I’m trying to get a handle on this:
assume there’s a C170B (to bring the question into bcp terrain…) with 700h SMOH for sale for 45k.
How much should a ‘identical’ plane with 1300h SMOH sell for?
An attempt at a pedestrian calculation, assuming 25k for a MOH of an O300 with 1500 TBO: at full TBO it would have cost about 17$/hr. So 600 hrs (difference in the example above) would amount to 10k towards the OH. Would that make the plane with 1300h SMOH worth 10k less than the 700h SMOH plane?
I don’t think so, but what’s a more realistic estimate?

In my opinion, age and condition of overhaul need to be considered, and your potential use per year. We all want to fly 300 hours a year, but how many really do?
A mid time engine might last you the life of the aircraft, while the one 200 away from TBO might need work at the next annual, and anything less than a full overhaul is probably gonna be a waste of time and money.

How many of us get to wear out an overhaul?

There is no right answer to this question as there are far too many variables even with the impossible identical in every other way airplanes.
When was it overhauled? Who overhauled it? How many hours has it flown in the last 5 years each year. How has it been flown? Does it have an oil filter? Do they have trend data from oil analysis? Too many.

If you are saying they are 100% isentical in every single way - because we grow airplanes in labratory environments where that is possible....

(Cost of overhaul / TBO ) * SMOH = difference in price.

But your question is not a practical discussion as you will never come close to it in the real world for it to matter.

Bagarre wrote:There is no right answer to this question as there are far too many variables even with the impossible identical in every other way airplanes.
When was it overhauled? Who overhauled it? How many hours has it flown in the last 5 years each year. How has it been flown? Does it have an oil filter? Do they have trend data from oil analysis? Too many.

If you are saying they are 100% isentical in every single way - because we grow airplanes in labratory environments where that is possible....

(Cost of overhaul / TBO ) * SMOH = difference in price.

But your question is not a practical discussion as you will never come close to it in the real world for it to matter.

Yep.

Approved data from the manufacturer is the best place to start when you’re considering all the variables involved with valuing an engine and it’s remaining life to TBO.

One of the most important things besides hours SMOH is the calendar years since that overhaul was completed, and how many hours a year/month the engine has flown.

Continental thinks so too. Read Continental Service Information Letter SIL98-9C for some more insight.

The key items to note in this SIL:

TCM recommends that the following information
be used, along with the engine’s published TBO,
in determining the engine’s continued
airworthiness:

1. Whether the engine has been operated
regularly or has been in storage; gaskets, seals
synthetic and natural rubber goods deteriorate
over time. Environmental corrosion can occur
internally and externally on the engine. This
naturally occurring process can inevitably affect continued airworthiness of the engine
and engine mounted components and
accessories. For these reasons, overhaul the
engine at least every twelve (12) years, or on
accumulation of the operating hours listed for
the engine model.

2. For aircraft used in aerial spraying, overhaul
the engine after the accumulation of 1200
hours or twelve (12) years whichever occurs
first.

3. Aircraft used in parachute jumping, glider
towing, banner towing, etc., may require more
frequent engine overhauls than listed for the
specific engine.

4. The quality of parts, accessories and
workmanship utilized during routine
maintenance, engine top overhaul and major
overhaul directly affect the service life of the
engine. The TBO’s listed are predicated on
the engine having been maintained in
accordance with TCM’s FAA approved
instructions for continued airworthiness
specified in the engine Maintenance Manual,
Overhaul Manual, and Service Bulletins and
operated within the limitations published in
TCM’s Engine Operators Manual and the
aircraft manufacturer’s Aircraft Flight Manual
/ Pilots Operating Handbook (AFM / POH)

- If an engine consistently accumulates 40 or more hours per month since being placed in service, add 200 hours to recom- mended TBO.

————

On an O-300, I’d value a 1300 hour SMOH engine with a fresh top end that flies regularly much higher than one with 700 hours and original cylinders but has sat for many years. I have never seen the top end on an O-300 make it to 1800 hours without problems. They usually need a top end refresh about halfway to that mark.

Thanks for all replies and info, very helpful!
I'm approaching (power-) ownership for the first time, so some of the engine stuff is new to me and I'm trying to educate myself as best as I can. For the same reason (...ownership) I don't have a go-to A&P, IA to get answers and/or recommendations.
Which means you can expect a call from me anytime..

According to the replies my decision to not pursue two candidates, one w/ 129hrs SMOH in '71 (!) and the other w/ 280hrs SBOH after prop strike in 2000 was a good one.
Looks like there a quite a few planes for sale, close to, at or past TBO, with >asking< prices not reflecting time on engine. Or looking at it the other way: if one added the cost of an overhaul before selling it would be way out of a reasonable price range?

Using the V-Ref calculator on the AOPA website provided the following:

SMOH Engine #1: 700. Effect on valuation=$2,600.00 at $13.00/hour.
This was the added value to a generic 1954 C170B.

SMOH Engine #2: 1300. Effect on valuation=($5,200.00) at $13.00/hour.
This was the reduced value to the same generic 1954 C170B.
(use of parentheses denotes a negative number)

Total difference in value between these two "identical" aircraft is: $7,800.