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Lycoming Conundrum

ArlingtonRV

Well Known Member
I found an interesting thing during this year's Condition Inspection, which is still ongoing. I had a leaking valve cover on #2 and possibly #4 that I replaced about a month ago and thought just for grins I would change the ones on #1 and #3 also. When I pulled the #3 cover I found a plastic disc stuck to the valve cover and the aft end of the rocker shaft was full of carbon chunks and plastic debris.

A little background, the airplane is a 2001 RV-8 with a Lycoming O-360 A1A that was modified before installation with 10:1 pistons, constant speed, fuel injection and an angle valve sump (among other things). All work done by Lycon in 1999-2000(ish). Engine currently has just under 2,250 hours.

This is what I found in the valve cover.

IMG_4551-XL.jpg


It turned out that the forward Thrust Button, as Lycoming calls them, P/N LW-12892 for those of you taking notes, was in the same condition, but still in place. I was a bit surprised to see what a new looked like.

New ones.
IMG_4565-XL.jpg


I was also surprised to see that what I thought was a dimple in the end was actually a hole. I installed the new ones.

New one going in.
IMG_4574-L.jpg


I got a little paranoid after talking to Carl, so I took the #1 valve cover back off to look at it and both buttons were just like the new ones, except much browner.

The big question that neither of us can answer is what could have caused this. It appears to only have affected #3. I wonder if it is possible that the holes in the ends became clogged with carbon debris and air pressure trapped inside the shaft forced them outward, but that doesn't make much sense since they don't fit all that tight.

Has anyone seen this before and/or know what could cause it?

Thanks
 
if, what I suspect, cylinder #3 is your hottest running one, heat is probably the cause. Proof would be the related carbon build-up.
As they age and are subjected to heat cycles and oil, these "plugs" become brittle.
Had some break apart when pulling them out to perform the 388C SB...
 
The fact that it was stuck to the VC is an indication that the shaft is not moving around. It could have been one cold start or it just broke on it's own. Who knows . . . it could have been cracked from the beginning.

If that head is any indication of the insides of that engine it looks pretty good for 2200+ hrs. No indication of persistent high temps. Curious . . . what oil do you use? Do you have any air relief for the back of #3, chute, washer etc?

Sorry for questions.

Hopefully, the sump oil suction screen was checked/cleaned too. ;)
 
if, what I suspect, cylinder #3 is your hottest running one, heat is probably the cause. Proof would be the related carbon build-up.
As they age and are subjected to heat cycles and oil, these "plugs" become brittle.
Had some break apart when pulling them out to perform the 388C SB...

#3 and #4 do run a little warmer than #1 and #2, but CHT's are usually in the low to mid 300s, a little cooler in the winter. Heat and time are indeed good probabilities.
 
The fact that it was stuck to the VC is an indication that the shaft is not moving around. It could have been one cold start or it just broke on it's own. Who knows . . . it could have been cracked from the beginning.

If that head is any indication of the insides of that engine it looks pretty good for 2200+ hrs. No indication of persistent high temps. Curious . . . what oil do you use? Do you have any air relief for the back of #3, chute, washer etc?

Sorry for questions.

Hopefully, the sump oil suction screen was checked/cleaned too. ;)

I currently use Phillips Victory 20W50, but I have switched between Phillips and Aeroshell over the years.

No, I don't believe there is any relief on #3, but CHTs are really not a problem, if anything, they run a little too cool, particularly in the winter.

Yes, the suction screen is cleaned/inspected regularly.

Thanks
 
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