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  2500 Hour Club - Myth Busting TBO!


As owners as operators of aircraft engines we are (or should be) quite familiar with recommended TBO values for the engine(s) we fly behind or between. In my case, my pair of IO470's have a factory TBO of 1500 hours. It should be noted that Part 91 operated aircraft are under no obligation to overhaul their engines at the manufacturer's stated TBO hours. The annual inspection by your A&P/IA will determine if your engine is airworthy for continued service.


With the advent of the front alternator mounted IO520 permold engine design around 1965 with the S-35 Bonanza, the TBO for this NA engine was set at 1700 hours and upon the debut of the NA IO550 power plant it too carried a 1700 hour TBO. Sadly, the IO470 power plant TBO was never revisited from its original 1500 hours. With some exceptions noted in Continental SIL98-9C (, the majority of the ABS Fleet sits at: 1500 and 1700 hour TBO values.


In 2012, however, Continental began manufacturing what they called "Gold Standard" Rebuilt and Factory New engines and in April 2013 announced TBO increases of up to 400 hours for their "Gold Standard" engines with flyers of 40 hours/month receiving the 400 hour TBO increase and most others receiving a 200 hour increase. Complete information on the Continental TBO increase can be found at which will include a link to their SIL98-9C that details the engine model and TBO values currently in effect.


To quote the Continental SIL98-9C:


"TBO periods were established on most CMI engines beginning in the 1960s. Since that time, CMI has made significant engineering improvements to virtually all major engine components. CMI has refined manufacturing processes and implemented computer numerical controlled (CNC) machining tools enabling CMI factory engines to meet higher standards than possible when CMI engines were originally granted FAA Type Certificates. These improvements have enabled CMI to increase TBO limits for many of our new and rebuilt engines."


Indeed, many component manufacturing process, diagnostic and analytical improvements have been made at the component levels in all areas from metallurgy to equipment manufacturing tolerances and as well the lubricant folks have not been sitting idle either. I seriously doubt that they are making the exact same product they made in the 1960Õs. Suffice it to say that we have a considerably different level of sophistication going into the components and lubricants we experience in our engines today.


There is another key quote from SIL98-9C that IÕd like to draw upon:


"An engine's published TBO DOES NOT mean that every engine will operate the number of hours or years listed without requiring component replacements and/or unscheduled maintenance events. Noncompliance with CMI instructions for continued airworthiness, operational and/or environmental factors may necessitate repair or replacement of the engine, engine components and accessories earlier than the published TBO."


I have supplied the bold emphasis on a particular part of the text which many of us know to be true due to the wide variability of factors affecting engine TBO. Just as there are outliers on the left side of the TBO bell curve, it stands to reason in my mind that there must be outliers on the right side of that bell curve.


As I began to query the Beech List forum about folks whose engines had delivered far greater than TBO hours, I was pleased to hear that there were several reports of Continental engines reaching in excess of 2500 hours. I was so impressed with what I was hearing from these owners that I decided to create the "2500 Hour Club" here at for those owners who have reached or are on their way to reaching this engine-operating milestone without splitting the case for bottom end work.


If you have an engine bottom end that is myth busting TBO and worthy of the 2500 Hour Club,

E-Mail me

and I will add you to this distinguished club.



A36 owner and ABS Member, Marc Charron, is sporting a FRMN TN IO520 from 1994 that has 1941 hours on the bottom end (2541 on the case). Here are Marc's engine experiences:


Marc reports using Aeroshell multi viscosity oil until about 500 hours of use when the starter adapter began slipping. He then moved to Phillips multi for a while and for his last 300 hours has been using Aeroshell 100W and oil and filter changes between 45-50 hour intervals. For quite a few years Marc operated in the deep cold climate of Canada and made extensive use of engine pre-heating below 40F for multi viscosity oil and at 50F and below for straight weight oil. Along the way there was a top overhaul at 800 hours on the case and some cylinder replacement and cylinder repairs to date. Marc reports operating this TN engine lean of peak throughout his ownership. Oh, and it's sporting a sweet but modestly priced paint job from Hawk Aircraft Painting in Tampa, FL.





Baron E55 owner & ABS Member, Stuart Spindel, has operated two IO550 engines on his aircraft lean of peak with current total time of 2625 hours. Over the years he has replaced both starter drives. After about 500 hours Stuart reports that one of the adapters began slipping. He switched from Aeroshell 15W50 to Phillips 20W50 and the slipping stopped. He reports no further issues for the past 1000 hours and began using Cam Guard a couple years ago.



