Understanding valve malaise
Among the piston designs to power light aircraft, a handful of Lycoming's engines have gained the distinction of being the most reliable and durable powerplants around. In particular, certain versions of the O-320, O-360, and O-540 engines reign supreme in hangar talk — you hardly ever have to open them up, and running to and beyond TBO is a common occurrence, so the hangar chatter goes.
That the overall package is so stout very much focuses the attention of owners and mechanics on the engines' one obvious weak point: the valve train. More to the point, the parallel-valve Lycomings have the mostly deserved reputation of being hard on valves and valve guides (exhaust, primarily). Moreover, the parallel-valve versions of these engines suffer more from premature valve and guide wear or sticking of the valves than the angle-valve iterations. (Lest you think we're picking on Lycoming's engines, Continentals have their share of valve-guide troubles, but the malady is arguably more profound on the smaller Lycomings.)
It shouldn't surprise anyone that the valve guides in any engine — but particularly an air-cooled one — are prone to wear. On the exhaust side, the guide has the unenviable task of positioning the valve, persevering through metal-to-metal contact with the valve, and providing a critical heat-dissipation path. Moreover, the guide must keep the valve in an exact position so that the valve head correctly contacts the seat; any mismatch here will quickly stress the valve and lead to leakage. Any leakage in the exhaust valve/seat path will quickly lead to further damage. Worse, this guide must run with a minimum of lubrication and at remarkably close tolerances; typical valve-to-guide clearance is 0.004 inches. You might guess that formulating the right material for the valves and guides, walking the fine line between good wear characteristics and effective heat dissipation, is a real engineering challenge. An ongoing one, too; late in 1996 Lycoming introduced a guide of slightly harder material than the longstanding component.
For many years, the common complaint from the maintenance quarters has been of sticking valves. Excessive combustion byproducts and lubricating oil would eventually migrate into the guide and oxidize in the intense heat encountered at the valve stem — typically, the valve head will see about 1,150 degrees F, with the inferno tapering to less than 900 degrees F at midstem. (These figures are for Lycoming's sodium-filled valves; Continental's solid-stem valves tend to run 100 to 200 degrees F hotter.) This coked oil is not oil soluble, so it stays there until something mechanical breaks it loose. Mechanics call sticking valves the "morning sickness" syndrome, because the engine with incipient sticking valves will cough and run rough right after startup.
More often these days, now that most owners are leaning the fuel mixture aggressively on the ground and keeping the combustion byproducts from forming in the first place, the more common failure mode is excessive valve-guide wear. Given the high temperatures experienced by the guide, it's not surprising to find that the guides failing today tend to bellmouth at the valve-head end, very often forming an oval in the plane of valve actuation. Naturally, when this happens, the valve is not positioned as effectively, and, more important, the heat-transfer path constricts. Now the heat of combustion wicked out by the valve is concentrated across a smaller surface area within the guide, forcing more heat to stay in the valve for conduction into the head through the valve seat. Suddenly a reasonably well-balanced load sharing has become lopsided.
Distinguishing Lycoming models, you often hear of parallel-valve and angle-valve cylinders. These are not interchangeable, and they often show up on what seem to be similar engines. For example, an IO-540-D is a fuel- injected parallel-valve engine with an 8.5:1 compression ratio producing 260 horsepower at 2,700 rpm. Compare that with the IO-540-K, a similarly injected angle-valve model with an 8.7:1 compression ratio and capable of turning out 300 hp at 2,700 rpm. The differences, both in the hardware and the results, are significant. (Confusion is common, too; the Mooney TLS's 270-hp Lycoming TIO-540-A1FA has been reported to be a cousin of the 350-hp TIO-540-J2BD in the Piper Navajo. They are, in fact, two vastly different powerplants.)
It's also worth understanding, for the purposes of the discussion to follow, that the O-320, O-360, and O-540 parallel-valve engines share the same basic cylinder assembly; there are subtle differences among the various dash numbers, and there are fuel-injected versions of each family. The O-360 is just an O-320 with a longer stroke. Also of note: The Lycoming O-320-H2AD and O-360-E engines are significantly different from the other parallel-valve models, with integral accessory cases, automotive-style lifters and rocker arms, and a host of smaller changes.
As the name implies, the parallel-valve cylinder places its valve stems parallel to each other, which results in a combustion chamber with a flattish top. This type of combustion chamber dates from the earliest internal-combustion engines and works fine up to a certain specific output; that is, for a given amount of power from a given engine displacement. In addition, the parallel-valve layout forces the valve stems — and therefore the springs, guides, and associated actuating hardware — closer together. As a result, there's not as much room for head material or finning between the valves, and the heat-dissipation path of the valves is somewhat truncated.
