Engine Overhaul Basics for the Mooney M20J


November 2002

In our last ShopTalk, the engine overhaul experience for the Mooney M20K was shared. This month the same will be done for the M20J. Again, dos and don’ts and other specifics for this engine will be emphasized.


One of the things that personifies the M20J is the 200 horsepower four cylinder fuel-injected angle valve Lycoming engine, the ubiquitous IO360. On the M20J, the propeller governor is mounted on the accessory case on the back of the engine. The governor is supplied with high-pressure oil through a stainless steel line run from the front of engine. This line is the subject of a decade-old AD notice. Also on the accessory case are a spin-on oil filter, a mechanical diaphragm fuel pump, a dry vacuum pump and the dual magneto.


The Bendix (now TCM) dual magneto utilizes a single impulse-coupling drive. This drive and the magneto, over the years, have developed a dubious reputation for poor reliability. Confirming that reputation, this system has garnered more than one Airworthiness Directive. M20J owners are not the only ones on the product development team for Bendix. In 1979, while working at a Cessna dealership, I recall all nineteen of the factory new 172s being grounded due to an emergency AD on that Bendix dual magneto.


This dual magneto has now been around for twenty-five years and I believe the development period is just about over. However, this accessory is not trouble free and must have regular preventative maintenance. One AD against the impulse coupling requires a visual and measurement check every 500 hours. To insure the ignition system will continue to function, along with the coupling check, the magneto also requires 500 hour servicing and inspection. Accomplishing the periodic maintenance, along with using the correct gasket to reinstall the magneto, should improve reliability.


If an M20J owner wishes to have the same peace of mind of M20E and M20F owners, there is an option to convert to two separate magnetos. Mod Works holds an STC to install a Lycoming IO360 with two Slick magnetos in place of the Bendix dual setup. This option will add a few thousand dollars to an overhaul, but may be worth the enhanced reliability and elimination of the 500 hour ADs.


If M20E & F owners gloat about magnetos, J owners can regain face by recounting the early E & F engine mount breakage problems resulting in two separate Service Bulletins to beef-up the mounts, now required by a 1975 AD. The M20J has always had the improved engine mount. Nevertheless, now is the time to clean and inspect the mount after removal from the engine. Look for any dents, cuts or cracks. After correcting discrepancies, the mount should be repainted to protect it from corrosion.


As the mount is detached from the engine, carefully note the arrangement of the special steel washers (shims) at each attach point. The Mooney factory shimmed the engine so that the cylinders on both sides would be level (both pitch and roll axis) when the aircraft is level on jacks. The shims are available in varying thicknesses and are easily mixed up. The engine must line up with the ram air intake in the cowl. An improperly shimmed engine can cause aircraft rigging problems and decrease efficiency. You may assume that the original shim arrangement is correct, but verifying the engine is level sometimes reveals the folly of assumption. Always install new engine mounts. The engine level is, of course, checked after the new engine mounts are installed and the engine is bolted to the airframe. Adjust the shims, if necessary, before installing all the other items that need to be hooked up or bolted on.


A brief word about my product endorsements: all recommendations are based on observed performance in the field, real airplanes flown by real pilots. If a product works, I’ll use and recommend it. If I see it doesn’t, I’ll let you know. On that note, here are some pointers learned from many years and many M20J engine overhauls:


  • Use only new Lycoming cylinder kits. Due to the tight cowling on the M20J, the cylinders heat up unevenly. For channel-chrome, Cermichrome, Cerminil or Nu-chrome cylinders, this will probably result in high oil consumption, one quart per three or four hours is typical. Also, unless you want to participate in the Superior Millenium Cylinder product development program, stick to Lycoming. New Lycoming cylinders, properly broken-in, will use one quart of oil per twelve to twenty hours.

  • Never install a reground camshaft in this engine; use a new Lycoming camshaft. Lycoming has proprietary lift and duration data that is not available to other manufacturers. A new Lycoming camshaft allows the engine to breath at its maximum; gas exchange is optimized over normal operating ranges. This will allow for a higher service ceiling and better speed and fuel efficiency.

  • Replace the hydraulic lifters with new Lycoming lifters. Have the lifter bodies reground or replaced with new ones. Maximum oil flow to the rocker arms and valve guides will result helping to prevent premature valve guide problems.

The engine replacement and overhaul market is extremely competitive. A factory rebuilt or remanufactured engine through a wholesale reseller is usually a best buy for a run-out engine. However, if the engine is a first run from the factory, then you can save one or two thousand dollars with an overhaul from a reputable overhaul shop. You can get a good engine if you and the shop follow the above pointers. This will cost more than the normal price quoted by a shop. You must dictate your demands up front and follow up to verify the engine is being overhauled to your specifications.


If the engine is other than first run, I recommend a Lycoming factory overhaul, remanufacture or, rarely, a new engine. All overhaulers will comply with ADs and Service Bulletins, but the factory will also carry out service letters. Rather than performing an overhaul and return-to-service, many parts will be replaced, often with the latest version. Most engine shops won’t be able to compete with Lycoming and provide these new parts during an overhaul.


