March 1, 1999
By Peter A. Bedell
Aircraft owners dread engine overhauls — especially if they are premature. The thought of spending thousands of dollars for a new or overhauled engine 700 hours before the manufacturer's recommended overhaul time generally doesn't sit well with most owners. If cylinder compression starts heading south and oil consumption is on the rise, the signs are leading you to an important and expensive decision — to top or not? Several factors affect this decision and, when that time comes, a thorough assessment of your aircraft's overall health is necessary.
Everything outboard of the crankcase (save for the alternator, starter, and other accessories) of typical aircraft engines is referred to as the top end. The basic structure of a cylinder consists of an aluminum head screwed onto a steel barrel. The smaller-outside-diameter portion closer to the crankcase is the barrel and the outboard section with the larger cooling fins is the head. Inside the head, of course, is where the majority of the commotion in the engine occurs. Given the different metallurgy present within cylinders, each part has different wear rates. When the aluminum head changes temperature, it contracts more rapidly than does the steel barrel, setting the stage for uneven wear patterns and eventual failure in the form of a crack or other anomaly.
Several signs will lead you and your mechanic to believe that the cylinders on your engine may need reworking or replacement. Obvious indicators, such as a crack in the cylinder or oil seeping out of the head-to-barrel joint, call for immediate action. However, it is the subtle clues that make diagnosing the problem a little trickier. Compression scores, oil consumption, oil analysis, and aircraft performance are all additional clues to determine whether an engine is getting tired.
Using techniques that are comparatively archaic in this age of computer diagnostics, a mechanic can accurately find the source of the problem. Take the differential compression check as a prime example. Using a two-part pressure gauge, a mechanic can fill the cylinder with 80 psi of air, which is read off one gauge, and determine how much of that 80 psi the cylinder is containing by looking at the indication on the other gauge). For example, a reading of 75 over 80 indicates the cylinder retained 75 psi. Often, mechanics will scribble the numbers on the rocker covers as a reference for the next time.
If a cylinder holds little of the pressure, you can listen (or feel, in the worst cases) where the air comes out. If it comes out of the exhaust stack, you've got exhaust valve or valve-guide problems. If you hear or feel air coming out of the oil filler neck and/or breather, air is likely escaping past the piston rings. Depending on how bad the leakage is, this type of top-end distress may eventually require the replacement or reworking of one or more cylinders after further investigation.
How bad does the compression have to be before pulling a cylinder? That depends on each engine type. To make matters more complicated, Lycoming and Continental have different acceptable limits for low compression. While 65 over 80 psi can be considered good for a Continental IO-470, that figure would not sit well with a mechanic testing a Lycoming O-320. Continental owners should obtain a copy of Service Bulletin M84-15 that details methods and techniques used in determining proper compression ratios of Continental cylinders. Copies of service bulletins can be obtained through AOPA.
Another telltale sign of top-end problems can be found in the engine oil. Is oil consumption on the rise? Does the oil turn black soon after an oil change? Worn or broken piston rings allow combustion gases to blow by and contaminate the oil, turning it dark prematurely. When the engine is not running, the poor seal between the rings and cylinder allows oil to seep into the bottom spark plugs, filling them with oil and eventually fouling them. Blow-by can also be traced by simply smelling the oil. If it smells like combustion byproducts rather than just plain oil, you've likely got some blow-by.
Continental Service Bulletin M89-9 points out another way to check for excessive crankcase pressure caused by blow-by. By plumbing an airspeed indicator to a crankcase vent — whether it be where the oil breather attaches to the filler neck or some other direct line to the crankcase — a mechanic can run the engine and determine the amount of blow-by depending on the indicated "airspeed." According to the service bulletin, 90 mph (78 knots) is the limit for Continental 360- through 550-series engines. For the A-65 to IO-346 engines, 44 mph (38 knots) is the limit.
