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Author Topic: Why simply doing a compression test might not be enough  (Read 2919 times)

ghancock

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Why simply doing a compression test might not be enough
« on: October 28, 2016, 10:07:52 am »
So we all do compression tests during annuals and we all have been told that anything about 60 or so is runnable.  We hear the leaking past usually the exhaust valve but figure if it aint broke don't mess with it.  Well, I wanted to show you what I've found on a cylinder that had 70/80 on the last check but had been as low as 65/80 on previous checks.

First, here is a picture of the inside of the cylinder and you can see an absolutely burnt exhaust valve.  Again, this cylinder was producing power and working.



So while the cylinder had compression, it obviously wasn't very happy about it.

Once I pulled the valve cover off after my last trip and figuring compression tests were not really enough, I discovered the inside of the rocker box to be full of burnt oil.




Note: the above picture was from another cylinder but they looked similar.  The #1 cylinder however had a broken valve cap that looked like this and had stopped rotating which in turn was wearing down the rocker arm.



This in and of itself was bad, however, the broken pieces of metal had to go somewhere right?



If you look in the hole on the left you'll see a large chunk of metal that had gone down the oil drain line and into the engine, luckily it got jammed in the hole, but the point is still the same.

In my opinion, if you have a valve leaking more than usual, you really should at least pull the rocker box cover and see what it looks like.  If you can remove the rocker arm which only takes a minute and try to wiggle the valve back and forth as an extra check, that would be a good idea as well.  The fact that the cylinder has ok compression based on Continental or Lycoming doesn't necessarily mean all is good on the inside.

Just my 2 cents,

Glenn
« Last Edit: October 28, 2016, 12:44:00 pm by ghancock »
--glenn
You can't win an argument with an ignorant person,  they'll just drag you down to their level and beat you with experience.

Badger

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Re: Why simply doing a compression test might not be enough
« Reply #1 on: November 28, 2016, 12:27:58 pm »
Great info, Glenn.  Thanks.

I got a call from my mechanic last week and he found a leaking exhaust valve in each engine (#6 on L, and #1 on R) during my Annual.  He's pulled them and having them repaired.  Fortunately, he's confident that the engine will do fine after those repairs. 

I've also got a couple of magnetos that need replacing on the Left engine (which might just explain some "minor" issues I've noted when in the air).

And, my Ameri-King ELT battery needed replacement....which, since that company cannot sell batteries anymore, means a new ELT.

All are minor issues, though, so I am pumped to get the aircraft back SOON (hopefully, by the end of next week...)

Ed
Ed

ghancock

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Re: Why simply doing a compression test might not be enough
« Reply #2 on: November 29, 2016, 07:40:24 am »
Yes, it is nice to hear them purring.   I am about to start replacing 3 more jugs on my right engine and then starting on the left.  I have decided I am just going to do a top on both engines so all the cylinders are mine instead of having to pull along old problems.  Then I can tell for sure if LOP is hurting anything or not.

Glenn
--glenn
You can't win an argument with an ignorant person,  they'll just drag you down to their level and beat you with experience.

Badger

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Re: Why simply doing a compression test might not be enough
« Reply #3 on: November 29, 2016, 10:07:32 am »
Great plan!  Good luck!
Ed