September 30, 1996 Continental's O-470 Series

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by	Paul Brevard
	This article first appeared in the July 1996 issue of LIGHT PLANE MAINTENANCE and appears here by permission of Belvoir Publications.

The 0-470 is a rugged engine that's capable of producing solid and dependable power. It distinguishes itself from other six-cylinder carbureted engines by its characteristic lope at idle and throaty exhaust stack burp. When balanced, an O-470 will run at cruise power without the least bit of burble or vibration.

There are 15 models of the O-470 engine flying around in Beech Debonairs, Bellancas and Cessna 180s, 182s, 310s and various AG models. The single common denominator in all models is the bore (five inches in diameter) and stroke (four inches in length). Cylinder and crankcase castings are the main differences between the various models. Other than these, the basic engine has remained the same with only minor changes made to accessories and peripheral equipment. To its credit, the engine's magnetos, carburetor, starter and charging system have all been upgraded and improved over the years.

The predecessor to the O-470 was TCM's old E-225 series. And though the E-225 enjoyed a successful career with Bonanzas and some models of the Navion, an engine that had a better weight-to-power ratio with a simplified carburetion and lubrication system was needed. The E-225 had a separate accessory case, dry oil sump and dated accessory configuration that became a liability to the design requirements of newer, more streamlined planes. In 1952 that need was satisfied when the 0-470-A was given its birth certificate: Type Certificate E-273. The subsequent 14 additions to this TC include five models which are eligible for an upgrade to a fuel-injection system in place of the original equipment Bendix pressure carburetor.

Of the 15 originally certificated, only eight models, the O-470-G, -J, -K, -L, -M, -R, -S, and -U, remain in the current TCM engine specification list. The most popular of the eight are the 0-470-L, -R, -S and -U models which power Cessna 180s and 182s. There were over 21,700 of these planes built between 1953 and 1986, so it stands to reason that these last four models would comprise the bulk of the O-470s flying today.

Cases & Cylinders

There are few issues concerning 0-470 crankcases because this engine is not under the same kinds of stress an I0-520 or turbocharged engine is. And while all crankcases crack, the O-470's case is remarkable because TCM's SB #M90-17 — which recommends the replacement of engine crankcases with newer and stronger styles during overhaul — will accept any part number and casting number assigned to the O-470 engine without requiring an upgrade. In contrast, larger, higher horsepower engines have had to change their case design in an ongoing effort to eliminate deck cracking, (with limited success). Thankfully, replacement-before- TBO type of crankcase cracks are somewhat rare on O-470s.

However, while this engine may not need upgraded cases, factory remans are currently using a reinforced casting which, in large part, appears identical to the sand-cast 520 case.

Cylinder castings for this engine have changed in style and structure eight times since the late 60's. Beefed up spark plug bosses, improved exhaust port walls, combustion chamber profile changes and threaded barrel engagement areas have all been altered by subsequent cylinder improvements.

The basic 0-470 engine consisted of two separate configurations identified by the shape and style of the cylinder head. Models -A, -E and -J all incorporated a parallel valve (also known as a straight valve) cylinder head which is easily recognized by the small rocker box cover and short, thick cylinder fin castings. Characteristic of these models is the large, round intake port and the small, two-bolt, exhaust port and stack assembly. These engines are still seen today on Beechcraft Debonair and very early Cessna 180 models.

The angle valve cylinder used on all other models is the present standard issue for O-470 engines. This style cylinder's fin area is 75% greater than the parallel valve type and the induction and exhaust ports are standardized with a four bolt pattern. Due to differences in the deck hold down stud configuration, angle valve cylinders can't be fitted onto parallel valve cylinder crankcases.

Cylinders are also classified as light or heavy. A light cylinder is one that depicts a casting manufactured in the late 60's and early 70's and is similar to the old E-225's cylinders. Later models of 470 engines use the heavy style casting made available around 1978 and subsequent cylinder part numbers are an improvement on that design. Heavy cylinders are physically identified by a thicker fin just below the top spark plug hole and a vertical bridge below the bottom spark plug hole. While these are not the only identifiers, they serve to assist in a definitive assessment of cylinder and casting age. You may consider upgrading at overhaul to a heavy cylinder appropriate for your engine if you are currently running light cylinders on their second run.

