time cylinders is a terrific way to save big money on engine maintenance -- provided you know what you're doing. If you don't, it's a good way to get into trouble.

Aircraft owners who attend my Savvy Owner Seminars come away with dozens of specific ideas for saving a bundle on maintenance. One subject I always discuss is the use of reconditioned or continued-time cylinders rather than new ones when the need arises to replace cylinders between major overhauls. I consider this to be a particularly sensible option when jug replacement is required on a relatively high-time engine that is beyond mid-TBO. In my view, it's a waste of money to buy a brand new replacement cylinder when all you really need is a few hundred hours of service.

A "continued-time" cylinder refers to one that was removed from another engine after just a few hundred hours of service, and still has a substantial amount of useful life left. A "reconditioned" cylinder is a worn-out cylinder that has been returned to new fits and limits by means of chrome- or nickel-plating, rebarreling, or oversizing. Both kinds can generally be purchased for 30% to 50% less than the price of a new jug. With the price of new cylinders today ranging from $1,000 to $1,400, the potential for savings here is obvious.

Whenever I discuss this subject, I always stress the importance of using only cylinders with a known history -- either by reconditioning your own first-run cylinders or purchasing exchange cylinders from a trustworthy source who can vouch that the cylinders they sell you are first-run. (A "first-run" cylinder is one that has been in service for one TBO or less.) Or to put it more succinctly, make sure you don't buy junk.

An email I received recently from a reader provides an excellent illustration of this:

We had a Cessna 182 on leaseback to our flying club. The owner was "economical" regarding maintenance in order to keep the airplane flying. During the airplane's tenure at the club, pilots experienced two in-flight cylinder failures. The first was a catastrophic head-to-barrel separation, but luckily it occurred in the traffic pattern at our home airfield and the crippled airplane was landed without incident. The owner purchased and installed a reconditioned cylinder, and all was well.

The second in-flight failure occurred several months later, a circumferential crack of another cylinder about halfway around the head just above the head-to-barrel joint. That failure was discovered quite fortuitously during a post-flight inspection by our club's mechanic, who spotted the telltale white soot. It was clear that cylinder would have failed catastrophically very soon thereafter.

At this point our club's mechanic began inquiring where the owner obtained the cylinders when the engine received its last major overhaul, and learned that they were all reconditioned by a nearby shop. We also had the good fortune to have access to the metallurgy lab at Marshall Space Flight Center, so we sent them the failed cylinder for analysis before the owner sent it in for core credit.

As you might imagine, the results of the metallurgical analysis showed the failure was caused by embrittlement and fatigue from prolonged use; i.e. many thousands of hours. Clearly, the fact that these overhauled cylinders met dimensional specifications did not tell the whole story. Based on the metallurgy report, the club asked the owner to replace the rest of the cylinders to insure a similar fatigue failure would not happen again. The owner declined, and chose to remove the aircraft from the club instead.

To make a long story short, we learned that that there is no way to determine how many hours in service a reconditioned cylinder has experienced once it has been separated from an engine, or how many times such a cylinder has been reworked and returned to service. An exchange cylinder you purchase might have 2,200 hours and been reworked once, or might have 8,000 hours and been reworked a half-dozen times. As long as it meets dimensional specs, it's yellow-tagged and put on the shelf, ready to ship to the next customer.

Don't get me wrong. I'm not suggesting overhaul shops are doing shoddy work. To the contrary, these craftsmen often work miracles with the used items they repair. The problem is that once a cylinder is removed from an engine and placed in the pool of reconditioned cylinders, there's no requirement to keep track of its service history.

The reader is spot-on. It's easy to measure a cylinder barrel and determine whether or not it meets the dimensional tolerances published in the engine overhaul manual. But there's no easy, non-destructive way to check a cylinder head to determine how close it is to the end of its useful life, or how likely it is to fail catastrophically if continued in service.

