As engine parts go, cylinder assemblies are a commodity item that most owners will have to buy sooner or later, as part of a major overhaul or perhaps for a mid-time top overhaul.
For the past decade, at least, the new cylinder market has become fiercely competitive but many shops recommend and owners buy reconditioned, yellow-tagged cylinders anyway, to save a few bucks.
Naturally, these are cheaper than new and if bought from a reputable shop, theyre usually a good value. But not always. Quality varies widely and the only protection you have against buying junk is a pre-installation inspection of the cylinder that your mechanic may or may not be doing. And that advice applies even if the cylinders are bought from a shop with a sterling reputation.
In repairing and restoring Mooneys, we have occasion to see dozens of cylinder assemblies a year, many of which are yellow-tagged overhauls. For the past two years, we have been documenting the results of our own in-house quality inspection of reman cylinders received from major repair facilities.
Our inspection entailed disassembling these cylinders by removing the valves and measuring and closely examining the adherence to OEM specs of the valve, valve guide and valve seat as well as the machining fit and finish of the valve components. (For this article, were focusing on valves only; there are problems with barrels, too.) Our findings havent been encouraging.
Our nit picky A&P, who was trained as a manufacturing quality control inspector, tore down and inspected some 50-plus cylinder sets, carefully measuring the valves, guidesand seats for conformity to Lycoming and Continental specs. Simply put, our field inspection of yellow-tagged cylinders found that some 60 percent didnt conform to OEM specs. A sampling of the problems uncovered:
A lack of uniformity of valve seat widths; some larger than OEM specs and some smaller. We found the outside diameter of the valve seat ground area to be in excess of OEM specs, probably caused by excessive grinding to achieve proper seat widths when dealing with a seat which was not concentric to the valve guide.
We found some seats had an improper finished angle ground into the seat area, which was probably caused by not having the grinding stone dressed properly prior to the grinding operation.
On some valves, we found only a thin line of contact area between the valve and the valve seat in portions around the circumference of the seat. This was probably caused by the lack of concentricity between the valve guide and seat.
We found a lack of uniformity in the valve contact-to-seat area in many valves, again due to the lack of concentricity of valve guide versus seat. With some valve seats being ground to nearly half the width of others, its apparent to us that workmanship and quality control in the cylinder industry leaves much to be desired.
What It Means
Unfortuately, this is not big news to shops which inspect yellow tagged cylinders before installing them or suffer the results of not doing so: Customers that come back with valve or cylinder problems of some kind. Quality control has always been an issue with reworked cylinders. Whats disturbing to us is that QC may be getting worse but it certainly is not getting better.
And theres undeniably debate about how closely a reworked cylinder should match new OEM specs. Were sure some of these off-spec cylinders would soldier on to TBO with no problems.
On the other hand, if the tolerances are too wide of the mark-and in our view, a substantial number of cylinders have such flaws-the likelihood of reaching a reasonable TBO is greatly diminished, with valve seat or guide problems virtually assured in some cases.
Unfortunately, this is what we call the tombstone effect. It may take several hundred hours for the problems to emerge and by then, in many cases, the airplane has been turned over to another owner.
Because Mooneys tend to be retained by the owners longer than other models, weve noticed a definite trend toward valve-related cylinder problems. The reasons are easy to understand. The typical cylinder assembly is somewhat of an unholy alliance of steel barrels screwed into cast aluminum cylinder heads, with each metal having a different co-efficient of expansion when heated and cooled. Inevitably, this causes uneven stresses to build. Heat transfer is a critical process in engine longevity.
Since the exhaust valve lives in a high-temperature flame front as hot as 1500 degrees F, consistent surface contact between the valve and its seat is critical in order for the cylinder mass to carry off the heat the valve accumulates. If the valve seat angle is incorrect, the seat width too narrow or the valve installed so that it seats non-concentrically, the heat exchange surface is reduced and the valve runs hotter. Perhaps much hotter.
Continental and Lycoming have published guidelines on valve machining and re-working procedures that are supposed to yield the ideal dimensions. But both companies fully realize that some cylinder shops seem to favor narrower valve seat mating dimensions and some produce wider seating.
Neither is necessarily good. A too-wide seat may help transfer heat from the valve, but it also reduces the closed-valve seating pressures and thus possibly wont maintain a clean seal, allowing carbon build-up under the seat on the stem. A too-narrow seat means a hotter exhaust valve which doesnt bode well for durability.
Hitting the Mark
OEM specs are generally considered to be fairly loose and easily obtainable by shops who do this sort of work. Yet in our sampling of 50-plus cylinders, dimensions ranged all over the map.
Frankly, even factory new cylinders are occasionally out-of-spec and our A&P inspector retains an especially dim view of Continentals new cylinder assemblies. As the recent fiasco with TCM crankshafts shows, factory quality control is far from perfect. Its no different among cylinder and engine shops.
Average TCM valve seat specs for the 360-series engine, for example, are: Valve seat dimensions: .070 to .120 inch, for both intake and exhaust. These are hard specs which many shops aim to hit square in the middle, giving a little margin on either side of the ideal.
Yet on the cylinders we examined, we found dimensions ranging from .047 or less to .123 inches. We uncovered similar dimensional shortcomings in Lycoming cylinders.
Moreover, these dimensions werent out of spec uniformly, but varied considerably around the diameter of the valve seat, from paper thin contact, to the proper width to slightly wider than specified, all on the same valve assembly.
Generally, speaking, by the way, the shop doing the cylinder R&R wont be capable of correcting these flaws, although a few may have both the equipment and the talent to do so. A decade ago, when it was economical for small shops to compete in the cylinder overhaul business, a shop might have been capable of this level of work. Its less common now. Most shops simply buy the yellow-tagged cylinder and install it on the engine, relying on the overhaul shops reputation and warranty to protect the customer against future problems.