Valve Guides

Older O-470 cylinders were equipped with aluminum-bronze intake and exhaust valve guides. And while the intake guide performed well and is, in fact, still used today, the exhaust guide didn't hold up to the heat created in the exhaust port. When 100LL fuel was used, the exhaust guides failed even faster due to the prolonged burning process of higher octane fuels. The exhaust valve guide was then changed to a Ni-Resist steel guide which is better suited to the higher temperatures encountered. After some years of success with the Nitralloy guide in the 520 series engines, the 0-470 was given the option of using the same top end parts as the 520. As a result, the use of the pre-sized Nitralloy guide, pre-sized aluminum-bronze intake guide (with seal) and improved four-ring pistons became the standard at engine overhaul.

In recent years, the Nitralloy guide has been replaced with a new version of the old Ni-Resist guide due to some concerns about accelerated stem wear and isolated cases of valve sticking due to corrosion in the guides of inactive engines. TCM's SB #M90-13 talks about the corrosive environment in which exhaust valves must operate and states that accelerated stem wear is caused by these elements. More to the point would be the elevated temperature the working Nitralloy guide creates by virtue of the nitrided surface on the guide's inside diameter. This higher temperature tends to flash (carbonize) the lubricating oil — especially at shutdown — and can cause immediate and permanent valve seizure in the guide.

Current issues of TCM's SB #M85-8 and #M85-19 define the options for superseded piston assemblies and valve guide combinations. It's important to be aware that older, outdated parts can still be purchased but they are probably not the best option given the expectations put on our engines today.

Pistons And Rings

The biggest change in pistons occurred with the issuance of the four-ring piston with semi-keystone (half-wedge) compression rings. This affected most of the 470 series with the exception of the 0-470-U. The -U model uses a cam-ground, steel-inserted piston. Clearance between piston and barrel became a serious concern with this cam-ground piston and SB #M90-5 R1 was issued to clarify concerns for skirt clearance on affected engines. Field experience found that these cam-ground pistons would vary on skirt diameter as much as 0.0015 of an inch. This caused problems when setting up clearances during reassembly. If skirt-to-barrel clearance on a steel barrel drops below 0.008 of an inch, the piston will rub in the bore causing scoring, metal contamination and a number of other unpleasant things. (Burnt paint on the barrel is sometimes an outward indication of the piston rubbing in the bore.) The old, all-aluminum piston used in all other O-470 engines doesn't have the same close-skirt clearance and thus, isn't susceptible to this problem to the same degree.

Another problem associated with older cylinder castings is discussed in SB #M85-9 R1. (This SB identifies cylinder combustion chambers that are not compatible with newer, high profile pistons.) The piston's outer circumference contacts the cylinder head's inner bore when at top-dead-center (TDC) on some 470 models. This is a particular concern for the 0-470-U which uses a tall, high compression steel-insert piston rather than the domed, low compression style. If you change your cylinder or piston model during overhaul, be sure to check component compatibility.

As with many Continental engines, TCM SB #M85-15 should be used to establish acceptable criteria for cylinder compression checks and this is particularly true for the O-470-U engine. The -U engine is a low RPM, high compression ratio (8.6:1) engine which develops 230 HP at only 2400 RPM. This added stress on the ring and barrel has accelerated a wear pattern commonly found in all 47Os and results in sagging compressions much like those found in the IO and TSIO-520 engines. As always, if valve leakage is present, repairs must be made, but if ring blowby is experienced and falls within the guidelines of the Service Bulletin, then the cylinder is considered a serviceable part.

