Engine rebuilds - how many times?

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Sundancer

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Considering an 0320 with about 3200 hours, 500 SMOH, so probably at least one overhaul and perhaps one more prior to that (or maybe not) - what's a reasonbale/rational limit on the number of overhauls?
 
I've worked at a couple of schools that worked on the principle of overhauling the engine twice and then replacing with a factory new engine. I always thought this worked well and I can never remember any cracked crankcases.

Another school near to us used to overhaul indefinitely but after a run of cracked crankcases they switched to the above and the problems went away.

Sadly factory new and overhauled engines are silly money these days.
 
Sundancer said:
what's a reasonbale/rational limit on the number of overhauls?
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Infinite. However there tends to be a big "depends" with this. And at the top of the list is how you maintain the engine over its OH cycle and who performs the overhauls that usually determine the number of OHs a specific engine can endure.
 
What size main bearings were installed on the last overhaul? That might help you determine if the crank is already heavily worn.
 
Also, there may be case modifications that could be a benefit. I adhered to the "overhaul twice" rule.
But I agree, the Rebuilt prices are insane. I watched the price increase almost 20% while I was waiting over a year for mine (ordered in 2021). At the current price for my engine, I would probably sell the plane.
 
I’d want to see the engine log. An O-320 with two majors 3200hrs would be….impressive.

I’d think it more likely be 3200 w/500 on a fresh top end and nothing done to the case/crank.
 
The factory NDIs everything being reused. Everything. They use magnaflux, zyglo, eddy current, dye-penetrant, and other methods to detect tiny cracks that will sooner or later turn into big problems. They then measure everything, and I mean everything. The Lycoming Direct Drive Overhaul manual has a whole section on dimensions, tolerances, even spring forces at certain compression heights. Lyc Direct drive O/H

Scroll down to page 103 of the .pdf. It runs all the way to the end of the book, at page 146. Continental has similar manuals.

Now, field overhaulers are supposed to adhere to this manual, but do they all, at all times? Suppose the crank is worn just the tiniest bit past the minimum? A new crank is REALLY expensive, so the temptation is to reuse it anyway. Do they use all the NDI stuff they're supposed to? They do often reuse cylinders; Lycoming never does. They never reuse cams or lifters, either. Or worn cranks. You get what you pay for.

We had a freshly field-overhauled O-200 making trouble on a customer's airplane. I found numerous problems, including badly worn (like rattlingly loose) rocker bushings. They sure didn't get like that in 20 hours.
 
Just like Abe Lincoln's axe... the handle was replaced five or six times and the head was replaced a couple of times, but it's still his original axe, right?
 
Lon Stratton said:
Also, there may be case modifications that could be a benefit. I adhered to the "overhaul twice" rule.
But I agree, the Rebuilt prices are insane. I watched the price increase almost 20% while I was waiting over a year for mine (ordered in 2021). At the current price for my engine, I would probably sell the plane.
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This is generally the outcome of practicality... meaning generally 2 majors for an engine.

As @Bell206 mentioned the engine can be overhauled unlimited times, from a practical standpoint eventually so many parts are below tolerances a person is buying 80-90% new parts the tipping point to just a buy new engine. Crankshafts can only be turned once or twice, cases age out, and frequently all new cylinders are purchased anyway.

If an overhaul requires a new case, crank, cam & lifters, and cylinders; there not much left to carry over. Starter adapter wear, oil pump wear limits, new mags... The math starts to point to new verses overhaul.

The math sells itself after 2 overhauls. Given this is POA people will post about overhauling some engine 4 or 5 times, which that might have worked for them or if only overhauled to service limits rather than new limits.
 
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The last overhaul I did I replaced everything but the case. If I replace the case next time, is it an overhaul or a new engine? Does it matter? Replace what's out of tolerance or not expected to remain in tolerance until the next overhaul interval. My overhaul was still significantly cheaper than new, but given my circumstances new was certainly an option.
 
Update - one overhaul only, was done at 2700 hours - I guess someone treated it gently. It has about 500 since, with two cylinders replaced. It's well past calendar TBO, has good compressions, and the logs (engine & airframe) are complete. A&P/IA is the original and current owner. I might be talking myself into something. . .
 
Sundancer said:
Update - one overhaul only, was done at 2700 hours - I guess someone treated it gently. It has about 500 since, with two cylinders replaced. It's well past calendar TBO.
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Wait; only 500hrs in 12 years? That’s less than 50hrs/year on average. Not necessarily a bad thing, but be aware an OH may be on the menu, so price it accordingly.
 
TCABM said:
Wait; only 500hrs in 12 years? That’s less than 50hrs/year on average. Not necessarily a bad thing, but be aware an OH may be on the menu, so price it accordingly.
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Oh, less than 50 per year - it's 15 years past calendar TBO. Guesstimating about $30K to do the deed, overhaul and labor. That would hurt. A lot.
 
Sundancer said:
Oh, less than 50 per year - it's 15 years past calendar TBO. Guesstimating about $30K to do the deed, overhaul and labor. That would hurt. A lot.
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I’m partnered in a 172M.

Our O-320 was just OH’d via exchange; that’s about $35k. Then there’s another $10K in labor R&R the engine along with a few thousand for various firewall forward stuff like hoses, etc. that gets done.

We sent the prop off and off for IRAN; it was beyond limits on one blade, so that was another $8K for a new prop.
 
TCABM said:
along with a few thousand for various firewall forward stuff like hoses, etc. that gets done.
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I spent almost 10K on that stuff, though admittedly I spared no expense and my aircraft has a more complicated FWF. Hoses, oil cooler, mount OH, shocks, prop gov OH, boost pump OH, baffle kit, each of those starts at 1 AMU.
 
I understand “pricing as if needing an overhaul” but imo in a 12 yr 500 hr engine that wouldn’t be a huge concern.
I had a AA1B for sale with 1930 original hr on it (1974) so that definitely needed serious thought as to overhaul.Had a guy call me that proceeded to tell me to sit down and pencil out what a new engine costs and think about it carefully and get back to him on a price what I really wanted for the airplane.Sorry buddy.Lol I got full price for it from another gent.
Not at all saying anyone is wrong for taking that into consideration but thinking someone is going to knock a huge chunk off may be wishful thinking unless overpriced.Again,don’t take this to the wrong way,im not trying to be a pot stirrer but this made me think of my experience.
 
Sundancer said:
Update - one overhaul only, was done at 2700 hours - I guess someone treated it gently. It has about 500 since, with two cylinders replaced. It's well past calendar TBO, has good compressions, and the logs (engine & airframe) are complete. A&P/IA is the original and current owner. I might be talking myself into something. . .
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Probably ran it decently hard and often to get to 2700 hours. Not uncommon for that engine.

Steel parts may have nearly indefinite service life if in spec. The cases have a limit as they are ground at the mating surfaces and eventually the deck height is no longer in spec regardless of the pushrods chosen. Cam journals are also line bored after the case is ground, so eventually run out of meat at those bearing supports as well perhaps.
 
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