Compression values

J

JOhnH

Touchdown! Greaser!
Joined
May 20, 2009
Messages
14,210
Location
Florida
Display Name
Display name:
Right Seater
I'm looking at a '67 Bonanza with an IO520.
The engine was rebuilt in May 2012 with 2991 tach hours, including new cylinders.

On October, 2013 the engine showed compression of 72-72-70-68-72-72/80 Tach = 3034 (or 43 hours tach time)

Is that a warning or a problem?
 
Last edited:
Not really, 68 isn't that bad, also if its been sitting for a while the numbers will be lower, but often will come back up with some use.

Did you do a leak down and borescope?
 
James331 said:
Not really, 68 isn't that bad, also if its been sitting for a while the numbers will be lower, but often will come back up with some use.

Did you do a leak down and borescope?
Click to expand...
They are delivering the plane to my mechanic in a few weeks. That will certainly be checked.
The plane has been doing a good bit of sitting which is why it is being sold. It also had a prop strike recently. It had a full tear-down inspection and had the crankcase and crankshaft repaired.
 
Compressions are fine. The prop strike can make it a bit of a wildcard, but it should be priced accordingly.
 
JOhnH said:
They are delivering the plane to my mechanic in a few weeks. That will certainly be checked.
The plane has been doing a good bit of sitting which is why it is being sold. It also had a prop strike recently. It had a full tear-down inspection and had the crankcase and crankshaft repaired.
Click to expand...

Id highly recommend being there for the inspection and putting hands on the plane.
 
Good numbers. Nothing above ~55 would bother me.
 
JOhnH said:
They are delivering the plane to my mechanic in a few weeks. That will certainly be checked.
The plane has been doing a good bit of sitting which is why it is being sold. It also had a prop strike recently. It had a full tear-down inspection and had the crankcase and crankshaft repaired.
Click to expand...

I can see a crank repair.... BUT,,, the crankcase ??:dunno::eek:
 
James331 said:
Id highly recommend being there for the inspection and putting hands on the plane.
Click to expand...
I'll be there. Also, the shop has a Bonanza experienced CFI on staff that will go up for the test flight, and who will be providing the HP/Complex training.

This from the shop's web page
In October 2013, we were proudly awarded with the America Bonanza Society's Center of Excellence Accreditation. We are the second shop in the country to receive this award and we will work hard to maintain this level of Beechcraft customer service far into the future.
Click to expand...
I don't know how important that is, but it sounded impressive to me!
 
docmirror said:
Good numbers. Nothing above ~55 would bother me.
Click to expand...
I'm glad to hear that, but does that still go for an engine with only about 50 hours on it?
 
Last edited:
JOhnH said:
I glad to hear that, but does that still go for an engine with only about 50 hours on it?
Click to expand...

In my opinion, yes. Low compression on the big bore Conti is related to valve recision, loose rings, poor cyl wall surface, etc. All things associated with hours on the engine. You'll typically find that one or two jugs will get weak enough around 1000 to require repl or refurb. That doesn't cover all situations, but is what I find has been typical behavior.
 
JOhnH said:
I'll be there. Also, the shop has a Bonanza experienced CFI on staff that will go up for the test flight, and who will be providing the HP/Complex training.

This from the shop's web page

I don't know how important that is, but it sounded impressive to me!
Click to expand...


Sounds like you're set!

Post some pics and let everyone know how it turns out.

While you're in there, make sure your crank case breather tube has the extra vent holes drilled in (it is winter after all), also I'd discount the plane if it has slick mags if they look like anything but brand new, even then... It's 2k to swap to Bendex when the slicks crap out.

Good luck
 
Last edited:
Prop strikes prior to a tear down and inspection are a non event.

And yes the cases will have been lapped and re-line bored each time they are split. (that's a repair)

Had the engine not been inspected, then I would worry.
 
Tom-D said:
Prop strikes prior to a tear down and inspection are a non event.

And yes the cases will have been lapped and re-line bored each time they are split. (that's a repair)

Had the engine not been inspected, then I would worry.
Click to expand...

Hmmmm..

I never knew that..:no:
 
compression values are over-rated...I'd rather "see" what everything looks like.:frown2:
 
Compression is much more useful as a trend. A single reading tells you what it was that day at that time. Asking what the compression was for the last few annuals gives you a trend. Is it holding steady or just slowly declining, as you would expect. Or was it 78 across the board last year and the thing is tanking.
 
Pedals2Paddles said:
Compression is much more useful as a trend. A single reading tells you what it was that day at that time. Asking what the compression was for the last few annuals gives you a trend. Is it holding steady or just slowly declining, as you would expect. Or was it 78 across the board last year and the thing is tanking.
Click to expand...