It should be noted that the steel-insert piston allows for a steel ring to ride in a steel ring land. This combination stays amazingly free from any real carbon buildup and allows the ring to float freely in the ring land. However, because of this condition, compression checks on the -U engine will be somewhat erratic and can vary substantially from previously recorded compressions. 470 engines using the all-aluminum piston don't seem to have this problem and compression results are generally higher and more consistent. The lower compression and stability of the piston ring in the ring land due to normal carbon buildup around the ring also probably contributes to the higher results.

Accessories, Coolers And Pumps

Oil cooler configurations have allowed for the installation of a variety of oil coolers. There are five manufacturers of oil coolers for the 0-470 and four different styles to choose from. Originally, a small, congealing cooler was installed for early -L models. The subsequent need for additional cooling brought about a large capacity short cooler, then a large capacity, tall cooler and finally, a large style, short and long non-congealing cooler.

It's rather confusing but the easiest way to order a cooler is by engine specification number — i.e. 0-470-R spec (1) — or just call your local oil cooler facility and explain to them what you have and what you want by bolt configuration and cooler dimensions.

Oil pump assemblies have also made some advancements in the O-470 engine. Early model -L and some -R engines were produced with oil pumps supporting an acorn-style relief valve which didn't allow for oil pressure adjustment. Later versions incorporated an oil pressure plunger and jack screw arrangement which allowed for easy oil pressure adjustment.

Current production offers as optional equipment a new one-piece pump assembly which, in addition to an adjustable oil pressure relief valve, houses an oil filter pad as an integral part of the casting. During overhaul, we recommended that either of these oil pumps be used in place of the older nonadjustable style.

When Continental purchased the Bendix line of magneto ignition systems, it became necessary to request which magneto type you wanted when purchasing an engine. Both Bendix and Slick magnetos work well with the engine, however, in our opinion, you're better off going with Slicks. Bendix mag ADs have been coming out with annoying regularity over the years and there's no reason to believe we've seen the last of them.

The ignition timing indicator plate attached to the starter adapter should be removed in accordance with SB #M84-8 and setting the magneto timing should be accomplished per SB #M88-8 R1. (This uses a timing degree wheel to verify piston position and can be much more accurate than the plate and starter adapter pulley marking.)

SB #M87-15 requires the use of a ground strap on alternators that are shock-mounted to the engine.

There are several bulletins regarding Marvel-Schebler carburetors and part throttle mixture control. Service Bulletin #M90-18 discusses replacing the composite float installed in the early 80's with a newer metal version.

Also, some Cessna 180s and 182s are experiencing a greater than normal amount of automatic leaning after the engine comes off of full power. Some owners are finding cruise conditions that will not allow for any leaning at all. This problem surfaced in the early 70's and Service Bulletin #M74-15 revised the economizer setting for the carb. However, the problem began resurfacing over the last two or three years. Some in the industry believe that changes in 100LL are the cause. (If you think it may be the venturi, forget it. The O-470's MA4-5 carb has had a one-piece venturi for over eight years.)

Operation and TBO

Graphic engine monitors, EGT/CHT analyzers, fuel-flow transducers and digital tachometers give exact readings for every engine parameter measured and can be great aids when trying to troubleshoot engine problems. However, a single-point EGT is the only tool needed to sufficiently lean your carbureted 0-470 engine. This is because fuel and air distribution in the engine's induction system is grossly inefficient and monitoring of single-degree changes and measuring fuel in ounces rather than gallons is unnecessary. Adjusting the fuel mixture for a particular power setting can be easily accomplished with a single-point EGT using TCM Service Bulletin #M89-18 as a guideline. Percent power and knowing where you are in relation to peak EGT is then all that's necessary for adjusting the mixture on an 0-470 engine. (Keep in mind that too much fuel is as destructive as too little fuel.)

As one would expect when such a large displacement engine draws its air through a carburetor, O-470s have a propensity for carb ice. The large volume of air that passes through the carb's venturi causes a significant temperature drop along with the desired pressure drop. In fact, many owners install a kit to read direct carb venturi temps in an effort to keep the icing condition down to a minimum. Chief Aircraft (800-447-3408) has an OAT/Carb Temp gauge kit for about $330. San-Val (800-423-3281) has two Carb Ice Detectors available, both are around $280. Give them or the other dealers in Trade-A-Plane a call for more information.