Bonanza V35A owner and ABS Member, Bo Harper, currently has 2000 hours total time on his Ultimate Engines IO-520BA since 1995. Bo has owned and flown the airplane 700 hours since 2009 from his base in the Chicago area. The aircraft and engine has been operated on Aeroshell 100W itÕs entire life with engine sump pre-heat pad and an engine pre-oiler. The pre-oiler is run prior to every start to pressurize the engine with oil prior to start. Being located in the engine bay the pre-oiler adds a smidge of help with CG. Bo reports a couple of cylinder replacements have been done since the 1995 overhaul.





Here is Norm H.'s over TBO story:


My engine came new with my 1997 36. I installed GAMI injectors with less than 100 hrs, then went lean of peak operation. At 800 hrs all the cylinders were under 60 with some under 50, so I did a top. I now have 2350 hrs on it and the cylinders are holding up fine. It is normally aspirated.


I fly about 150 hrs a year, use 100W oil with changes between 50 and 100 hrs. After the top I went to ROP operation. The oil analysis doesn't show any problem nor does the filter cutting. My home airport is at almost 5000 ft and the weather here is very dry, frequently giving RH in the single digits. I preheat on cold days.






Bert Weinstein's 2500 Hour Pirep



Just read about your 2500 Hour Club in the ABS newsletter.  It's a great idea!  My IO-520-BA engine made it into your club this past year.  It is currently at 2525 hours and running great.  I wrote about this engine in the ABS newsletter September, 2011, shortly after replacing the #2 cylinder after more than 2300 hours.  


This engine was overhauled in May, 1987 by Schneck (which went out of business shortly afterward). About 2 years later I installed flow-balanced fuel injectors using off-the-shelf Continental injectors of different sizes.  (Described in Light Plane Maintenance, Dec. 1989 and Jan. 1990.)  This saved fuel and made the engine run much smoother.  I started running lean-of-peak after George Braley's many articles on the subject.


Since replacing cyl. #2, the engine is operating almost like new.  Oil consumption is about a quart every 15 hours, and here's a quote from the most recent oil analysis at 2520 hours and 48 hours between oil changes:


"BERT: You noted four months of inactivity for this IO-520, but we never would have guessed. Metals look great compared to the last report, even though this oil saw a bit more use. Less wear after more hours is an excellent indication that this engine is wearing well with no mechanical problems in the works. The only "issue" on this report was a low flashpoint, indicating fuel present in the oil. Fuel was found at just a trace amount, though, and that's harmless. All in all, this is a fantastic engine -- keep up the good work!"


Cylinder #1 is starting to show some of the same wear signs that led to replacing #2:  step wear at the top of piston travel and thinning of the chrome channel pattern.  We are monitoring it and will replace it when the wear becomes more extensive, perhaps replacing other cylinders at the same time, depending on their condition.  


The ABS article describes my operating habits, so I'll only recap a few points.  I have used Phillips XC (20W50) oil from the beginning, and installed a pre-oiler at 1400 hours.  The two main pieces of advice I would give others are exhaust valve lapping and flow-balanced lean-of-peak operation.  Experience has convinced me that Continental's specification of "no leakage past the exhaust valve" is important.  If there is more than a whisper audible at the exhaust when doing a compression check, and the leakage doesn't go away after 10 hours or so, it is important to lap the valve (with the cylinder still in place) to reseal it.  Do it early and they will last almost forever.  Let the leakage go and the valve face and seat will quickly become so damaged that the cylinder must be removed.  Plenty has been written about lean-of-peak operation, so I will simply add my vote for its benefits—lower fuel consumption, cleaner cylinders, no spark plug fouling.


After replacing cylinder #2 I installed the GAMI baffle for that cylinder.  It works great, brought the temperature on #2 down by about 30°F.  It is still the hottest cylinder, but no hotter than #6, and only about 15° above the average.  If that baffle had been in place from the beginning, I'm pretty sure #2 would have lasted significantly longer, probably about like #1.


In case anyone asks, I started using AvBlend at around 2200 hours and Camguard at around 2400 hours.  From what I'm reading it sounds like they can't hurt, and may help, but clearly they aren't necessary for long engine life.


Best wishes for your push to encourage others to get long life from their engines!


- Bert Weinstein

ABS Life Member #11843




Cam Gulley



I have a 1968 36 Bonanza (serial number E-44) and installed a factory new normally-aspirated IO-550B engine in it back in 2004. The engine was rated at 1700 hours TBO.