They are a very good tool to find one of these. :)
 

Attachments

  • 20140411_131727.jpg
    20140411_131727.jpg
    1 MB · Views: 45
Pedals2Paddles said:
Compression is much more useful as a trend. A single reading tells you what it was that day at that time. Asking what the compression was for the last few annuals gives you a trend. Is it holding steady or just slowly declining, as you would expect. Or was it 78 across the board last year and the thing is tanking.
Click to expand...
the problem is.....poor repeatability.

Take the reading again next week....and each week there after and you'd notice a wide range of noise in the data. It's a gross error check at best. It either leaks......badly.....or it doesn't. And if it does leak....where is the leak coming from?

Beyond that.....it really is meaningless.

Buy a $50 el-cheapo borescope, USB dental camera. Have a look see inside and that is far more information than the bad data from compression gage.

So....what if your valves are burning?....but, there's no leak. Wouldn't like to know in 30 or so hours you'd likely have an impending valve failure?
 
Last edited:
Tom-D said:
They are a very good tool to find one of these. :)
Click to expand...
What am I looking at, besides just the inside of a cylinder.

Checkout_my_Six said:
the problem is.....poor repeatability.

Take the reading again next week....and each week there after and you'd notice a wide range of noise in the data. It's a gross error check at best. It either leaks......badly.....or it doesn't. And if it does leak....where is the leak coming from?

Beyond that.....it really is meaningless.

Buy a $50 el-cheapo borescope, USB dental camera. Have a look see inside and that is far more information than the bad data from compression gage.
Click to expand...

Yep. This is why I try not to read too much into any single reading. There are obvious signs of trouble that a single reading can indeed tell you though.

If you have one that is completely crapped out and/or drastically different than the others for example. If you have 74/76/74/62, something is wrong. You don't need to trend that, it's bad. But if you have 72/73/71/75, that doesn't tell you a whole lot. It is a good reading, but you can't infer much detail beyond that.

Or if the air is blowing out the exhaust valve. That is unrelated to the numeric value. If it is blowing out the exhaust or intake, you have a valve problem. Again, no trending or question.
 
Last edited:
Ron Levy said:
Have you read what Continental has to say about compression test values on its engines?
http://www.tcmlink.com/pdf2/sb03-3.pdf
Click to expand...
I guess I failed that quiz.
I found the article instructional and somewhat interesting about how and why to perform a compression test, but I must have missed the part that explains their position on the results of the test.
 
Crankcases needing repair following a prop strike aren't uncommon. The case gets a lot of stress and may crack. Not all cracks are catastrophic.

Personally, though, if I get a crack and the engine is apart, I want a new case. Not new to me, brand new.
 
Ted DuPuis said:
Crankcases needing repair following a prop strike aren't uncommon. The case gets a lot of stress and may crack. Not all cracks are catastrophic.

Personally, though, if I get a crack and the engine is apart, I want a new case. Not new to me, brand new.
Click to expand...

Yup...... Me too......:yes:
 
JOhnH said:
I guess I failed that quiz.
I found the article instructional and somewhat interesting about how and why to perform a compression test, but I must have missed the part that explains their position on the results of the test.
Click to expand...

The gist of the service bulletin is to establish a leakage using their master orifice leakage fitting. It's going to be around 47PSI, but they make you calibrate the compression tester using their calibrated leak. Once you establish the calibrated leak,(around 47PSI), then you move on and run the normal comp tests against each jug. Any that don't meet the established minimum, you go fly for 20 minutes then come back and test again. If it still won't reach the minimum, you have to remove the jug for repair.
 
28 years on a '75 172M, all with the same compression gage:

Cyl 1 70/72/67/66/71/71/70/74/72/68/72/72/72/72/68/73/73/73/74/71/66/70/65/75/76/75/72
(1977/2004__)
Cyl 2 70/73/70/66/73/72/68/74/70/68/66/71/71/67/72/70/7271/71/71/75/63/71/64/71

Cyl 3 70/74/75/68/71/68/70/75/70/66/68/62/71/61/71/75/75/74/66/64/72/76/64/67/72/62

Cyl 4 71/70/70/66/70/69/67/70/67/68/66/72/69/71/64/70/70/73/68/70/72/75/65/67/65/72

Note the numbers go all over. Occasionally one would be under 60 & we'd recheck in 10 hrs, recording the higher number.
 
Ted DuPuis said:
Crankcases needing repair following a prop strike aren't uncommon. The case gets a lot of stress and may crack. Not all cracks are catastrophic.

Personally, though, if I get a crack and the engine is apart, I want a new case. Not new to me, brand new.
Click to expand...

Then never buy a factory re-maned engine.
 
Tom-D said:
I thought that big crack was rather obvious.
Click to expand...
It is, but I didn't know if that was a defect or design. Not familiar enough, but it did make me scratch my head.
 
You must log in or register to reply here.
Back
Top