When oil burn rates get worse than two hours per quart in typical GA piston engines, it's time for a look-see. In fact, most mechanics would rather the investigation start when the engine burns more than a quart every three hours. Another variable in the tracking of oil burn is the type of coating your cylinders have. Those with no coating or "steel" cylinders will optimally burn about a quart in 10 hours. Cylinders coated with Engine Components Inc.'s (ECI) Cermicrome, on the other hand, may go as many as 25 hours or more without requiring a quart of oil. Cylinders with a channel-chrome coating generally burn the most oil with typical burn rates falling in the quart per four-to-eight-hour range. Of course, all engines are different and burn rates may vary. What's important to note, though, is whether the rate takes a noticeable swing away from its normal trends.
For about $20 per sample, an oil-analysis program may provide an excellent source of advanced warning. Spectrographic analysis of the oil provides the owner or mechanic with a breakdown of which materials were found in an oil sample and in what quantity the materials were found. An abundance of one type of metal can pinpoint items that are wearing abnormally inside the engine. For example, high amounts of chrome could be attributed to the accelerated wear of chrome rings in a steel cylinder. If your airplane has chromed cylinders, the chrome could be getting scraped off by the steel rings. High amounts of aluminum could be the result of the piston's scuffing along the cylinder thanks to a broken ring(s) or excessive heat and consequent out-of-round bores. There are many wear metals tracked by oil analysis and a good analyzer with a knowledge of piston aircraft engines can even suggest where the metals are coming from. Otherwise, your mechanic can look at the results and formulate a theory as to the source. As with tracking oil burn, oil analysis works best when a trend is established. Simply sending in a sample whenever you suspect something's wrong won't do much good if there is no baseline to compare it to.
After it has been determined that there is a problem in one or more of the cylinders, the time comes to take stock of your engine, as well as your airplane's overall condition. First, are you looking at an anomaly within one cylinder or have all of the compressions started sagging? If only one cylinder has gone bad, it's probably best to just pull the affected one off and have it reworked or replaced. If the cylinder is attached to a Continental IO-520 in a Cessna 210 and is 500 hours into its second run to TBO, it would be prudent to replace it with a new assembly, given the service history of those cylinders and the time left before the engine's TBO. On the other hand, a cylinder on a Piper Archer's Lycoming O-360 has a good shot at continuing service to the end of a second run with just a repair job. The advantage of going with new is the knowledge of the cylinder's history. A repaired or overhauled cylinder could be on its first run or fifth run.
Also consider the perceived value of a top overhaul. Replacing all of your cylinders with new assemblies doesn't carry any clout with the FAA and that attitude follows suit with those looking to buy an airplane. Would you rather buy an airplane with an all-new engine or one that was recently topped? Some view top overhauling as a way to mask other engine problems. Though that could be true in some cases, it should be understood that top overhauling is very common on high-horsepower/turbocharged engines as well as those inactive engines whose cylinders have rusted.
Before deciding on which type of cylinders to purchase, you must consider how much your airplane is expected to fly. If you fly more than 100 hours a year and live in a dry area, plain steel cylinders would likely be a good choice. However, if you fly less than 100 hours per year and live along the humid Gulf Coast, you may be better off with coated jugs. A chrome or nickel coating combats the rust that would readily set up in steel cylinders on an inactive airplane. Your mechanic should be able to recommend the best cylinders for you and your flying style.
If you decide to top your engine with something other than the manufacturer's jugs — such as Superior Air Parts' Millennium cylinders or ECI's Classic Cast offerings — consider what will happen to them at TBO. If you send the entire engine to the factory in exchange for factory remanufacture or overhaul, the relatively youthful aftermarket cylinders will be gone and your engine will return with a set of new OEM cylinders. To keep them, a field overhaul is one option, or a not-so-easy option is to purchase a set of derelict OEM cylinders to swap with the aftermarket cylinders prior to shipping your engine off. With junked cylinders costing relatively little, the swap can be made for a little more than the mechanic's labor cost.
The good news is that new cylinder kits are priced fairly low these days, making new cylinders a more viable option than they were in the past. For about $1,100 to $1,300 you can get a new cylinder kit instead of spending $800 on an overhaul-exchange jug that contains only new innards and an unknown past. Spending a few hours with the latest issue of Trade-A-Plane will get you a good deal on new or used cylinders.
