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Cylinder boroscope tutorial?

alpinelakespilot2000

Well Known Member
Anyone have a link to a tutorial that covers how to boroscope cylinders to complement the annual condition inspection compression test?

Need to know what, where, and how to boroscope. Then ideally how to interpret the resulting pics.

Thanks.
 
There’s one which I’d recommend https://m.youtube.com/watch?v=1i-uovqvLaI

A good presentation here https://m.youtube.com/watch?v=DnOlrqReesk

Our homebuilder association offers practical maintenance courses in which we teach borescope inspections. Guess we are located slightly too far ;)

Borescoping an engine ain’t black magic. All it takes is a good borescope, practice, and yes, enough knowledge to decipher what you looking at.

Great videos. I'll have to check that snapshot button on the VA-400 - I had not noticed that.
 
The exhaust valve is the most important thing to look at and quite honestly pretty easy to interpret. No green and no asymmetry. The rest I was still a little clueless on so I posted my results here and got some opinions from more experienced. Here is that thread.
 
The videos done by Dave P are outstanding. He's a great example of a tech who is thinking about the issues he sees and working out an explanation for them. No need to dig into the engineering (although he touches on some of this), he just knows from experience what he needs to be looking for. His diagnosis of the rotator issue is insightful, really good to see that kind of forward thinking. He shares some serious experience from previous work ... well done!
 
There’s one which I’d recommend ...

I'm going to play the devil's advocate. Pretty much every time he says, whatever you see, it's fine as long as there is no metal in the filter and the compression is good. About the exhaust valves: he seems to make a decision whether to lap in place or yank the cylinder based the borescope picture... Yeah, only when the compression is low. And even in this case the first remedy is to fly for an hour and recheck...

So, if the compression is good, why bother?
 
I'm going to play the devil's advocate. Pretty much every time he says, whatever you see, it's fine as long as there is no metal in the filter and the compression is good. About the exhaust valves: he seems to make a decision whether to lap in place or yank the cylinder based the borescope picture... Yeah, only when the compression is low. And even in this case the first remedy is to fly for an hour and recheck...

So, if the compression is good, why bother?

As I understand it, if a valve is not rotating, which causes the discoloration to be uneven, that valve will fail. Even with good compression today, it's better to replace what's needed to get the valve rotating properly. I could be wrong, but this seems to be more common in Continental engines due to the design of the rotocoil. Lycomings have something that causes the valve to rotate, but I don't know how that works or if it's reliable.

Here's a picture of an example from a Continental and the article I stole it from:

https://airplaneownermaintenance.com/174-turbo-normalized-bonanza-exhaust-valve-rescue/

IMG_9111.jpg
 
out of the IPC, items 10 & 11 are the rotators on a Lyc
 

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Quote from your link:

we did a cold compression check and could not get more than 30/80 psi, with significant leakage past the exhaust valve

Sure, the cylinder had low compression which warranted further investigation with borescope... And only in order to be able to determine if lapping in place makes sense instead of removing the cylinder completely.
 
So, if the compression is good, why bother?

Checking the compression is, as for instance oil analysis, only one of the tools available to assess and monitor your engine’s health. As Mickey points out, I’d rather discover that failing valve in the warmth of the hangar than sweating it out whilst airborne ;)
 
As I understand it, if a valve is not rotating, which causes the discoloration to be uneven, that valve will fail. Even with good compression today, it's better to replace what's needed to get the valve rotating properly. I could be wrong, but this seems to be more common in Continental engines due to the design of the rotocoil. Lycomings have something that causes the valve to rotate, but I don't know how that works or if it's reliable.

Here's a picture of an example from a Continental and the article I stole it from:

https://airplaneownermaintenance.com/174-turbo-normalized-bonanza-exhaust-valve-rescue/

View attachment 17856

Discoloration is usually from inconsistent heat transfer. The colored spot in your picture is due to heat, not lack of rotation. There is likely no contact between the valve and the seat at that spot and therefore it is getting too hot (valve can't transfer its heat to the seat in that spot). This is from abnormal wear or a bad valve grinding, not from lack of rotation. Sometimes rotation can help to prevent this type of wear, but not always. In your pic, the valve likely has the deformity and therefore rotation keeps the heat in the same spot on the valve. If the seat has the deformity and the valve was rotating, the whole valve would eventually be burned. A picture showing the open gap would tell you if either or both of the surfaces are not making contact. Any non-shinny part of the valve face or seat face indicates poor contact.
 
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Here’s a good guide from the Air Safety Institute
 

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Discoloration is usually from inconsistent heat transfer. The colored spot in your picture is due to heat, not lack of rotation. There is likely no contact between the valve and the seat at that spot and therefore it is getting too hot (valve can't transfer its heat to the seat in that spot). This is from abnormal wear or a bad valve grinding, not from lack of rotation. Sometimes rotation can help to prevent this type of wear, but not always. In your pic, the valve likely has the deformity and therefore rotation keeps the heat in the same spot on the valve. If the seat has the deformity and the valve was rotating, the whole valve would eventually be burned. A picture showing the open gap would tell you if either or both of the surfaces are not making contact. Any non-shinny part of the valve face or seat face indicates poor contact.

Thanks Larry - that helps my understanding. I was under the impression that the inconsistent heat transfer was caused by the valves not rotating and carbon and lead deposits being left unevenly on the valve/valve seat, but I can see how what you describe can happen and trigger the uneven heating.
 
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