My '68 model 0-470-R was removed for overhaul in '93 with 1984 hrs on it.  The engine was running fine with good compression in the low 70's.
As I recall, one cylinder had been repaired due to a bad exhaust valve
This is only my personal experience.  It's not a suggestion or recommendation to push limits.

My mechanic and I decided that the "R" model was for "RIGHT"

Michael

If you have a lot of resources, I'm a believer in predictive maintenance.  Oil anaysis, vibration analysis, thermography, compression checks, NDE exams, volumetric exams etc.  But is it enough and even practical on the airplanes we fly.

There is an article on TBO's in the August Cessna Owners magazine that discusses some realistic performance parameters and some cautions about waiting too long.  But it only discussed excessive hours not excessive time.  I didn't think it was very comprehensive but adressed some issues.

Aging from excessive time depends on the environment and materials.  If the plane is hangared and kept cool not too cold in a moderately dry environment, the hoses and elastomers will last longer.  But will the cylinder gaskets dry out from lack of use?  Was the engine oil changed regularly (4 month intervals) even though the engine didn't run 50 hours?  Was the openings to the engine covered if left in a climate with high humidity?  I would guess it is hard to imagine one answer to your question as each plane "lives" its own life.

A lot of questions and few answers.....  You and your mechanic know the plane best.  I guess use common sense and save your pennies for your reserve fund.  If you have been keeping one, then the decision isn't so painful. 

BOTTOM LINE:  I guess if you are right in your decision, you should always do your Major Overhaul too soon.  Being wrong and doing it too late can be harmful to other parts of your plane or yourself, if the engine quits in flight.  If you wait too long, at best, it may be a more expensive overhaul than you though it would be.

An O-300 that has lasted 34 + years, working just fine, and it's reward is.....the trash heap?  I'd rebuild it.

Yeah, I guess "trash heap" was a little strong,....we all know you'd really exchange it and it'd go back into the parts-pool somewhere.  But you don't say which airplane you've got this in.  The IO-520 is almost the top-end of TCM's engine line, with hp ratings running from 285 - 310 and up.  You might find an STC or get a field approval to change the prop, prop gov, and re-schedule the fuel flow and thereby convert the engine to a higher hp "dash" number.  TCM has SB's that address engine conversions to other dash-numbers, and perhaps your engine might qualify.
Otherwise, the only engine that would put out more hp would be the IO-550 series, and that would also entail prop, gov, and other changes including the engine, exhaust, intake, mounts, etc,  ...you can imagine what that costs. ;Þ

Dave: I've flown in 4 banger conversions... be sure you are ready to accept the vibration and accelerated airframe deterioration. You might look into the Continental 360 6 cylinder conversion. Better yet, trade for a Square Tail 182... Bob

Bob: What can you tell me about that conversion?

  Dave

Hi All: xpmods have got an STC for Continental 210 HP 6 cylinder O-360 into the 170/172 airframe. Lee, I am not sure about Skylark 175's. Maybe you can be the first? Check out this site: <http://www.xpmods.com/enginemods/default.asp> Also I've heard that there is a stc for putting a O-470 into 175'S. I have a 182 Squaretail, never flew anything but 150's and 172's until I went to look at it as it was priced the same as some of the 172's I was shopping. With twice the effect HP and constant speed prop, it was love at first flight. Absolutely no comparison...
Blue skies to all, Bob

Price out some of those conversions....If you can get the engine cheap and the STC, maybe still around $30K for that O-470 conversion.

Bill

From TCM Service Information Letter SIL 98-9A, "Time Between Overhaul Periods":

"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."