I currently have just shy of 2000 hours on the engine (actual current time is 1950.2). I just replaced both mags about 3 months ago because they were beginning to have issues. I’ve also replaced 2 cylinders during the life of the engine as needed. We inspect and replace hoses and seals at every annual as needed. I use AeroShell 15W-50 multi-grade oil, do oil changes every 50 hours, and do an engine oil analysis every oil change. I haven’t run LOP operations with my engine until recently when I began experimenting with it due to installation of a new JPI 930 engine monitoring system. I have normally run 50-degrees ROP which keeps the engine cool with Beryl D’Shannon baffling. GAMI’s are next on my list of engine improvements.


The most major repair was a top overhaul at 1300 hours due to a landing-gear extension failure where the gear locked approximately 10% into their extension while preparing to land. Additionally, the manual crank also locked up making any gear extension impossible. So, after about 2.5 hours circling the airport trying to break the gear free, I ultimately had to perform an engine-off landing. The prop stopped in the worst possible position and drug the ground after landing which required a new prop and an insurance-required engine inspection and top overhaul. My incident caused a Service Bulletin to be issued because it was the first of its kind (where a washer had dislodged in the manual crank case and lodged in the gear mechanism causing complete lock-up of the gear-box).




Cam Gully

Eastland, TX

ABS Member #52952





Having two ongoing experiments in TBO with my IO470-L engines on my B55 that I have personally run and maintained for just over 1600 hours and 10 years now, I can report a right engine FRMN from 1999 that now has nearly 2000 hours on it and a left engine from 2004 that has about 1600 hours SMOH on it. My engines histories include a promotional install of Superior Millennium standard cast cylinders in 2007 at 800 and in 2006 at 400 hours, respectively. Aeroshell 100W oil (sans additives) and spin on filters with 50-hour oil & filter changes.


My cold weather operations in Wisconsin (and to a lesser degree Dallas, Texas) are always accompanied by generous Tannis full engine and cylinder preheats. The onset of pitting lifters was nipped in the bud on the right engine during a top overhaul when we examined the engine internals in 2007. Last year's 2013 annual included new left engine hoses and an overhauled fuel pump. This year's 2014 annual plan calls for new right engine hoses and an overhaul of the fuel system. See fuel system components overhauled HERE.




In my opinion, going long service hours on the bottom end makes it more important to pay attention to the external items that can cause an engine to go quiet at a very inopportune time. I consider engine hoses and the fuel pump/system to be tops in this critical category. See the fuel system overhaul page HERE. Additionally, you folks with Barons and un-feathering accumulators, give a special watch to that decades old hard line and hose that runs from the prop governor to the accumulator ball.


They are single points of failure and as such, command a great deal of my attention as I climb past TBO and toward the 2500 Hour Club. Accessories like the starter, alternator, magnetos, vacuum pump and cylinders can be overhauled/changed on condition or inspected at specified hours with much less economic fanfare than a case splitting overhaul event.


It would seem likely, and dare I say normal, that any run to TBO and beyond on the bottom end of our Continental engines may result in some cylinder repairs or replacements. This is a critical engine repair and one that should be undertaken by mechanics skilled in the art. I have read far too many NTSB reports of catastrophic engine failures not too many hours AFTER a cylinder replacement maintenance event. The engine gurus that I trust use and recommend that torque plates be employed for cylinder changes. In order to change the cylinders, the crank must be turned to orient the connecting rod to such a place in its travel that allows for the piston and piston pin to be assembled onto the rod. While turning the crank and having little or no clamping force on the case through bolts, it is possible for the crank bearings to "spin" or shift in their locations. This event misaligns the oil passage hole in the bearing with the oil feed hole in the case, which can be the makings of a catastrophic engine failure by starving the main bearing of oil.


Key factors that seem to put the odds in our favor for greatly exceeding TBO on our engines are:

  1. Regular and frequent flight operations (infrequent ops consider the use of a corrosion preventing oil additive)
  2. Quality lubricating oils and filters changed at or below recommended intervals
  3. Cold WX ops utilizing engine preheating
  4. Lifter inspection diligence
  5. Proper cylinder replacement and inspection procedures
  6. Proper front-mount alternator inspections and replacement procedures
  7. Careful monitoring and replacement of high time engine hoses (time or age limits may apply) and fuel pumps


With the cost of engine overhauls continuing to increase, getting the most out of our engines safely can afford us a much more pleasing financial experience with which to enjoy flying. If you are over TBO and approaching the 2500 Hour Club, please contact me with some of your details so that your experiences can be added to this page.


Happy Skies!



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