By their very nature, air-cooled airplane engines are a hand-crafted liability. From the very first hour of operation to the last gasp, aircraft engines suffer a baffling combination of wear patterns, which makes the prospect of making TBO a crap shoot.
Clearly, most of the critical wear happens in the cylinders and the savvy aircraft owner has to resign himself to some mid-time attention to the jugs, a process known less-than-affectionately as topping.
As routine maintenance goes, this is a big-dollar item and probably one on which more than a few owners waste money. Sometimes topping a wheezy engine makes sense, sometimes it doesnt. Sometimes its a close call either way. In this article, well hit the high notes on why and when it makes sense to pay for a top overhaul.
The top end of any air-cooled engine is everything outboard of the crankcase. This includes cylinder barrels, heads, valves, guides, pistons, piston pins, pin plugs, rings, rocker arms, shafts and a handful of springs, retainers, washers, studs and the like. In the typical overhead-valve configuration, its a Rube Goldberg arrangement of many moving parts, all with different wear rates.
The term top overhaul has no meaning to the FAA or any other regulatory entity. Unlike a major overhaul or remanufacture, the condition of the top end-rebuilt or not-is unfettered by bulletins or AD note mandates. A top overhaul is not required by time-in-service, nor is cylinder condition necessarily the only criteria used to determine when a top is due.
The term top overhaul is often tossed around to describe a broad range of maintenance remedies, from simply throwing a handful of Borax into the carb airbox to replacing all cylinders with factory-new assemblies. So what does determine when a top is in order? You may not like hearing this, but its a vague, hard-to-define confluence of circumstantial evidence that loosely defines another fuzzy term: airworthiness.
One reason for the squishy definition of topping is that it encompasses so many variables. Factory-new cylinders with 200 hours of time-in-service might benefit from ring replacement if the barrels are glazed. But valve and guide work wouldnt be worth the time and money.
Conversely, replacing all the internal workings with new, along with head welding and barrel chroming might be the only solution for a worn-out top end. Are both of those fixes top overhauls? Probably.
Do You Need It?
Whether to forge ahead with a top overhaul is a function of cylinder condition, engine time-in-service, cylinder total time-since-new (calendar and actual hours), flight hours and years remaining on the engine measure against how the owner uses the airplane and, of course, how much hes willing to spend.
Cylinder compression and oil consumption are big players here. Continental and Lycoming give maximums for oil usage but the general rule is that if youre burning more than a quart every two hours, you had better start shopping for cylinder work. Further, if your spark plugs are oil fouling in flight-regardless of the actual oil consumption-a top may be the only solution. Compression sometimes sheds more heat than light on the cylinder wear issue. Worn out barrels burning copious amounts of oil can yield compressions in the high 70s. Whats more, a broken ring or burned land can escape a compression check if it happens to be flooded with oil during the check.
On the other hand, Continental cylinders with compressions in the high 50s and with little or no oil usage can be misinterpreted as weak or deteriorating when the opposite is true. The standard compression cut-off used to be 60/80. If the static cylinder compression check dropped below that, the cylinder was pulled, otherwise, it remained in service.
No more. Continental cylinders should be checked using Service Bulletin M84-15 with a commonsense approach to valve leakage. Using this procedure, cylinder compression on a 520-series engine might sag as low as 48/80 and be acceptable.
And even though the bulletin tolerates no valve leakage, apply some judgment before yanking the head. A 520 compression reading of 72/80 with a slight hiss from the exhaust valve is not cause to pull the cylinder. In-transit debris caught in the valve seat area will be pushed overboard with the first four-stroke exhale and its presence has little to do with the bulletins intent.
Compressions in Lycoming engines are easier to understand and simple to interpret. Readings during a compression check of the typical Lycoming engine should remain in the 70s regardless of the time in service or barrel configuration, except for some engines equipped with silica-impregnated barrels. Compression results below 70/80 should be investigated-especially on high-time, straight-valve Lycs. Any reading in the low 60s to high 50s is courting exhaust valve breakage.
Some engines are more likely than others to need a top prior to a major overhaul. For instance, its generally accepted that a TSIO-520 will need topping after 600 to 900 hours of time-in-service. Some may make it to 1200 hours, some will wear the barrels out in as little as 300 hours and a hearty few will make TBO or beyond.
