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CONTINENTAL ENGINE OVERHAUL CONSIDERATIONS
ShopTalk - October 1999


MAPA Editor's Note: Kerry McIntyre continues his two-part discussion of engine overhauls. This month, he takes a look at the Continental series of engines that power the Mooney M20K and M20R series. Kerry has maintained and rebuilt many Mooney airplanes from his shop over the years and does superb work. He can be reached in Wyoming at his shop at (307) 789-6866.


With this second in a two-part article, I would like to continue with our discussion on aircraft engine overhauls. In the July issue of the LOG, I wrote about the Lycoming engine overhaul process and some basic items that are common to both Continental and Lycoming engines. This month I will discuss Continental overhauls, particularly the TSIO-360, TSIO-520, and the IO-550 engines. All of these engines are very different. I will start with the TSIO-360 series since they are the most common engines in the current Mooney fleet.


The TSIO-360 series engine has been around a long time and has been used in everything from the Cessna Skymaster, the Piper Turbo Arrow, and Seneca and of course the Mooney M20K series. The TSIO-360 is a very smooth, low compression engine that when trimmed properly will provide hundreds of hours of trouble-free operation. However, there are several points I want to make that I believe are very important to getting a proper overhaul on the TSIO-360.


The first point is that new Continental cylinders are a must for the proper overhaul of the TSIO-360. Because this engine has been around so long, overhauled exchange cylinders you get may be from a 1971 Cessna Skymaster that was used for charter and got a thousand hours per year put on them. It is now twenty years later and you are the lucky purchaser of these completely worn out but freshly overhauled beauties! Don't waste your money unless you can verify the total time on the cylinders is less than 1800 hours. To be safe, my suggestion will always be to purchase new cylinders at overhaul time.


The second point is to stay away from chrome cylinders at overhaul time. With any turbocharged engine, you don't want any type of chrome cylinders. Chrome won't dissipate the extra heat created by the turbo system. The last thing you want is a new engine that runs at red-line CHT all the time.


Again, I would like to repeat my primary point here - bite the bullet and buy new cylinders at overhaul time. Boring the steel cylinder oversize works very well in the short term but will cause you grief around mid-life when they are worn out. By starting with new cylinders at overhaul, at mid-life when the cylinders are worn out and lose their choke, you can then pull them all to be bored oversize so they will make it to TBO.


All this discussion about buying new cylinders at overhaul time comes down to this: I have never seen a Continental cylinder on a turbocharged airplane make it unscathed to TBO. Most don't make it past mid-life. As a matter of fact, even most non-turbo TCM cylinders larger than the 0-200 do not make it past 1200 hours when operated from sea level.


This brings up an interesting story I'd like to share with you. One day I had a brand new Beech A36 in my shop with three of its six IO-550 cylinders worn out with less than 900 hours total time since new. The owners were furious. I insisted we call the Continental representative so he could answer the big question of why the cylinders had worn out so soon. The technical representative pointed out that the baffling on the A36 was why the cylinders on this particular IO-550 did not make it to 900 hours (Beechcraft was at fault). I said to him, "No Continental cylinder makes it to TBO without being removed at least once." The Continental rep said I was wrong. In fact, he said the California Highway Patrol's 18 Cessna 185s (IO-520 and IO-550) all make TBO without pulling any cylinders. I was surprised to hear this and decided to investigate.


One day a CHP Cessna 185 taxied by my maintenance hangar, so I chased after him and questioned the officer about cylinder life on their engines. I asked him about power settings. His reply was, "80% of our flying rime is on patrol. During these patrols, we fly at 1900 RPM and 19 inches of manifold pressure with 10 degrees of flaps at about 70 MPH." No wonder they are getting full TBO with their cylinders! If you operated your Continental engine at 45% BHP for 80% of the time you will not need a top overhaul either.


However, for most of us in the real world, that operate our Continental engines at 70% BHP (or higher) 80% of the time, had better plan on a top overhaul at mid-life (engine, not owner). Indeed, the fleet of CHP Cessna 185's never needs a top overhaul at mid-life. Again, this is not a realistic expectation for the majority of us.


Another unusual item with the TSIO-360 is the forward main bearing seal. It is made of nylon and if this seal gets damaged, your prop will go to flat pitch as the oil heats up. This seal must be installed properly. If it is damaged, you must split the crankcase to replace it.


Continental engines are not known for eating camshafts as long as proper dry tappet valve clearances are maintained. So you can get away with an overhauled camshaft but always buy new lifters.


Another interesting item on Continental engines is the starter drive. Most overhaulers will just replace the spring in the drive but the expensive part is the drive gear shaft as it also wears out. Try to make sure that both the spring and the drive gear shaft are new in your starter drive. The #1 alternator on the TSIO-360 is gear-driven from the back of the accessory case and has a shock drive on the shaft. Always get a new shock drive and a quality overhauled alternator.


