Acceptable compressions on Continental TSIO 360 MB

[MooneyNoob]

I was talking to a seller of a m20K 252 with 400 SMOH who was showing me recent compression numbers.  A couple of cylinders were in the 63, 64 range (dropped from 72 in two year during which the plane only flew 30 hours).  He was saying that for a Continental those compressions were still totally fine, and would even go back up by themselves if the plane flew more.  Is that realistic?   It kinda feels like that engine is heading for a top overhaul at only 500 or 600 hours but I'm not an expert and could use advice. 

Thanks in advance!

[Airways]

That's pretty much standard for Continentals.

[toto]

This isn't exactly answering your question, but I'll just mention that some shops really "phone in" compression tests. I've used one shop that insists on doing compression tests *cold*.. Even after I shared Lycoming guidance that compression should be checked on a warm engine shortly after shutdown, they still test on a cold engine that's been sitting for several days. 

The results are predictably bad, and they dutifully write them in the log. 

If the compressions are a concern, I'd take it to an engine shop and get an assessment from an expert before making any expensive decisions. 

[LANCECASPER]

1 hour ago, MooneyNoob said:

I was talking to a seller of a m20K 252 with 400 SMOH who was showing me recent compression numbers.  A couple of cylinders were in the 63, 64 range (dropped from 72 in two year during which the plane only flew 30 hours).  He was saying that for a Continental those compressions were still totally fine, and would even go back up by themselves if the plane flew more.  Is that realistic?   It kinda feels like that engine is heading for a top overhaul at only 500 or 600 hours but I'm not an expert and could use advice. 

Thanks in advance!

Those are actually good compression numbers on a Continental. You can probably do the test ten times and get ten different results. Borescope pictures might be nice also on a pre-buy.

You will find discrepancies with any airplane, so don't let two cylinders in the 60's stop you from buying an airplane where you like everything else. You're asking some of the right questions, but if it checks all of the other boxes for you, remember that too much analysis creates paralysis and if you don't pull the trigger someone else will. In today's market if you are serious, put down a deposit and have a pre-buy done by the mechanic of your choice.

[Davidv]

There is one extremely meticulous Mooney mechanic I know who doesn’t even do compressions on all pre-buys but opts for comprehensive boroscopes instead.

[MooneyNoob]

@Davidv Would you be able to share the mechanic's name and contact info, and what part of the country is he in?

And thanks, all, for the great info!

[GeeBee]

https://www.aircraftspruce.com/catalog/pdf/servicebulletin.pdf

 

 

[Davidv]

14 minutes ago, MooneyNoob said:

@Davidv Would you be able to share the mechanic's name and contact info, and what part of the country is he in?

And thanks, all, for the great info!

Brian Kendrick, but just to clarify I saw him not do compressions on one prebuy but he did on a recent one using a better method.  His schedule is very full and he travels but if there is one screw out of place on a Mooney he will find it.  Mooneysupport.com.

[MooneyNoob]

@DavidvPerfect, thank you!!

 

[philiplane]

I check compressions cold first, borescope, then do them again when hot. Hot means hot too, not just run for 5 minutes. CHT's must be 300+ in order for a hot test to mean anything. A lukewarm engine will generally test worse than hot or cold.

The borescope inspection will reveal the condition of the cylinder walls. Pitting or scuffing will cause low compression that won't improve. Light rust on the walls will polish off and compressions will improve. Leaky valves may stop leaking if there is only lead contamination from idling & ROP operations. LOP flight will cure that. A good borescope inspection is the only way to know if the cylinders will get better or if they're on the way out. 

[MooneyNoob]

@philiplaneIncredibly detailed and helpful.  Thank you!!

[RLCarter]

Continental spells it out, 6-4.11.2 of the Standard Practice Manual... easy to Google and download a copy. Conti’s can get really low and still pass, 

[kortopates]

those compressions are normal but the data is useless about telling you the condition of the cylinders. A pre-buy and annual should always include a borescope inspection which is much more telling.


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[PT20J]

At the museum, most of the engines are P&Ws. We end up checking compression cold most of the time because no one wants to waste the time to tug  three warbirds out of the hangar to get the one in the back out on the ramp to run it up. If a cylinder is bad, you can hear the air rushing out. If it's good, we get good compression (70s) cold. Radial cylinders are all rebuilt and chromed since they haven't been made for decades.

When we mounted the new Lycoming IO-360-A3B6 on the Mooney, we checked the compression before start up (because the engine replacement finished off an annual inspection and you are supposed to check compression as part of an annual). The engine had been run in by Lycoming and all we had done at this point was drain out the preservative and put in oil. Stone cold and not having been run for a couple of weeks, it had 79/80 in all four cylinders.

I always try get the engine hot to check the compression and drain the oil, but I'm not convinced it makes that much difference.

Skip

[EricJ]

6 minutes ago, PT20J said:

I always try get the engine hot to check the compression and drain the oil, but I'm not convinced it makes that much difference.

Skip

My IA has always done it cold, and I've been doing it cold as well.   I don't think it makes much difference, since a compression check is not a very precise process, anyway. 

I don't drain the oil hot, either, and just let it drain over night instead.   It makes less of a mess taking the filter off cold and drained as well.

 

 

[GeeBee]

Continental once filed the end gaps on the rings until an engine had zero compression on the differential. Put the engine on the dyne and it ran at full rated horsepower. Used a lot of oil, but it ran fine. It is not what is the compression, but what is causing the readings you are getting. 

 

[philiplane]

And the limitation they have on oil consumption isn't because consuming oil alone is bad. It's because they want a safe amount of oil left in the sump, at the same time the gas tank goes dry. 

Edited July 30, 2021 by philiplane

[carusoam]

Similar experience…

I was looking to buy a Missile with an IO550…

Annual had just been completed and the compressions were all typical for Continental engines…

 

Lycoming compression tests may have some more meaning…

I let that opportunity go to spend more time learning about continental engines…

 

It turns out… compressions were about all we had… to know anything about our cylinder walls and valve health…

Now we all have dental cameras, with pics of the valve’s pizza image, and cross hatches on the cylinder wall…

That Missile was really nice too….  

 

Do what I did….  Let that one go… study up on Continental engines. Get a nicer M20R instead….  
 

It’s your PPI… you decide what it takes to be separated from your dough… share that with the current owner… write it down in a purchase agreement so the details don’t get lost… memories are hare to remember some days… yours or theirs…

Don’t be afraid to pay asking price if the engine looks pristine on the inside… after you have taken pics…

PP thoughts only, not a plane sales guy…

Best regards,

-a-