compression: one cylinder low OK?

Ted DuPuis said:
I'd venture a guess I know a thing or two about compression, the numbers, and what actually happens inside the combustion chamber.

I'm also not sure whether you're trying to disagree with me or not, because the summary of my post was "Don't worry about it."

You are correct I don't service these things daily. But, I've spent a few thousand hours running these engines on dynos, as can be proven by my hearing loss. Really shoulda been wearing my Zulus in the lab, not just in the plane.
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Again poor wording on my part. It should have read "when one understands" It wasn't just a response to you.
The 0-300 should run great with one cylinder at 60/80.
I an surprised that you had 0-300s on the dynos, since the factory hasn't supported them for many years.
 
Just to chime in - don't do anything drastic.
Yeah, I'd rather the leakdown number on one cylinder wasn't lower than the others, but not a show stopper yet.
Your likely suspects are a leaking valve (exhaust usually) and stuck rings.
If it were my engine I would throw a can of AVBLEND in the oil and immediately fly the heck out of it at max continuous power for a couple of hours, leave it for a day or two and repeat, (even more than once) then recheck the leakdown on that cylinder while it is hot. I might hit a home run for a few bucks.
If push comes to shove, pull the cylinder, lap the valves, ball hone the bore and a fresh set of rings. A few more bucks but will usually solve it assuming no cracks across a valve seat, etc..
 
Checkout_my_Six said:
School us bout dynamic compression.
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The short answer for the novice, is it requires time to pass a gas thru a hole, while running it does not have time to do that thus the cylinder will continue to preform normally.
To continue, I prefer to do cold compression checks. When the engine has what we know as morning sickness it will show up when cold. because the valve will stick partially open when cold. and not when hot.
 
Tom-D said:
I an surprised that you had 0-300s on the dynos, since the factory hasn't supported them for many years.
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When I say "These engines" I'm referring generically to the legacy piston aircraft engines that we all fly behind (albeit in many different forms).

I generally avoid posting specific engine models I did or did not run on the dynos on here, especially since I never worked for Continental. :)
 
You might try a ring wash procedure on the low cylinder. Don't cost but a couple of bucks, might just free up a stuck ring and bring the number back up.

Always go with cheap and easy first.
 
Chip Sylverne said:
Always go with cheap and easy first.
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Unless you had a failure that involves seeing the crankshaft spinning (as in the parts that are normally inside the crankcase). If that happens, there ain't no cheap and easy. ;)
 
Ted DuPuis said:
When I say "These engines" I'm referring generically to the legacy piston aircraft engines that we all fly behind (albeit in many different forms).

I generally avoid posting specific engine models I did or did not run on the dynos on here, especially since I never worked for Continental. :)
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Show me some one who knows it all, I'll show you some one who scares me, but there are a few here.

I didn't think you had a whole lot of experience with the 0-300.:) I have overhauled a few, now I only have 5 on my customer list that I service and inspect each year.
Of course when you have a catastrophic failure you'l not have any compression of either kind. Ya gotta keep it real.
 
I'll sell you one, same year, but with a garmin 430, clean interior and no low compression. I'm deploying for 10 months and going to sell the airplane and upgrade to a PA-32 when i return
 
If the leak is caused by the exhust valve, fly it with a can on valve cleaning additive for cars. The chance that the valve is working after cleaning is 50%.
 
whereisrandall said:
Hey gang, I'm looking at getting a 1956 Cessna 172 with a 6 cylinder Lycoming 300. Compressions at last annual (February 2016) were 76-74-75-76-62-78. Any reason to be concerned about cylinder 5? Thanks as always,
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Not really a concern... Just keep an eye on it. My #5 was similar when I purchased it in 2009, in 2015 (~400 hours later) it was very low and I removed it from service and replaced with an overhauled one. This year everything was peachy! The O-300 powered Skyhawk is the smoothest flying C-172 there is...
 
gitmo234 said:
I'll sell you one, same year, but with a garmin 430, clean interior and no low compression. I'm deploying for 10 months and going to sell the airplane and upgrade to a PA-32 when i return
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Uhhuh. My situation has changed, so I won't buy now. But I've thought seriously about your bird.
 
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