Low compressions on 0-300D

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Flybo01

Filing Flight Plan
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Flyby30
Hello,

So recently I had a A&P check my engine compressions and was sad to find that they were low 45-60-60-70-75-68. He repeated the procedure in front of me and it showed as he noted. But the weird thing was that a/c just had an anual done 2 months ago and there were all 72-76-76-69-70-78......I looked up the compressions check pattern last 12 years on the engine and they are very similar to last anual. There are 420 hours flown since 12 years ago, engine at TBO So not sure what happens all of the sudden.....I want to add that this A&P replaced the push rods and gaskets not sure if this anything to do with it. We run the engine for 4 mnts then check compressions and note they showed low. Thoughts?
 
Rgbeard said:
Carbon chip wedged under the exhaust valve, holding it slightly open?
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not sure....how do I go about it? He said I may have to pull the cylinder out and get it replaced , I need to know be sure that this is indeed the problem cz is not cheap
 
Flybo01 said:
Thoughts?
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Curious, why did you check your compressions if they were okay 2 months ago? But before you rip and tear, is this the same compression tester used for your annual? If not, have your cylinders rechecked with that tester first, since the previous readings matched prior tests. There are usually different readings between different compression testers due to various reasons: different size master orifice, gauges not calibrated, etc.
 
Not looking for shortcuts at all....if I need to replace I have no choice, I am just trying to get an second opinion of what could be the issue here....Do I really have a problem or was the compressions checked erroneously by the A&P, someone told to check if he has his system properly calibrated....I just don’t want to start opening the engine to justify someone’s mistake
 
Bell206 said:
Curious, why did you check your compressions if they were okay 2 months ago? But before you rip and tear, is this the same compression tester used for your annual? If not, have your cylinders rechecked with that tester first, since the previous readings matched prior tests. There are usually different readings between different compression testers due to various reasons: different size master orifice, gauges not calibrated, etc.
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engine is at TBO and I’d like to keep a close look of what’s going on...since he was working on it he offered to check even though was done recently....
 
Fly it for two hours at full throttle. Circle the airport at pattern altitude + 2000 feet. Really heat that engine up! Then check your compressions again. -Skip
 
I am not a mechanic and I didn't stay at a Holiday Inn Express last night, but I have seen compressions on my own plane change due to different testing equipment or improve with more flying, for what it's worth.
 
Why were the push rods changed? Not a normal wear item. If he unwittingly installed oversize ones, they might be holding the valves open slightly and causing loss of "compression".

Did you listen to where it was leaking? Hissing at exhaust means leaky exhaust valve, at air intake (with throttle open) means leaky intake valve, at oil filler cap means leaky piston rings.
 
See TCM Standard Practices Manual for the compression check procedure. You are allowed to go way below 60 before having to pull cylinders.

As stated, go run it hard for an hour or two and then check again. There was another thread not long ago with one cylinder at 20 PSI. 45 would be ok with me, 20 not so much.
 
Pretty much been said already, but follow the CMI guidance on allowable leakage. Continental service bulletin SB03-3 is a great place to start... Many a cylinder has been pulled out of service prematurely using other guidance, but this SB is a pretty common sense approach.
 
thomasdr72 said:
Pretty much been said already, but follow the CMI guidance on allowable leakage. Continental service bulletin SB03-3 is a great place to start... Many a cylinder has been pulled out of service prematurely using other guidance, but this SB is a pretty common sense approach.
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Looks like SB03-3 has been superseded by the latest Maintenance Manual M-0, Rev 1, September 2019
 
wilkersk said:
Awfully curious about why the pushrods were "changed".
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One reason could be the mechanic checked valve lashe and it did not meet spec. You can change the lashe by using a longer or shorter pushrod.
 
Don't sweat it you have six cylinders on the o-300 at least 5 are good.:D We have a couple of leaky push rods on ours we are going to work on soon. Probably just re-swage the lip on them.

Personally I would fly it hard a couple hours then check again.
 
For a cylinder leak-down test to suddenly deteriorate on one cylinder (it's not "compression" dammit), even on a Continental, there almost has to be an audible airleak.
While running the engine hard to get things to seal is a time honored tradition I have to disagree in this case.
Get a second opinion and a borescope on the cylinder before leaving the ground.
It is not airworthy under the regs for one thing - besides the obvious risk of being sued for everything you own if it causes injury while being flown illegally.
And NO, I am not a compulsive nitpicker about regulations, but there are times they make a lot of sense.
 
Witmo said:
One reason could be the mechanic checked valve lashe and it did not meet spec. You can change the lashe by using a longer or shorter pushrod.
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Those have hydraulic lifters, don't they? Does the valve lash suddenly go out on those? Wouldn't that mean likely serious valve seat recession?
 
Flybo01 said:
Push rod seals.....they do leak from time to time, common problem with the 0-300
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I'm not a Connie guy. But, when the push rod tube seals leak on my bug, I make sure that after I change them, the push rods go back exactly the way they came out.
 
wilkersk said:
Those have hydraulic lifters, don't they? Does the valve lash suddenly go out on those? Wouldn't that mean likely serious valve seat recession?
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That's one possibility. The rocker arm face might also have worn down. Some rockerarms have rollers to prevent wear. I had to replace a rocker arm on an O200 when an oversized pushrod wasn't enough.

No one's mentioned this but sometimes the gaps in the piston rings align and can cause a temporary loss of compression. That's why you stagger the gap in the rings when you install a piston. I wouldn't go pulling cylinders on the basis of a single low reading during a leakdown compression test if the engine still produced the minimum static rpm at full throttle.
 
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