The A&P is on the hook for screwing up the installation of a cylinder but the shop that overhauled it will have to stand behind its work, too. All too often, a valve or cylinder problem turns up several hundred hours down the road and the pilot-who could very well be another owner-takes the rap for having operated the engine incorrectly when in fact, faulty parts were to blame.
Of course, given the way cylinders go south for no apparent reason, theres no guarantee that an out-of-spec valve assembly wont motor on right up to TBO or that one thats bore-sighted on the middle of the specified tolerances wont go to pieces in 10 hours.
But our view is that the specs are the specs and if youre paying for them, you ought to get them. In the long run, valve tolerances that are in the middle of the published range give you more margin against overtemp events caused by bad fuel, improper leaning or just bad luck. Its not unreasonable for shops to deliver and to reject the work when they dont.
And what of those shops? A number of the out-of-spec cylinders we found came from Engine Components, Inc., a San Antonio, Texas trade house thats well known for its Cermicrome and, more recently, CermiNil cylinder treatments.
ECI advertises widely and is well known. Still, the company has had its share of missteps, not the least of which is problems with the Cermicrome process, a cylinder treatment it developed but has since discontinued. It appears to be having better luck with the replacement process, CermiNil, which thus far hasnt drawn many complaints. ECI at first expressed some surprise that our measurements were accurate. We explained how our examinations were done and actually sent some of the cylinders back to the company so they could conduct their own inspection.
ECIs Ed Salmeron admitted that some of the cylinders were slightly wide of the mark. However, in ECIs view, not all of these specs were sufficiently out of kilter to be meaningful. (Were not sure we agree with that.)
When we pointed out that some custom shops seem to producing cylinders that are consistently within OEM tolerances, Salmeron replied that this is difficult or impossible to do in the kind of production volume that ECI turns out.
In fact, says Salmeron, ECI is rapidly reducing its involvement in the cylinder overhaul business. Just three years ago, some 70 to 80 percent of ECIs dollar volume was cylinder work of some kind.
Over the next three years, the proportion will reverse itself: ECI has invested heavily in equipment to produce new cylinders, giving both the OEMs and Superior some competition.
Speaking of Superior, they may not be immune to out-of-spec valve assemblies. When we examined a set of brand new Millennium cylinders, we found that the exhaust valve seats were misground on all four. (Our A&P corrected them by regrinding.)
We contacted Superior to ask about this and were told theyre aware of no consistent problems with valve specs on new Millenniums. However, the company says it will correct any problems that do surface under warranty.
ECI says the same. To its credit, the company will rework any of out of spec cylinder to suit a customers demand and will stand behind any that fail due to poor workmanship or materials. Our talks with mechanics find few complaints about ECIs warranty policies.
Still, in fairness to customers, we would be happier if the cylinders came out the shop with properly specd valve dimensions in the first place.
What to Do
Does all this mean that every overhauled cylinder out there is a disaster waiting to happen? Not quite. But theres enough substandard work to make us worry. We think many reworked jugs are probably a good value, even those that may not perfectly meet Lycoming and Continental specs. Prices are competitive on overhauled jugs across the board.
But that very same competition has a downside. Ever since the OEMs lowered the price on new cylinders nearly a decade a go, overhaul shops have been feeling the pinch. Many have been driven out of business and those that remain enjoy thinner margins than they did in the 1980s.
While their entry into the cylinder business has proven a bonanza for the OEMs-theyve upped their parts sales volumes while reducing the liability exposure of junk cylinders-customers have fewer choices if they dont care to buy new. We see this as a mixed blessing. Because of the tight margins, some cylinder shops simply dont commit the necessary resources to quality control and post machining inspection.
What should a customer do when buying replacement cylinders? This is very much a case of relying on the reputation of the shop, as recommended by either your overhaul facility or the A&P doing the cylinder R&R. There are still some shops that do general maintenance and are still capable of doing cylinder work in-house, but these are getting few and far between. Most shops farm out the work to outside sources.
Although new cylinders arent perfect, they will at least have new parts, no welds and probably no cracks around key areas in the head, which is more than you can say for any overhauled cylinder.
On a major overhaul, youll have to make the call on whether new cylinders are worth the price but in most cases, theyll be too competitive in price not to at least consider.
For a top overhaul or cylinder replacement, an overhauled or repaired cylinder will probably be the better choice. What you should you look for? Its no secret that many cylinder overhauls are done to service limits, with serviceable used parts.
We think at the very least, any overhauled cylinder should have all of its replaceable parts-including valves, seats, guides and springs replaced, even if the originals are in perfect condition. Ask the cylinder shop or your A&P about this point. Steer towards a source that provides such new parts.
Second, its reasonable for an A&P to disassemble a yellow-tagged cylinder assembly for inspection, rather than merely slapping it onto the engine and hoping for the best. At least some of the problems we have described here-mis-installed seats, lack of concentricity and so forth-will be visually obvious to an experienced A&P who knows what to look for, even one not equipped with the necessary measuring equipment to pin down the fine details.
It may cost you an extra $100 to have this inspection done but consider it cheap insurance against an out-of-warranty cylinder failure.
Last, if you decide to forego the inspection, understand this: Even if the cylinder has been bought from a reputable shop, it could have problems. Good shops have bad days. Price doesnt necessarily have much to do with it, either. Weve seen cheap reworked cylinders that had closer valve specs than factory new jugs.
Unfortunately, if you want a decent cylinder thats likely to go the distance, youll have to do your own quality control, in the form of an inspection by a competent A&P. Its sad but true: These days, youre on your own.
-by Coy Jacob
Coy Jacob owns and operates The Mooney Mart in Venice, Florida. Hes an Aviation Consumer contributing editor.