Another design drawback of the O-470 is that the front two cylinders tend to run excessively rich. This is due to residual fuel that collects in the balance tube located across the front of the engine. In the late 70's, a Cessna kit, now discontinued, was made available which wrapped this tube in insulation for winter use. The kit usually helped to alleviate the problem somewhat. (If you plan on installing a kit of your own design, be sure to use insulation that's fireproof!)

All 0-470 engines carry a TBO of 1500 hours except for many 0-470-U models. TCM SB #M89-13 (note four) is very specific about which -U engines hold a 2000-hour TBO and what requirements must be met to allow for the extension on other -U engines. This means that the -L,-R, -S and any other models you choose to list will always be 1500-hour engines. Period.

Also note: The 2000-hour TBO extension is only allowed for engines using cylinders equipped with steel barrels, not chrome. If your engine was originally manufactured at Continental as a 2000-hour engine, then, presumably, chroming of those cylinders would be considered acceptable. But plan to use new steel cylinders if making the change up to the 2000-hour TBO with an originally 1500-hour TBO -U engine.

And Now The Problem Areas

Probably the deterioration most expected with the 0-470 would involve sagging cylinder compression and high oil consumption. Exhaust valve deterioration, exhaust guide wear and piston ring and barrel wear contribute to high crankcase pressure, oil blowing from the breather tube, fouled plugs and high oil consumption. Continental has changed a variety of cylinder parts in an effort to keep these calculated wear patterns to a minimum but the average engine will face a certain amount of deterioration. Enough, in many cases, to require top end attention between 900-1100 hours, especially for those engines which take many years to acquire that much time in service.

Oil consumption in O-470s can be particularly annoying, and in our opinion, especially for engines fitted with channel chrome cylinders, with a quart every three or four hours being common. Cermichrome, Nu-Chrome or any of the silica-carbide processes seem to perform well from the standpoint of oil consumption. A quart every 12-20 hours is not out of the ordinary, at least in the initial hours after overhaul. However, after accruing 600-800 hours, many times these chrome barrels see an oil consumption increase of as much as 50% or greater, depending on engine model. The -U engine, for instance, with the higher compression piston, has a better-than-average potential to wipe the barrel clean of the impregnated silica-carbide, thus increasing oil consumption and decreasing compressions. Generally speaking, the -L, -R and -S engines do fine with silica barrels and have much less of a potential for silica failure.

If the option is there, steel cylinders are preferred over any chrome process. Oil consumption will run one quart in 18-25 hours until midway through TBO at which point consumption might deteriorate to one quart each eight to twelve hours.

In contrast to a Lycoming engine, camshaft failure is not expected but it happens with at least some frequency. The 470 has no greater potential for this kind of failure than does any other Continental and in most cases, the 470 engine will continue in normal service without recognizing cam profile loss due to the slow rate of wear and the very small particulate produced. When cam failure does occur, the front shared lobe between the number five and six cylinder is usually the culprit. More often than not, this wear pattern begins with minor pitting of the cam and follower due to corrosion. This degradation of the hardened surface continues until the soft layer beneath the hardening is breached. At this point, spalling occurs and material removal accelerates in rate of wear and particulate size.

Some speculate that the front cam lobes are affected because of the proximity of the breather tube to this area of the cam. Whatever the cause, the cam can be checked by removing the pushrods and cam followers and visually inspecting it for wear.

What's New For the O-470 Engine?

Aside from changes to some accessories and the possible installation of some form of electronic ignition, the basic O-470 will probably remain the same for many years. And I, for one, hope that the changes that do come are minimal. You see, my first airplane ride as a kid was in a barn-stored Cessna 180B that roared as it took-off from a small grass strip less than five miles from my home. It's an adventure I relive each time I hear that high, throaty rumble from a passing Cessna powered by the venerable 0-470.