Question for cylinder head gurus

[flyingriki]

Originally Posted by skylor

The valve seat should not be moving. Period!

Skylor

Can't imagine thinking differently! My engine was acting funny at startup, but only sometimes. I found movement in one of my seat on the offending cylinder. Pulled the cylinder, reached in and pulled the seat out with my fingers. Was a cracked head. Turned into an overhaul based on other things.

[scsmith]

Thanks everyone.

I don't have any specific knowledge of the particulars. My friend has a Cirrus, but he also uses the boroscope to look inside other engines for friends. So I don't know if this is his or someone else's.

I can ask him about the brand/specs of the boroscope. I'll post back here.

These are on you-tube, so if you would like to see more, there are a couple more. One of the others is definitely a valve-guide wear example-you can see the valve shift sideways as it seats. two others show other valve seats moving.

In the one I linked to, I disagree that there is valve-guide wear. It pretty clearly looks like the valve does not move laterally as it contacts the seat, it moves straight into the seat, and then the seat squirms a little because it is loose in the cylinder. It is kind of a moot point though, a new cylinder will fix both issues.

The very best part of this story is that without the boroscope inspection, one would have no indication of anything wrong here, and would run the engine until something failed. This kind of periodic inspection seems very valuable.

But I wonder.....how many engines are out there with slightly loose guides like this, and might run for a long time, maybe even to TBO, and how would anyone would know? I think that this inspection is not widespread, and so there is very little data. But this winter I hope to get my friend up here with that scope to take a looke at mine.

[scsmith]

In case anyone is interested (a few expressed interest) here are more particulars on the engine in question, and the borescope.

First, the borescope is a Snap-On BK8000. My friend said that it is difficult to see minor things like this looking at the display because of screen glare. He recommends wearing a black shirt and closing hangar doors. But the best thing to do is record the images and view playback on a computer.

The engine is an IO-550 in a Cirrus SR-22. It was a factory-new engine with now 1700 hrs total time.There are no running symptoms that would indicate a problem.

He has been doing borescope inspections every other oil change for several years. Early this year, he observed one very slightly loose valve seat, and decided to monitor it over time. At the latest inspection (most of a year since first discovery) that seat had become noticeably more loose, and two other cylinders now show slight looseness of the seats.

The engine is getting a top-end overhaul now. The shop said that they do see this from time to time. Sometimes when they turn the cylinder upright the seat falls out.

Just think how many thousands of aircraft engines are out there that never get this type of borescope inspection! How many have slightly loose seats and run until some other issue requires cylinder change? How many run until the seat falls out while the engine is running and breaks a valve off?

Is this something more common to Continentals than Lycomings? Is there something unique about the design of the seat that makes it prone to loosening? Given the absence of inspection protocol for this issue, there is just no meaningful data.

I'm going to borrow my friend's borescope and take a look as soon as I can.

[Mike S]

I had an intake valve seat come clear out of its machined cavity, and it got cocked a bit sideways and prevented the valve from closing/sealing off the combustion chamber. This made for an extremely rough running engine

Luckily, it happened as I pulled the power back to land---on a flight to San Antonio in my old Stinson.

Franklin 165 engine, so it is not just a Lyc or Conti issue. I suspect any engine with a steel seat inserted into an alum head is a candidate for this kind of thing.