Your best offense in making a wise top overhaul decision is a good trend-monitoring program. Watch for compression trends. One fluke compression reading below specs shouldn't result in a cylinder's being yanked off without further investigation and subsequent checks. Carefully track oil burn from day one by noting the exact time that a quart of oil was added to the engine. Oil burn rates combined with an engine oil analysis and/or an oil-filter inspection at each oil change should spot any potential trouble before it becomes critical. If a trend is spotted early enough, it will buy you a little more time to make an educated and economical decision as to the best course of action.
A couple of examples involving top-end problems recently surfaced with two airplanes that I regularly fly. The first involved a Lycoming O-320 installed in a 1975 Cessna 172. The O-320 has a reputation for being a nearly bulletproof engine that can make several runs to TBO without ever requiring the removal of a cylinder between overhauls. So it was to my surprise to learn that a cylinder had gone totally flat — no compression. The problem wasn't easy to spot, either. At low rpm, the engine would run a little rough, like it had a partially fouled plug. During the runup everything would check normal and in flight the engine seemed to deliver smooth normal power. Upon shutdown, however, the propeller would tick over somewhat erratically as it came to rest. With his interest piqued, the more mechanically savvy partner in our airplane pulled the propeller through by hand and discovered the flat cylinder.
After removing the cylinder, our mechanic found that the exhaust valve was being compressed into the valve seat at an odd angle, damaging the seat as well. Combustion gases were escaping past the cocked valve in what should have been the closed position and going right out the exhaust. Under higher power, such as during runup and in flight, the valve sealed in enough of the combustion to produce smooth operation and perceivably normal power.
Given the fact that this particular engine had slightly more than 600 hours since a major overhaul, the only decision to make was whether to rework or replace the cylinder. Chalking our incident up to bad luck and considering the O-320's excellent cylinder durability history, the decision was made to ship the one ailing cylinder out to a rebuild shop, where a new seat was pressed in. Typically, when a cylinder gets reworked it also receives new valves, piston, rings, and other valve-train hardware. The total cost of reworking and reinstalling the cylinder was less than $500. A new cylinder kit costs about $1,300.
Another occurrence involved a Continental IO-520 installed on the right side of a Beech Baron. This engine had 900 hours in 16 years and several bits of evidence were pointing to top-end problems. Compressions were erratic, often sagging into the upper 50s and lower 60s. Oil burn had steadily worsened from a quart in five hours to a quart every three. The oil reeked of blow-by and oil could be found in the bottom spark plugs of the inboard cylinders of the canted engine. It was becoming obvious that the rings were doing a poor job of sealing against the cylinder walls. Engine Oil Analysis of Tulsa, Oklahoma, began to notice higher levels of iron and aluminum in the oil samples. In the remarks section of one report, owner Howard Fenton wrote, "iron marginal, aluminum high for channel-chrome cylinders. Levels may be indicator of broken ring (s) or worn ring lands. I think a TOH [top overhaul] is in the near future."
Some months and less than 20 hours later, the icing on the cake was the discovery of a tar-like substance seeping out of the head-barrel joint of the number-six cylinder, indicating the early stages of head-barrel separation. That cylinder was definitely junk, but if we replaced only that one would the others soon follow? Given all of the indications, and the fact that we still had 800 hours until TBO, the decision was made to pull all of the cylinders and replace them with new ones.
First to come off was the number six cylinder. As it was pulled off of the piston, pieces of one of the piston rings fell to the hangar floor at the mechanic's feet. Same deal with the opposing cylinder. Both of these cylinders exhibited the early stages of scuffing along the top and bottom of the jug, proving that the piston wasn't riding on the rings that were left on the pistons. The pistons showed corresponding scuffing evidence.
Oil analysis had caught this problem several months prior, leaving us time to order a set of six new Millennium cylinder kits from Superior Air Parts. Although it was painful to sink more than $7,000 into an engine 800 hours before TBO, we are now enjoying the predictable oil burn, clean oil analyses, and solid compression checks that the new jugs provide. Now for that other engine.... — PAB
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