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Stuck valve mystery

Mconner7

Well Known Member
I stuck a valve this week and bent a pushrod. I pulled the valve spring expecting it to be stiff or sticky but it seems to have a normal wobble clearance in the guide. I borescoped it and see no reason for it to stick closed.

I have a reamer and pushrod on the way but am baffled as to why it would stick closed like that.

Any ideas?
 
Most people will agree it's the luck of the draw and you got the short straw.
It would help to get a better answer if people knew engine, hours, systems and flown per year. Etc Etc.
Never had one so I must be getting long straws.
But I burn Costco premium so maybe that helps. And have SDS ignition and fuel systems.
Your luck obviously varies.
Art
 
Get a borescope and take a look at the piston. If a foreign object got trapped between the valve and piston, it would have caused this issue. There should be a witness mark or two on the piston if this is indeed what happened. The culprit could have blown out on a later stroke so there would be nothing floating around in the cylinder now.
 
Ok so the motor has just under 700 hours since factory new millennium cylinders. I fly at least 100 hours a year. I have had a total of four times that I got a burble that I now know was the valve. The last time as 11 months ago, almost 100 hours.
 
Get a borescope and take a look at the piston. If a foreign object got trapped between the valve and piston, it would have caused this issue. There should be a witness mark or two on the piston if this is indeed what happened. The culprit could have blown out on a later stroke so there would be nothing floating around in the cylinder now.

Thanks Mike, I have the scope, just didn’t look at the piston, good idea.

Mark
 
Did you do the wobble test and if so what was the clearance recorded? The stuff that forms in the guides is pretty hard (carbon based) and could be loose and therefore only creates sticking at certain times. I would continue along and get it reamed or ball honed.
 
Did you do the wobble test and if so what was the clearance recorded? The stuff that forms in the guides is pretty hard (carbon based) and could be loose and therefore only creates sticking at certain times. I would continue along and get it reamed or ball honed.

The wobble gauge in in the care package with the reamer and the pushrod and it’s housing. I will report back what the wobble was before and after reaming.

Mark
 
The wobble gauge in in the care package with the reamer and the pushrod and it’s housing. I will report back what the wobble was before and after reaming.

Mark

Don’t ream if not needed per wobble test right?
 
Don’t ream if not needed per wobble test right?

The reamer may find combustion byproducts. It’s like the Jewish mothers chicken soup, it couldn’t hurt. I will scope the guide once I drop the valve, if it’s shiny metal, no but if it shows deposits, no harm in cleaning the bore?
 
I just honed my exhaust valve guides using the ball hone mentioned previously. A 1/2" Flex-Hone in 320 grit was my "first pass". One valve guide cleaned up with this. Two others required stepping up to the same hone in 240 grit. I also used a 50/50 mixture of ATF and acetone as my hone oil as this stuff is pretty good at liberating deposits.
 
Valve guide photos

I pushed the valve in and found spalling of the guide. Makes me think there was some trash caught between the stem and guide that worked itself out or was crushed.
 

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I pushed the valve in and found spalling of the guide. Makes me think there was some trash caught between the stem and guide that worked itself out or was crushed.

What did the valve stem look like?
Did you reuse the exhaust valve?
 
There was some wear on the valve stem. Since this post, I found two failed lifter body’s and a cam lobe failing. I assume that was the source of the foreign object that scored the guide.

My engine is completely disassembled for overhaul now, I hope to start putting it back together in a couple weeks when the case is back. This is a long saga so PM if you want any details.
 

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There was some wear on the valve stem. Since this post, I found two failed lifter body’s and a cam lobe failing. I assume that was the source of the foreign object that scored the guide.

My engine is completely disassembled for overhaul now, I hope to start putting it back together in a couple weeks when the case is back. This is a long saga so PM if you want any details.

Just an opinion, but I don't think there was debris in the small space between the guide. My guess is combination of lack of lubrication and heat cause galling of the guide against the sliding stem of the exhaust valve. I would be sure to replace the valve and the guide in the overhaul process.

Lubrication for the upper end comes via the pushrod tubes. Oil pressure is supplied to an annular gap in the tappet. See the cutaway below. If the tappet was failing it could easily have restricted oil flow up to the upper end.

Screen Shot 2022-10-27 at 6.30.38 PM.png
 
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