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,