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Curious spot on an exhaust valve

A few days ago when doing the 50hr service I borescoped the valves for the first time ever.

The borescope (an expensive SKF unit) was really hard to see anything with but I managed to find this spot on the #1 exhaust valve

It is a crappy photo (taken with a phone, of the scope’s LCD, because the flash card was full and it was a borrowed unit so I didn’t want to erase it) but it worried me. It looks like there is a deep hole in the middle. It also looks like the remains of a washer, welded to the valve! I know exactly there are just two ways a washer could have got in there (during original assembly, or something has come off inside the air duct) and neither is particularly savoury. So I came back 2 days later with this chinese borescope and it was better, and I got these pics


I also brought some stainless steel wire, about 2mm dia (a TIG welding rod actually, for welding SS) to see how deep the hole was. Bending it suitably to reach the valve, I managed to scrape its surface


and it turned out that this was just more or less a soot deposit, with no step in the valve surface.

I send this to my US engine shop and they said categorically it is fine.

What is interesting is how this forms. Presumably, with a specific combination of component temperatures, the valve stem cools its centre just enough to create this pattern.

There is not a lot on google showing this phenomenon, except this which shows something circular at the bottom

There is otherwise nothing special about the #1 cylinder. Here is the typical EDM data (in a cruise, on a very cold day)

Has anyone come across this before?

Administrator
Shoreham EGKA, United Kingdom

This is one of my valves taken in 2018. In my case I think in my case these are lead deposits but I burn a bit leaner these days and get less of this.

I presume the rings are due to cooling from the sodium filled valve stem?

United Kingdom

OK @Peter or anybody else? I just did my 50 hour check today and this is a photo with the borescope through the top plug and bent 180 degrees to look back at itself. I had a brief panic when I saw these “cracks” on the top of cylinder 1. I am aware that cracks can develop between the spark plug and the valves so had a really good look.

I have inspected another 30 photos of the above and it is definitely not cracking. I’ve recently done 6 x long trips with 25 hours of flying in less than 2 weeks. I am guessing that this is probably chunks of “deposits” breaking off and like Peter’s photo, a tiny change in the surface levels looks massive. BTW Peter, I am still using the Vividia Ablescope VA-400 recommended by Mike Busch. You bought one about the same time as myself, I find the photos it takes are really good.

United Kingdom

It is prob99 just a crack in the deposit in the cylinder. You could do what I did and stick a piece of bent wire in there and scrape off the deposit. The “crack” is about 2cm away from the spark plug hole so this should work.

Your photo of that circle in the middle of the valve is just amazing!

I never got such pics with my endoscope but there is time to learn

Administrator
Shoreham EGKA, United Kingdom

I also use a Vividia Ablescope VA-400 boroscope. I did my 50 hr check 2 weeks ago and while at it, replaced all my 8 spark plugs with new Tempest ones.
My old champions were crap. I ran the resistance test on them and they were anywhere between 0 and 20k ohms. Anyway, while in there, I also boroscoped by cylinders
here is a sample of how my exhaust valves look like.

I almost always fly my IO-360 lean of peak. Pretty much 90% of the time except when taking off :)
The burnt pizza look is fine but any deposit on the exhaust valve indicates, you could actually run leaner. I am a firm believer of lean of peak flying and even more now, after I’ve boroscoped my cylinders.

EDTG, Germany, Switzerland

Beautifully clean valves @By9468840. You have the injected engine whereas mine has a carb. I run it as lean as I dare but can’t go lean of peak as some cylinders would be too lean. I am really envious of your clean valves!

Below is one of my valve seats, I’m quite pleased about how clean they are!

Last Edited by Archer-181 at 11 Jul 21:03
United Kingdom

Yes we should have a “EuroGA cleanest valves” competition

Administrator
Shoreham EGKA, United Kingdom

just sharing some more pictures to contribute to the beauty contest.
But I have a question. One of my intake valves had a bit of oil around it. See pic.
I could not find any reference online indicating if it is a problem or not.. but curious what you guys think.

EDTG, Germany, Switzerland

By9468840 wrote:

here is a sample of how my exhaust valves look like

Wow! What CHT’s are you running?

Antonio
LESB, Spain

Peter wrote:

Yes we should have a “EuroGA cleanest valves” competition

With a scoring system (average hours, CHT, EGT, FF), otherwise fresh hangar queens get the prize

Will be interesting to see pictures of valves from a FADEC engine consistently run at 75% power output, this will reveal most of myths on how to “properly run” an engine?

Last Edited by Ibra at 12 Jul 13:09
ESSEX, United Kingdom
12 Posts
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