A curious round spot on a valve, IO540-C4D5D

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peter-h

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peter-h
Hi All,

For the first time in 18 years of ownership, we borescoped the engine at the 50hr service.

The borescope was very hard to use but we did manage to check all the exhaust valves. All were ok except #1 which shows no signs of edge discolouration but in the middle is a spot, about 1/4 inch to 1/2 inch in diameter, which looks like a deposit but it could be something else.

The engine was overhauled 2.5 years and about 300hrs ago, by a top US engine shop. It is always
operated at peak EGT.

The metal on that part of the valve is quite thick so I can't see it could be a hole or a piece of metal which came out of it, but who knows?

Sorry for the really bad photo. I will try to get a better one if needed. It is fairly easy to do - remove top cowling and the #1 top spark plug.

20190626581072411.jpg


The spot didn't appear to be in the centre. But it was surprisingly circular itself.

I have a Chinese borescope; the one we used here was an expensive SKF one.

I have done a lot of googling and have seen somewhat similar things but mostly they were large spots caused by overheating.

Many thanks for any input.
 
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Wow, never seen anything like that. It almost looks like a piece of FOD, maybe a small washer welded itself to the valve face. Unusually symetrical circle.
Send a copy to the engine builder, see what they say.
 
Have done.

Yes I did wonder if it was a washer.

There are no such washers in the air intake duct, and I doubt anything could have been hanging around the "tunnel" of the RSA fuel servo.

What concerns me is what might happen if it comes off.
 
Well, you see something in the borescope. It seems to me the obvious answer is to yank that jug and put real eyes and fingers on it. Why borescope if not?
 
I see a pair of fingers holding a triangular shaped valve, is this the same pic as was originally posted? Also, shouldn't you be operating the engine either rich or lean of peak?
 
The pic is the right one. We could not take a photo with the borescope itself because the flash card in it was full and it was borrowed... So the pic was done with a phone, of the borescope's LCD screen, and it was outdoors, hence the poor quality.

I don't have maintenance facilities at the airfield. We could pull the cylinder outdoors but I need to be sure it is worth doing. It is a procedure not without risk; you have to tighten the through-bolts immediately, with a dummy plate in place, to prevent the bearing shells moving. There are all kinds of deposits inside cylinders, quite normally, but not usually a well defined one like this.

I do need to get a better photo.
 
Go to Wally-Mart and buy a SD card for $10. Repeat borescope, put SD card in computer and get good image.
 
Looks like the open ends of a lock washer tack welded at the 9 and 11 o'clock positions. You're correct in pulling a jug isn't without risk, but pulling a single jug usually isn't as fussy about torque plates and such. The risk is probably a lot less than leaving that thing in there, if it is what I think it is.
 
Could one try to scrape it off the valve, with some kind of a bent tool? And then lift it out with a magnetic tool?
 
peter-h said:
Could one try to scrape it off the valve, with some kind of a bent tool? And then lift it out with a magnetic tool?
Click to expand...
This. And it's likely just a deposit. I cannot see a washer staying welded on the valve, and I cannot see it getting there without peppering the top of the much softer piston.
 
I also cannot see a washer getting welded there because the valves are covered with an ash deposit. The only chance would be with a brand new engine.
 
Kenny Phillips said:
This. And it's likely just a deposit. I cannot see a washer staying welded on the valve, and I cannot see it getting there without peppering the top of the much softer piston.
Click to expand...

However, I can see a perfectly circular shape, with sharply defined inner and outer edge radii. Admittedly, this belongs in Ripley's believe it or not as a gazillion to one shot if it is a washer, but it doesn-t look like any naturally occurring deposit I've ever seen.
 

peter-h said:
Hi All,
... It is always operated at peak EGT.
Click to expand...

This is the point of highest cylinder pressure and maximum pressure on the engine. It's never a good idea to run peak EGT, it likely the worst possible lean setting. Consider running either rich or lean of peak. "back when" many of our POH's were written the engine instrumentation was not available we have today, both onboard or in the engine manufacturers labs. The POH is commonly wrong in this regard.

Search YouTube for the excellent Savvy Aviation EAA webinar on Engine Leaning Basic and Engine Leaning Advanced class.

-David
 
peter-h said:
Could one try to scrape it off the valve, with some kind of a bent tool? And then lift it out with a magnetic tool?
Click to expand...

You could try, but you'd leave hot spots on the valve. Curious if this jug ran hotter than the others?
 
I actually fly slightly LOP. Have GAMIs.

#1 cyl is not specially hot

20190626404823715.jpg
 
NordicDave said:


This is the point of highest cylinder pressure and maximum pressure on the engine. It's never a good idea to run peak EGT, it likely the worst possible lean setting. Consider running either rich or lean of peak. "back when" many of our POH's were written the engine instrumentation was not available we have today, both onboard or in the engine manufacturers labs. The POH is commonly wrong in this regard.

Search YouTube for the excellent Savvy Aviation EAA webinar on Engine Leaning Basic and Engine Leaning Advanced class.

-David
Click to expand...