With the angle-valve cylinders, Lycoming attempted both to improve the volumetric efficiency of the engine and to create a more massive head that could dissipate the heat produced by the additional power brought by that efficiency. Notice also that most versions of the angle-valve Lycomings carry slightly higher compression ratios than the parallel-valve cousins of the same displacement. The more hemispherical combustion chamber design allows for the higher compression ratio and the move plays into the desires of the engineers to generate more power. One side effect of the angle-valve arrangement is that the valves and guides are longer than their parallel-valve counterparts, affording a more generous heat-dissipation path.
It is this disparity in the two designs' abilities to remove heat from the combustion chamber that forms the basis for a new angle, so to speak, on what may be at the root of the parallel-valve engines' higher incidence of valve-guide wear. Excessive heat is, simply put, the enemy of long valve and guide life. And valve and guide failures have long been attributed by mechanics and Lycoming to high cylinder-head temperatures -- in turn blamed on over-leaning, use of high power settings, and poor baffling or cowling design. Never has the basic design of the engine been seriously questioned.
Until Bill Marvel and Bill Scott came along, anyway. Marvel's story is long and complex, involving his Grumman Tiger and a tale of unusually high valve-guide wear. Marvel is an A&P mechanic and intensively involved in the maintenance of his airplanes. He has had the opportunity to tap into the experiences of many Grumman owners through his contacts at the American Yankee Association; he was formerly president of the organization.
The story starts thus: Marvel purchased a factory-overhauled O-360-A4K (your basic carbureted, 180-hp parallel-valve four) for his Tiger in 1989. Two years later, at fewer than 400 hours since the overhaul, the O-360's number one cylinder failed a compression test; a leaking exhaust valve was the culprit. In the course of tearing down that cylinder, Marvel discovered that the rocker arms had been installed incorrectly. The rocker arm intended for the exhaust valve — it has a groove in the bushing and a squirt hole aimed at the valve — was on the intake side, and the rocker arm for the intake valve was on the exhaust side. Eventually he discovered that all four cylinders had their rocker arms reversed from the correct orientation. Marvel wrote to Lycoming and, to his surprise, was offered a factory-new engine; this despite the fact that the warranty had expired on the factory overhaul.
But by the time the second engine had reached 400 hours — about 2 years by the calendar — the number one cylinder had gone soft once more; again the exhaust valve was the culprit. (Marvel believes that excessive guide wear cut off one heat-dissipation path, overheating the valve; the valve then began to distort, and eventually a furrow was pounded into what should be a smooth head-to-seat interface.) Further investigation by Marvel and Bill Scott, owner of Precision Engine in Owensboro, Kentucky, revealed that the fit between the pushrod ball end and the rocker arm was poor and that any oil arriving at the pushrod end would as likely dribble off as travel through the rocker arm, into the rocker-arm bushing, and then out the squirt hole onto the exhaust valve. Thinking that he'd found the problem of early wear, Marvel had Scott hand-lap the pushrod ends to the rocker arm socket to improve oil transfer. The men also measured the guides on all cylinders and found the number three's guides out of limits. These two cylinders were refurbished to new limits, including fresh guides and valves.
Fewer than 200 hours later, Marvel observed that the exhaust-valve guides on the odd-numbered cylinders — on the copilot's side of the airplane — had once again worn beyond service limits. This discovery started Marvel and Scott down a long research path culminating in some surprising discoveries in the way the Lycoming valve train is constructed.
With few exceptions, Lycoming has used a hydraulic lifter known as the mushroom type; that is, the surface that bears on the camshaft is larger in diameter than the cylinder that resides in the lifter bore. This type of lifter cannot be removed from the engine without splitting the cases. (Lycoming used the alternative "barrel" lifter in the O-320-H2AD and O-360-E "76" series engines; Continental uses the same style in all of the larger engine families -- the 360, 470, 520, and 550 models.) These hydraulic lifters, or cam followers, are made to absorb any changes in the valve-train free play; naturally, when an engine heats and cools, the clearances among its many parts will change.
What Marvel and Scott discovered is that the current Lycoming mushroom lifter is extraordinarily close in design to early automotive tappets — which, when boiled down, meant something very important. Those early engines used the lifters directly between the camshaft and the valve stem — there was no pushrod or rocker arm. So there was no reason for the lifter to pass any oil for the purposes of component lubrication. And yet, the Lycoming design clearly intends to use oil bypassed by the internal tappet plunger; the hollow pushrods, drilled pushrod ends and rocker arms, and oil-squirt holes in the rocker arms all attest to this.