A Lycoming O-320, on the other hand, will likely steam right to TBO with little more than a slight increase in oil consumption. In a broad sense, Continental engines tend to require a top overhaul before reaching TBO and Lycomings tend to sag towards TBO with no cylinder work. As always, there are exceptions. A normally-aspirated Continental 360 or 520 might very well reach TBO without cylinder work and its likely that the Lycoming TIO-540-J and -S series engines will need exhaust valve guide attention long before its time to major the engine.
While cylinder condition can determine the need for a top, cost and asset gain rules the decision. An 1100-hour Continental turbocharged 520 flying 300 hours a year in Part 135 service may need a top overhaul because of decaying compressions or high oil consumption, but it would be dubious to undertake it, given the 1400-hour mandated TBO.
Likewise, replacing or rebuilding the cylinders at 300 hours into a 1600-hour TBO will leave the top end short as the engine approaches the end of its run.
In both cases, youll lose value in the parts replaced or the engine time remaining. But if your flying doesnt require a mandatory rebuild, its conceivable that the 1100-hour, 520 top overhaul will deliver several years of reliable service if you fly 50 or 60 Part 91 hours a year. And if you find yourself paying for a top well before mid-life, nothing says you cant repeat the procedure with an end-of-the-road top tune-up as the engine slides past TBO.
Lycoming engines face the same dilemma, but unlike the typical Continental, theres a less predictable and more insidious aspect to a Lycoming top. Since Lycs generally have a robust top end-at least compared to Continentals-those that need cylinder work are usually well into their TBO run. Thus, the owner of a high-time O-320 or O-360 might consider a top because a cylinder is oil-fouling plugs or perhaps the consumption has jumped to a quart every four hours from its normal quart in six. Or, perhaps performance has dropped noticeably.
Things to Consider
When moving forward with a top overhaul, consider two things: First, a high-time Lycoming will need lots of parts and rework procedures. A rebuilt Lycoming cylinder is generally equipped with new parts designed to go to TBO. Thats another 2000 hours for our 320s and 360s. For a 1700-hour engine, a top at this point is wasted money. The engine wont last long enough and youll lose a degree of trust in the engine overall long before those new parts wear out.
Second, many high-time Lycoming engines have taken decades to accrue those hours. Often, this calendar time-in-service results in pitting of the lifters and initial stages of spalling on the camshaft lobes. Removing cylinders to perform a top lays open all the valve train components and you might not like what you see. Lycoming provides criteria on whats considered acceptable corrosion for cams and lifters, but your mechanic might be hesitant to sign-off on any rust he sees. Its a judgment thing.
Bottom line: Youll face a premature overhaul when a simple rework was all you wanted. The decision, then, for a Lycoming top overhaul is a host of compromises. Its tricky business. Is the top-end weak enough, operationally speaking, to risk unearthing a cam that may not pass your mechanics eagle-eye muster? Thats not to say that a bad cam should be ignored or that an obviously deficient top end should sail through an annual. But you might be unraveling an ugly-and expensive-ball of yarn.
Check the oil filter and screens first. A camshaft in the initial stages of spalling will take up to 300 hours to declare itself via visible metal in the screens. That means that a careful analysis of the true condition of your cylinders-not to mention your bank account-should precede busting the first holdown nut loose.
Top overhauls are divided into two categories: Cylinder exchange and custom rework of your cylinders. Since most local maintenance facilities have neither the tools nor the expertise to perform valve, guide and barrel reconditioning, cylinder exchange is the popular MO.
The exchange route has a couple of advantages. For one, youll have a good idea what the top will cost and downtime will be predictable and minimal. The downside is that youll be paying for all new parts and all repair services the exchange shop provides, regardless of the condition of your old cylinders. Thats fine if your old jugs were on their third run with more welds than metal. But for relatively fresh castings and those 700-hours-since-new but serviceable parts, its a waste of core asset and hard-earned cash.
And if thats not enough, turning in your 1995 TSIO-520 casting for a heat-treated remake of a 35-year-old-chunk of sandstone with fins should scare the daylights out of you. Exchange your cylinders-Continental or Lycoming-if your old top end is high-time and of unknown origin and if you dont mind the additional cost. Have a custom rework shop overhaul your cylinders if theyre relatively low-time and on a first-run-since-new.