I thought that was peak CHT.
 
peter-h said:
I actually fly slightly LOP. Have GAMIs.

#1 cyl is not specially hot

20190626404823715.jpg
Click to expand...

On an absolute scale no, but relative to the others it's second warmest. My #1 is coolest being right up front. Look at your #2.
 
I knew this would lead to the lean of peak discussion :) I am familar with the issues: http://peter2000.co.uk/aviation/engine-management/index.html

Anyway, I went back with a better (chinese) scope and a length of 1/16 stainless steel wire and got some pics

Closeup:

20190626264833517.jpg


Further back:

20190626194843617.jpg


With the wire, one can just scrape it off. So it is just a thin deposit, with no noticeable thickness to it

20190626244853717.jpg


So unless this is a known manifestation of an impending disaster, I will fly with it.

Does the valve look like it is getting too hot? The colour of it is certainly even all round.
 
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I have to say that's the strangest deposit I've seen, but much clearer in this pic. Now that you've confirmed it's just schmutz, I'd say otherwise the valve looks very healthy.
 
upload_2019-6-26_13-28-25.png

Here we go again.

Peak EGT does not correspond to peak CHT nor peak ICP. Flying around in peak EGT is fine. 50 ROP is the "worst" spot, but that's only insofar as the actual temp values you're seeing on a CHT basis. If your CHT is below below 380-400, it doesn't matter what you run, even if it's 50 ROP.

People overthink this mixture knob thing. Another case for FADEC keeping the OCD types from confusing themselves out of house and home. :D
 
hindsight2020 said:
People overthink this mixture knob thing. Another case for FADEC keeping the OCD types from confusing themselves out of house and home. :D
Click to expand...

But then I am stuck with whatever they give me. Sometimes I want econ-o-cruise, and sometimes I wanna get there quicker.
 
Much better picture. Might be nothing, or in could be that valve is leaking enough to torch the outer diameter, while the inner circle is kept a lower temp by the valve stem. On the further picture there is a blemish at the 3:30 to 4:30 position while the rest of the seat is clean. What do the other valves look like?
 
hindsight2020 said:
View attachment 75455

Here we go again.

Peak EGT does not correspond to peak CHT nor peak ICP. Flying around in peak EGT is fine. 50 ROP is the "worst" spot, but that's only insofar as the actual temp values you're seeing on a CHT basis. If your CHT is below below 380-400, it doesn't matter what you run, even if it's 50 ROP.

People overthink this mixture knob thing. Another case for FADEC keeping the OCD types from confusing themselves out of house and home. :D
Click to expand...

I run 75 rop or 50 lop per the manufacturer's recommendation and the owner's request.
 
EdFred said:
But then I am stuck with whatever they give me. Sometimes I want econ-o-cruise, and sometimes I wanna get there quicker.
Click to expand...
You'll have a single lever: push it forward for fast, pull it back for cheap.
 
PaulS said:
I run 75 rop or 50 lop per the manufacturer's recommendation and the owner's request.
Click to expand...
Cool. And? The turbo arrow poh for instance prescribes a power table that is not altitude compensated, leading to a no kidding misrepresentation of rated power at altitude than that advertised, with deleterious impact to safety and longevity. IOW, Appeal to authority is a pretty old logical fallacy. but you're right, your monkey your circus, my monkey my circus. Its just a hobby.
 
hindsight2020 said:
Cool. And? The turbo arrow poh for instance prescribes a power table that is not altitude compensated, leading to a no kidding misrepresentation of rated power at altitude than that advertised, with deleterious impact to safety and longevity. IOW, Appeal to authority is a pretty old logical fallacy. but you're right, your monkey your circus, my monkey my circus. Its just a hobby.
Click to expand...
Yep, and all engineers are dumbasses.:eek:
 
Yes indeed; that's an excellent example. A fairly round spot in the middle. Thank you.

PaulS - the other valves didn't have that spot but then I was using the rubbish endoscope on them. Next time I will do it better...

There is quite a lot of skill in getting a good quality image. I think one needs a separate light source poked in through a second plug hole.
 
peter-h said:
Yes indeed; that's an excellent example. A fairly round spot in the middle. Thank you.

PaulS - the other valves didn't have that spot but then I was using the rubbish endoscope on them. Next time I will do it better...

There is quite a lot of skill in getting a good quality image. I think one needs a separate light source poked in through a second plug hole.
Click to expand...

The only other thing I would do next time is get pics of the valve stem and seats too, if possible, just make sure you don't crunch the camera in the process, that would suck, not so much for the camera, but pulling the cylinder to clean it.
 
Not sure I could get the endoscope under the valves. How far do they come out? The end is about 8mm diameter.
 
peter-h said:
Not sure I could get the endoscope under the valves. How far do they come out? The end is about 8mm diameter.
Click to expand...

Check out the video at the end of this article. Basically you turn the engine (spark plugs out) until the valve opens. Again, make sure you don't crunch the camera. You look from inside the cylinder. A good mechanic should be able to do this. I would look at the seats and valve stem.
 
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