Thinking that maybe the lifters weren't doing their part of the job, Marvel and Scott set about measuring the amount of oil sent to each rocker box (easily done in the Lycoming, given the external drainback lines of all but the 541-series engines). More surprises. After a 7-minute ground run, they measured as little as 38 cc at one cylinder. But the real eyeopener was the disparity between the odd-numbered cylinders and their counterparts; numbers one and three had less than half the oil flow of two and four. Why? Marvel points to the design of the oil system, noting that the gallery serving the lifters on the even-number cylinders does primarily that, while the opposite lifter oil gallery also serves the main and camshaft bearings. Further tests revealed that the average flow of oil to the rocker box in the O-320-H2AD engine — one of the two Lycomings to use the automotive-style barrel tappet — flowed on average eight times the amount of oil of the O-360-A. It is generally believed in the field that the H2AD, while still occasionally eating a camshaft or two, has remarkably little valve and guide duress.
Another bit of evidence plays into the Marvel-Scott hypothesis: On Marvel's airplane, the hottest- and coolest-running cylinders (by CHT) are on the odd-numbered side of the engine. Therefore, according to the theory, it isn't strictly cylinder-head temperature alone that dictates guide wear. Moreover, Marvel claims that his CHTs were moderate — in climb, typically 425 degrees F and 370 degrees F for the numbers three and one cylinders, respectively; in cruise, he sees CHTs of 400 degrees F and 350 degrees F for those cylinders. Lycoming sets the redline at 500 degrees F; it recommends maintaining less than 435 degrees F in high-power cruise and 400 degrees F at normal cruise settings. Given that the only cylinders to prove troublesome on his engine — and of several other parallel-valve Lycomings from whose owners Marvel has heard — were those tested to have lower oil flow to the rocker box, Marvel is a firm believer that the correlation between oil flow and guide temperature (hence wear) is solid.
Lycoming, not surprisingly, disagrees. According to the company, it could find no substantiation that cylinders in certain locations on the engines suffered guide wear more than others. What's more, Lycoming is sticking by its assertions that guide temperature is almost wholly dependent on overall CHT and is not significantly influenced by rocker-box oil flow. Lycoming's advice is to do all you can to make sure that the engine runs cool and to commit to employing service bulletin SB 388B, the aptly nicknamed "wobble test" for guide wear. This inspection is neither difficult nor terribly time-consuming. Lycoming recommends this test every 400 hours for all fixed-wing installations (300 hours in helicopters) of all its engines.
Marvel and Scott point to a move made by Lycoming last year to stem the tide of premature top-end overhauls on the Mooney TLS as proof that guide temperature is critical. The TIO-540-AF1A in the TLS is a parallel-valve engine turbocharged to produce 270 hp — among the most powerful variants of this 540-cubic-inch engine. (It is not entirely out of line, though, when you consider that the per-cylinder output is exactly that of the four-banger 180-hp O-360, which shares the head design and valve train components.) But a key ingredient in the TLS's hunger for exhaust valves is a maximum recommended cruise setting of some 90-percent power, approved by the handbook at peak turbine inlet temperature. (Most Lycomings carry the admonition to not lean above 75-percent power.) Guide wear has been high in this engine even though the CHTs are generally well-managed.
Lycoming's response has been to borrow a bit of technology featured on the 380-hp TIO-541 engines in the Beech Duke — use pressurized oil specifically to cool the exhaust-valve guides. Lycoming cuts a groove in the head around the midpoint of the guide and supplies it with engine oil; the excess oil returns to the rocker box, to be directed back to the sump through enlarged drainback lines. Though it's too early to tell, Mooney and Lycoming both hope that this will be the final fix. This new technology is on the current TLS Bravo and can be retrofitted to the existing fleet. (Of note: Lycoming rebuffs the notion that this form of guide cooling is needed or even desirable on any other parallel-valve engine than the TLS's.)
So, does this mean that any parallel-valve Lycoming is going to have premature guide and valve distress? Not exactly. The overhaul shops we talked with say that the incidence of valve problems — sticking and early wear — is not epidemic but about the same as it's been for several years. The research Marvel and Scott have conducted, however, suggests that versions of the engines that run in difficult environments — at high power, or on long cross-country trips where they spend most of their time leaned — need to be watched with more than the most casual eye.
More information on the research by Marvel and Scott is available on the Internet ( http://gtravis.ucs.indiana.edu/ Engines/Marvel/).