Generally speaking, mid-time-since-factory-new Continental cylinders will require the replacement of an exhaust valve and guide, re-facing of the intake valve, resurfacing of the exhaust port (with stud replacement), and oversize grinding of the barrel requiring new pistons and rings. The usual compliment of gaskets and keepers is assumed. Typical costs for parts and labor will run about $610 per cylinder.
First-run Lycoming cylinders will usually have more time on them than their Continental counterparts, but the parts needed for the average top overhaul will include a new exhaust valve and guide, re-facing of the intake valve, resurfacing of the exhaust port, finish honing of the barrel, new piston-pin plugs and new piston rings. Typical cost will be about $590 per cylinder. In some cases, the exhaust valve will be serviceable, saving $285 against the purchase of a new part. In most cases, a first-run Lycoming barrel will be standard steel.
For both brands of cylinders, required parts and procedures skyrocket when performing a top overhaul to previously overhauled jugs using multiple weld and chrome repairs. The rework shop then provides the same services it does for exchange jugs. But when the individual repairs are priced out, youll find yourself paying more than you would for an exchange.
New versus Rebuilt
A handful of years ago, financial commitments by the Big Two combined with methodical marketing and aggressive marketing strategies resulted in a price reduction for factory-new cylinder kits, as much as 40 percent in some cases. Spurred by the introduction of new cylinder castings from Superior Air Parts, and, more recently, Engine Components Inc., Lycoming and Continental retained their lower prices in order to make cylinder replacement rather than repair more attractive. They also wanted to get as many junk cylinders out of the field as possible.
And it worked. It worked so well that it drove many smaller cylinder shops out of business. Whats more, many salvage operators and those cylinder shops still in business found their core inventory to be worth a third of its previous value. In 1990, a first-run TIO-540 core cylinder was worth $400. Today, youre lucky to get $75 for the cylinder. (And you pay the freight.)
The difference between a new cylinder assembly and a rebuilt component has been reduced to the point that some cylinders, such as the Continental O-200, O-300 and 65 series are cheaper to buy new than they are to rework. Many cylinder shops wont even try to weld or chrome these small-bore cylinders.
Typical pricing on a factory-new Continental 520 cylinder assembly is around $1100. An exchange assembly with all new parts and a fresh CermiNil barrel will run about $800. The difference between new and rebuilt on the 360 and 470 cylinders is between $100 and $150.
If you send in your old cylinders for exchange on those rebuilt by a cylinder shop, its possible that at least one or two may be rejected. After tacking on a core charge to the rebuilt cylinder price, youll now be paying as much or more than you would have for a new cylinder.
The spread for Lycoming engines is generally greater with rebuilt cylinders running around $300 to $400 less than factory new depending on model. The issue of new versus rebuilt, then, becomes one of asset gain, expectation and pure dollars. Theres a limit on that last item, of course.
With few exceptions, engine overhaul shops recommend replacing your cylinders with new during the engine rebuild. The reason is simple. Its hard to justify spending $25,000 on the overhaul of your TIO-540-J2BD without adding an additional $2000 for new cylinder head castings. The rationale is clearer yet when you realize that a potential buyer will look on new cylinders as a big plus while used cylinders will be also-rans in the value equation.
Thats not to say channel chrome, silica-carbide chrome, nickel barrels or any other rebuild/rework process wont stand up to the competition; its merely a function of perception. And no matter how aggressive the marketing, a rebuilt, heat-treated cylinder using all new parts, cutting-edge barrel technology (well, not really), and fancy-sounding rework techniques is still a rebuilt cylinder.
Ultimately, its a salvage process that was originally designed as an economical alternative for those on a budget. Unfortunately, these days, the spread isnt wide enough. But money is money and saving a couple of thousand dollars on the top overhaul of your O-360 is attractive. If, on the other hand, the savings wont buy your way into brunch on Mothers Day, then take the obvious choice and enjoy the satisfaction of having done the best you could.
A word of caution: Regardless of how its marketed, sold or bought, a rebuilt cylinder isnt like a new one. Weld repairs and fatigued metal arent as reliable as new castings. This isnt subject to debate; its a fact pure and simple.
When choosing between new and rebuilt, assume that one cylinder will go soft at some point after the top and factor the cost of replacement into the spread between both prices. The mental exercise will automatically reposition your expectations.
Also With This Article
Click here to view the Topping Checklist.
Click here to view “Cylinder Sources and Supplies.”
-by Paul Brevard
Paul Brevard is editor of Aviation Consumers sister magazine, Light Plane Maintenance.