A factory overhaul will come with a new Lycoming engine logbook, but the total time on the crankshaft will be carried forward. A factory-remanufactured engine may have a used crankshaft also, but the log will indicate zero time since remanufacture. Both engines will have new cylinder assemblies – lifters, camshafts and possibly a new crankcase. Also, new sparkplugs, ignition harness and fuel pump. The fuel injection servo will be overhauled along with the alternator and dual magneto.


Lycoming has just started shipping engines with a new permanent-magnet Skytec starter. Unfortunately, the IO360 for the M20J has been sent out with the wrong starter. The solenoid interferes with the airbox. Exchange this starter with the Skytec 149LS-M, which is made specifically to fit the M20J. If you use an overhaul shop and make no special arrangement, you will get back the old Prestolite starter, overhauled. With a good battery, the Skytec starter will crank the engine about twice as fast as the Prestolite and will weigh ten pounds less. That is ten pounds more useful load for your M20J!


The vacuum pump, if it has any oil contamination, will need to be replaced. Please, never install a rebuilt vacuum pump if you fly any hard IFR. The rebuilt pumps are cheaper to purchase, but do not provide the reliability of a new pump. A new pump will go 800 hours safely if the filters are changed regularly and there has been no oil contamination. Determine the hours of service on the pump and project the hours to when the dual magneto needs servicing. Since the vacuum pump is removed during the magneto service, replacement now may save downtime and money.


While the muffler is detached, examine the flame tubes. Remove the tailpipe and insure the flame tubes are intact and not warped or missing. If your exhaust system needs repair, you may want to replace it with a new system. Knisley Welding of Loomis, California can provide a new M20J exhaust system for around $1,700. Don’t take chances with repair of the muffler as it is the source of cabin heat. The smallest crack or blister on the muffler can cause carbon monoxide to leak into the aircraft cabin.


The oil cooler will need to be sent out for flushing and overhaul. Also the propeller must have the old oil cleaned out. If the propeller is leaking oil or near TBO, it is wise to have it overhauled now.The prop governor will also need to be sent to a prop shop for overhaul.

If the age of the oil and fuel lines can be determined to be less than five years, a flushing and visual inspection may be all that is necessary. If older than five years or otherwise in need, replace them with fire-sleeved new style hoses. Because the M20J is not normally turbo-charged, 124H style hoses are not necessary. The hoses for the oil cooler must be 156F style with all others 111 medium pressure. The fuel pressure hose from the servo to the pressure transducer must be fire-sleeved and routed away from the exhaust system. This is critical to avoid fire as the hose can become heat brittle and leak fuel into the engine compartment.


If your operating climate warrants it, this is a good time to install an engine oil/cylinder heater kit. Also, anytime the engine is available for close inspection, examine the baffling. Repair or replace any baffles that are broken, folded or otherwise not making a good seal. Diligence here insures proper airflow through the engine and can prevent cylinder hot spots and costly early top overhauls.


The ram air door needs close inspection. Check the door seal and bushings for integrity. Seals have been found to be missing and bushings so worn that the door will not seal correctly. Inspect the flexible rubber seal on the ram air box and oil cooler box. Replace as necessary. The Scat hoses to the muffler can should be replaced with new ones as well as the ¼ inch rubber hoses for the induction sump and fuel pump drains and also the engine breather hose.


Before cowling the engine, the engine must be run to check for leaks and oil pressure adjustment. Uncowled engine runs should be kept to a minimum, so it is a good idea to do a dynamic balance at this same time. The balance does not have to be perfect, just close for now. After the engine is broken in, a complete dynamic balance is in order.

Here are some basics for completing a proper break-in on a new engine:

  1. For the first fifteen hours, try to climb at 100 knots or more to provide sufficient cooling for the cylinders.

  2. Use straight weight mineral oil for the first 25 hours. In warm climates run 50 weight and use 40 weight in cold, winter climates.
  3. Keep close track of oil level as you fly the 25 hours. Make a note of tach time and amount whenever oil is added. This will provide an indication of how the break-in is progressing.
  • Lycoming recommends changing the oil at 50 hour intervals. However, because this is a high compression engine (by aircraft standards) its oil will contaminate and thin out sooner than the oil in a low compression engine. I recommend 25 hour changes. Also, unless you operate in cold climates, do not use multi-viscosity oil. After break-in, clean fresh straight-weight detergent oil will reduce oil consumption. As said before, a properly broken-in IO360 will typically use one quart for 12 to 20 hours of operation. Therefore, only one quart of oil need be added between oil changes.

All this leads to the question, “What is this going to cost me?” Variable factors are age and condition of accessories, upgrades (magnetos) and unusable core components. Depending on the choice of overhaul or remanufactured engine, you had better plan for $24,000 to $28,000. If you have an additional $10,000 (engine core deposit) you may be able to pre-order (from Lycoming you can) your replacement engine and considerably reduce the aircraft downtime. When the old engine is sent to the overhauler/remanufacturer and the core components are accepted, the deposit is refunded.


As always, if you have a question about this article, you may contact me at my aircraft repair shop, 307-789-6866 or via e-mail. Until the next ShopTalk, enjoy flying your Mooney.