The cylinder assemblies used on our piston aircraft engines are constructed by joining a hardened steel barrel to an aluminum alloy head. Threads are machined into the top of the barrel and the bottom of the head casting. The two are mated together during manufacture by heating the head in an oven, cooling the barrel in a refrigerator, then quickly screwing the cold barrel into the hot head. As the barrel and head both return to room temperature, the threaded head-to-barrel interface tightens into an "interference fit" that joins the two subassemblies permanently.

When a cylinder is placed into service on an engine, its barrel gradually wears due to friction as the piston and rings move up and down in the barrel roughly 100 million times over the course of engine TBO. The amount of wear can be readily measured using a micrometer, and once the barrel has worn to the point that it exceeds published service limits, it is deemed unairworthy. Such a worn-out barrel can be restored to airworthiness by electroplating it back to new-limits dimensions with chrome or nickel plating, or by grinding the barrel to an approved oversize dimension (e.g., plus .005") and installing oversize pistons and rings. Alternatively, a few repair stations are authorized to de-mate the worn barrel from the head and install a new replacement barrel.

The cylinder head does not wear dimensionally. Because it is made of aluminum alloy, however, it does have a finite fatigue life. A fundamental principle of metallurgy is that ferrous metals like steel can last forever if operated within its fatigue limit, but that non-ferrous metals like aluminum or magnesium always have a finite fatigue life beyond which they will crack and fail. Since a cylinder head is made of non-ferrous metal and is subject to hundreds of millions of repetitive stress cycles, it will crack and fail eventually if kept in service long enough. Sometimes we get lucky and head cracks are caught during visual inspection before they progress to the point of catastrophic failure. Sometimes we aren't so lucky and experience an in-flight head-to-barrel separation. If such an event happens at a bad time, it can ruin your whole day.

Oddly, neither the engine manufacturers nor the FAA publish any clear guidance on how long a cylinder head can safely remain in service. Nor have I been able to find any good statistical information correlating cylinder head time in service with the probability of catastrophic fatigue failure. Conventional wisdom suggests that once a cylinder head has been in service for two or three engine TBOs, the chances of a fatigue failure starts to increase significantly. (If anyone knows of a more definitive guideline, I'd sure like to hear from you.)

That's why it's important for savvy owners who want to save money by installing continued-time or reconditioned cylinders to make sure that those cylinders are first-run jugs; in other words, that the cylinder heads have no more than one TBO's worth of hours on them. As pointed out earlier, there's no way to look at a cylinder and tell how much time is on the cylinder head. The only way to protect yourself is to use only cylinders that have a known and well-documented history.

There are only two ways to accomplish this. One was to send out your own first-run cylinders for reconditioning. This is a safe way to go, but it isn't always feasible. Sometimes, you can't afford the downtime it would take to wait for your own cylinders to be reworked (often a month or more for plating or rebarreling). Other times your cylinders are simply not repairable (e.g., a head crack that cannot be weld-repaired).

The other was to purchase an exchange cylinder from an overhaul shop that you trust to furnish a first-run cylinder, and relying on that shop's promise that the cylinder they ship you has a known history. The last two times that I needed replacement cylinders for the high-time engines on my Cessna T310R, for example, I phoned up Ken Tunnell at Lycon Aircraft Engines in Visalia, Calif., and asked him if he could sell me a continued-time jug with less than 1,000 hours on it. I felt okay about doing this because I know Kenny personally and have come to trust him. Not every owner has the knowledge or contacts to do that.

I'm pleased to report that for owners of TCM 520-series engines, things have recently gotten a lot easier. RAM Aircraft in Waco, Texas -- arguably the premier overhauler of TCM 520s in the world -- has a new program called "Valuetime" in which they remove first-run cylinders from engines returned to them for overhaul, have them nickel-plated back to new limits by ECi using the "CermiNil" process, and sell them for about $800 each (compared to $1,200 to $1,400 for new -520 jugs). I think this is an excellent program, and I hope that other big-name shops (Mattituck, etc.) follow RAM's lead and start offering reconditioned or continued-time cylinders for sale that they guarantee to be first-run. It would go a long way toward removing the stigma from using "previously owned" cylinders, and could save owners a lot of money on cylinder replacement.