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When evaluating the value of an aircraft, the total hours since major (or factory) overhaul (SFOH/SMOH) is an important indicator of price since it tells us how many hours the engine has operated since the last overhaul.

I read in another answer that the typical time between overhaul (TBO) interval for a piston engine is 2000 hours.

However, what is the best way to determine the actual TBO interval for a given engine model? I'm hoping someone might know of a handy list somewhere rather than having to dredge through service manuals for each model I'm interested in.

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2000 is common but that depends on the engine. The official TBO is what ever the TC/manufacture states, here is the list for of the Lycomings published by Lycoming and here is the continental list. These are manufacturer recommended TBO's and are presumably based on data and tests they have run but they are in the end an estimate and usually reflect a relatively cautious value.

The "actual" TBO is a somewhat subjective number that no one can give you. Internet data and general collaboration can help you get a picture of what to expect for a given airframe/engine combination but individual units must be handled on a case by case basis. When assessing any aircraft its good to look for statements like "generally makes it to TBO" or "this aircraft had a lot of engine issues" or "oh man that turbo charger made that a finicky power plant" then come to your own conclusion. Forums, message boards, and old APOA articles can be your friend on these matters. Oil analysis and compression testing is the best objective way to measure engine health and a good pre-buy is important for that. Some people say "if it ain't makin' metal keep on flying it"

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    $\begingroup$ I used to know an overhaul mechanic who said that Lycs will easily run 3000 hrs+ although they typically require line boring if run that long. Anyway, the other really big issue on a pre-buy is to review the logs for periods of long inactivity. In Canada aircraft have to have journey logs so it's easy. Not sure how that works in the US. $\endgroup$ – John K Jun 26 '19 at 13:02
  • $\begingroup$ Great point. Engines that run regularly live long happy lives. Engines that sit...not so much. $\endgroup$ – acpilot Jun 26 '19 at 14:19
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TSMOH alone doesn't tell you much. The condition is what matters. There are many examples of brand new engines failing within the first 100hrs and many more examples of decently maintained engines ticking along well past 3000hrs. 150hp mogas O-320s I flew as a CFI racked up ~3500 hrs in some cases and may or may not have had a dash of MMO here and there...who's to say? In fact, a low time engine is much more likely to fail than a high time example if the same engine (I'll search for the backup to this claim and post it here if I remember). Give me a 2000hr O-470 over a 10hr O-470 any day of the week!

TBOs for most horizontally opposed engines range from a low of 1200hrs on some of the older geared engines to a high of 2400hrs. The calander limit is generally 12yrs but nobody ever talks about that. TBO is not really a good indicator of engine life, though. In the US, we run "on condition" meaning that the TBO is merely a reference, not a limiter. If the engine is healthy and is not giving you any bad signs then there really is no reason to drop ~$30,000 or whatever on an overhaul.

Each manufacturer has a list of their hour and calendar TBOs but, again, they don't mean much to a part 91 owner/operator. Certain commercial operations must adhere to the hour and calender TBOs but even then the FAA will grant extension waivers in many cases.

Personally, I would not hesitate to buy a plane with an engine at or near TBO...after a thorough inspection, of course. I'm confident I can get another few hundred hours out of a healthy engine. And if I don't...fine. I priced the engine as a runout and will drop a mid-time engine in sourced from someone doing an engine upgrade!

Published TBOs for common piston engines: TCM - http://www.continentalmotors.aero/uploadedFiles/Content/xImages/TBO%20Page%20SIL98-9C.pdf

Lyc - https://www.lycoming.com/sites/default/files/SI1009BD%20TBO%20Schedule.pdf

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  • $\begingroup$ Would you please define MMO? $\endgroup$ – Michael Hall Jun 26 '19 at 15:52
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    $\begingroup$ Marvel Mystery Oil. Some people swear by it, others say it's useless. You'll need to determine if it is legal to use in certified aircraft. I've heard arguments both ways. $\endgroup$ – acpilot Jun 26 '19 at 16:00

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