Engine Overhaul Basics for the Mooney M20K

October 2002

This month’s ShopTalk will discuss some of the aspects of the engine overhaul of the Mooney M20K and what the aircraft owner must consider during this time. This article will detail the expenses and some dos and don’ts for this type of job. If you own or are in the market for this aircraft and the engine is near TBO, you’ll want to read this article very carefully. I believe you will be as surprised as I was at the costs involved.

The early M20K (231) came with a Continental TSIO-360 GB engine. Continental has since discontinued manufacturing certain parts for this engine. As a result, owners of this engine will be forced to upgrade to a TSIO-360 LB, which was used on later 231s. The 252 (also designated M20K) came with a Continental TSIO 360 MB. These three engines are all based on the same crankshaft, crankcase and cylinder configuration. The intake and exhaust systems, the turbo and fuel injection systems are what make these three engines different from each other.

In all high-powered and especially turbocharged aircraft engines, heat is the enemy of reliability. Close attention to heat control and management is the watchword here - close attention.

Intense heat generated by the exhaust system often causes erosion of the engine mount tubes and forward plates; a problem with M20K engine installations. It resembles corrosive pitting in the metal and weakens the mount. This is common in aircraft that have tubular engine mounts with turbocharged engines. Over the years, attempts to solve the erosion problem have mostly failed; epoxy paint, for instance. However, one solution seems to work reliably. The engine mount was sent to Kasola & Associates; a firm I have utilized before. They recommend a ceramic coating used on Indy car exhaust systems, which is made to continuously withstand 2000º F. After the engine mount was repaired, everything was coated with this special coating. The overhaul and ceramic coating will cost $900 to $1,300 plus the cost of removal and shipping.

The flexible hoses in the engine compartment that carry fuel and oil are a very important part of the engine overhaul process. There are twelve to fourteen of these hoses installed on the 231/252 and all are a potential fire hazard if not inspected and replaced on regular basis. Because turbocharged engine exhaust systems run cherry red hot, all rubber products in the engine compartment are subject to intense heat. Any critical hose should be replaced with a 124H style fire-sleeved hose assembly. The replacement cost for all these hoses will be between $1,200 and $1,600. Mooney also uses some very interesting SCAT hoses that are square in shape but have round ends. Save these hoses if at all possible as a single replacement can cost over $300. Small holes in these hoses can be repaired using fuel tank sealant or orange RTV. Rips and tears, however, will require hose replacement.

Your engine will not be returned with all the accessories overhauled or replaced. Here is a quick check list on how to deal with the accessories that won’t come with the engine:

1. Vacuum pump: always replace with a new pump unless it has less than 300 hours TT and has no oil in it. Never ever install a rebuilt vacuum pump on an airplane that is used for IFR flight. A new vacuum pump should last 800 hours safely as long as it gets no oil in it and the system filters are changed regularly.

2. Waste-gate and controller: if you have an automatic waste gate made by Merlyn send it back to them for cleaning and a function check. If you have the fixed waste gate make sure it is not damaged or clogged and that the bolt that adjusts the critical altitude has 9 to 11 threads showing. The 252 came with an oil operated waste gate and controller. Send both of these units to a FAA approved turbo accessory overhaul shop such as Approved Turbo Components.

3. Over-boost valve: the 231 and 252 use different manifold pressure settings for the same valve. Make sure the shop overhauling the valve knows your engine model.

4. Prop governor: get this item overhauled and have a new fly weight assembly installed.

5. Alternator(s): the 231 and 252 came with an engine driven alternator with a rubber shock drive system. Have the shock drive slip tested and perform at least 500-hour inspections and/or servicing. If the shock drive fails the slip test, a new replacement will cost $1,200. Some 252 aircraft came with dual alternators. The second belt-driven alternator and voltage regulator was an option. A 500-hour inspection/ servicing should also be performed on this alternator.

6. Turbo check valves: Both valves should be replaced with new ones at about $500 each. Be sure that valves are installed so the oil flows the correct direction and that the word “Hinge,” stamped on the valve, is facing up.

Your new engine should include the starter, magnetos, ignition harness, spark plugs, fuel injection system and turbocharger, either overhauled or new.

Rubber engine mounts need to be replaced if they are more than three years old or if they are damaged. The 231 and early 252 now have differing arrangements for the rubberAugust engine mounts. When you reinstall the engine with the new rubber mounts do not follow the old parts manual. Use a 1988 or newer M20K service manual for the correct orientation of the rubber mounts, bolts, washers and shim washers.

Inspect the exhaust system for any blisters or cracks and repair as necessary. Always use a FAA approved aircraft exhaust overhaul shop, such as Knisley Welding. Always replace the V-band clamps with new ones.

A Continental factory remanufactured engine comes with a new exhaust system, but this engine will cost about $6,000 more than a quality overhauled engine with new Continental cylinders on it.

I do not discourage anyone from buying a factory remanufactured engine but if you have a first-run engine this is probably a waste of money. The advantage of a remanufactured engine is less downtime, a new exhaust system and a zero-time logbook. The disadvantage is you get an assembly line engine model that has had four Airworthiness Directives issued against the crankshaft for manufacturing defects (1997, 1999 and two in 2000). Your original crankshaft may not need to be replaced providing it has no AD’s against it. Because it has been operated reliably it is unlikely to have a future AD issued against it.

Remember to send your prop out to have the old oil cleaned out of it. If it does not need an overhaul, a flush on a two-bladed prop will cost about $400. If it needs an overhaul, the cost will be about $2,000; even more if the blades are dimensionally too small andneed replacing.

If you chose to overhaul you existing engine, use a reputable overhaul shop and always have new Continental cylinder assembly installed. If you do your homework and know what you want in an overhauled engine then the warranty period will be a non-issue because you will never need it. There are only a handful of engine overhaul shops that could be considered reputable. You must do your homework and choose carefully. Use recommendations. You may call me for my opinions.

After an engine or prop installation have a dynamic balance performed. This makes a huge difference on this installation. Be careful when dynamic balancing this propeller; only utilize the placement for existing weights. If washers need to be added for fine-tuning, install them where the weights would normally be. Never, never, never drill a hole in the back plate of the spinner to install a screw, nut and washers to balance these engines. The hole will eventually crack out due to centrifugal force requiring purchase of a new spinner bulkhead and possibly a new spinner. Always install weights at the locations the manufacturer has established.

Always remember to double check and adjust the unmetered fuel pressure settings, idle mixture and idle speed per the latest Continental bulletin for your engine model.  Flight check the critical altitude per the M20K service manual and make the necessary adjustments.  Remember to correct for density altitude when doing this flight test.

By the time you do all these items and include your mechanic’s labor hours, you will spend $33,000 to $40,000. This is a big investment so make sure that the shop doing the work for you knows how to correctly spend your money. Try to find someone familiar with the M20K airplane and feel free to use this article as a basic checklist.

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.