The turbocharger must be overhauled by a reputable shop along with all the fuel injection components. Speaking of the fuel injection system, if you have a six probe EGT system installed, you can do your own GAMI type fuel injection adjustments. Do this by simply changing fuel injection nozzle sizes until all six EGT's match under normal cruise power settings. The only limitation is that you must stay within the manufacturer's limits on nozzle size for your engine model. Continental does this when they run in a new engine on the test stand, but by the time you add the airframe manufacturer's baffles and cowling, the EGTs have changed. Continental fuel injection systems and nozzles are very adjustable compared to Lycoming systems. My fuel injection nozzle trays are numbered 1-6. So, for example, I know that in #6 cylinder was a 13B nozzle and in #2 was a 13C, etc.


Early TSIO-360 engines did not have pressurized magnetos. This is a must-do item if you don't have them. Additionally, some of the early Continental engines didn't have plates bolted to the front of the engine to keep the crank seal from popping out. Make sure your engine has them if the parts catalog calls for them.


Later model M20Ks had an air-oil separator installed by the Mooney factory that can be retrofitted to earlier airplanes. I suggest you do this at overhaul time on the TSIO-360. It will really help keep the belly clean and free of oil streaks. Another small but important item is the automatic wastegate control retrofitted on some 231 models and standard from the factory on 252 models. The automatic wastegate control should be sent back to the manufacturer for inspection and cleaning before it is installed on a freshly overhauled engine.


I believe that the crankcases should always be lapped and line bored. All turbocharged engines should get a new oil pump and scavenge pump gears. Check the heater muffs for blisters. All heater muffs with more than 2000 hours TT are suspect, especially on the turbo side of the exhaust system. While the engine is off your airplane, this is a good time to treat any exhaust corrosion sometimes found on the forward and aft side tubes of the engine mount. Do not tape anything on the mount tubes in this area to protect them, as corrosion could form underneath and you will not be able to see it. Ignorance is not bliss in this case.


Always install a new oil cap on your TSIO-360 at overhaul. This cap must be tight or it may lift and pressurize the crankcase causing oil to pump out of the breather tube. One last item on the TSIO-360 and TSIO-520 engines - make sure you have the latest VAR style crankshaft or you will have to purchase a new VAR crankshaft at overhaul to replace your non-VAR crankshaft. The purchase of an aircraft crankshaft is not a wallet-friendly item.


Continental came out with a seventh cylinder stud hold-down location on later versions of the -520 and this was an important update. The -520 series are known for cracked crankcases, so be sure you get the seventh stud mod. The fuel pump drive shafts were another problem area with the -520. Make sure you get the latest drive shaft and it is lubricated properly.


A brief comment about the Rocket conversion that utilizes the TSIO-520. This is a tight-fitting installation. With the engine out for overhaul, this is a perfect time to closely inspect the engine mount and exhaust system that are normally hidden underneath and behind the engine. Make sure that everything is blister-free, clean, and in good condition before reinstalling the engine.


The normally aspirated IO-550 series are the latest generation engines and have most of the Continental upgrades. However, there have been some problems. Fuel pump drive shafts and prematurely worn cylinders have been some of the growing pains. On all Continental and Lycoming engines, make sure that the f1exible baffles are not torn or folded backward when the cowling is installed. On all turbo models, replace both turbo oil check valves at overhaul. Red silicone gaskets for the rocker boxes work well on the -520 and -550 engines, but they don't work on the -360. Of course, comply with all service bulletins and airworthiness directives during overhaul.


Always have your new engine fuel flows set up for the latest Continental specs. Just because all the fuel injection components are overhauled does not mean they work together on your airplane. And don't forget to strobe the prop and adjust the prop governor for maximum RPM per Continental specs.


I have left out the more technical items but have tried to hit the important highlights of Continental aircraft engine overhauls. I can not drive home the point enough about new cylinders. Always buy new ones at overhaul time. As far as using Superior parts versus Continental, I can say that Superior is just as good and usually a little more wallet-friendly than Continental parts. I will reserve judgment until enough hours are on the Superior cylinders to tell just what service they will provide.


I personally believe that a factory overhauled engine is the best bang for your buck. You get the resale value of the factory job and you get a lot of new parts that the overhauler just can't afford to install and stay competitive price-wise. I have found that there are no bargains in the overhauled engine business. Just good engines and bad engines. If you do it right, your Continental engine should use about one quart of oil per twelve hours (or better). Do not be surprised by the top overhaul needed at mid-life, it's just part of the Continental ownership experience.


With these two articles, I hope I have been able to further your understanding of Lycoming and Continental engine overhauls. If you have questions about this or any other ShopTalk article, please e-mail me at shoptalk@knr-inc.com or call me at my aircraft repair shop, 307-789-6866. All ShopTalk articles can be read here at www.knr‑inc.com. Until next time, enjoy flying your Mooney.

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