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Broken piston rings

CPSONE

Well Known Member
Not on an RV but worth sharing as it happened on TO-360's yes both!
Came back from a flight a while ago in my Turbo Seminole and found oil coating the bottom of one wing behind the left engine. Lost about 1/2 a quart in an hour flight. Upon inspection a very small amount of oil was found to have been seeping out the top of the dipstick (through the shaft retaining pin in the screw cap and out the top around the shaft itself, this was a big clue). Dipstick was installed and tightened correctly. The magnetos (dual mags in 1 housing) had oil on the bottom of the housing and been having problems with the mag seal so thought the seal went again. Sent the mag in for overhaul and turns out the previous engine shop put it together wrong and it was indeed leaking.
I had the magneto on my other engine also removed / overhauled / put back together correctly and it turns out it had just started to leak oil too.

Got it all back together, ran ok, mag drops ok so took it for a short flight.
Checked the bottom of the wings behind the engines, right one clean, left one covered with oil again! Bottom of the mag housing was dry. Oil must be blowing out the breather. Compression check was carried out and #2. I think it was had 0 compression. Cylinder came off and both compression rings were broken. It was a lycoming cylinder that had an eci cerminyl overhaul which takes a special piston ring. Turns out the engine shop that put it together used the wrong rings. I had all the other cylinders pulled $$$ and we found one more broken ring. Had all the rings replaced with the proper ones and it runs and flies better than ever. My wallet, lighter than ever...
 
Its not uncommon to see broken piston rings with worn-out cylinders. As the cylinders wear, the rings form wear steps at the top and bottom of the cylinders, and over time as the rings pass over these they become fatigued and break. You see it more often on chrome cyllinders since the rings are more brittle due to them being cast iron.
 
cylinders

Forgot to mention...Cylinders had only about 100 hours on them, they all looked great inside. Once piston had to be replaced (the one with two broken rings) but they said it was likely a result of the broken rings and not a cause.
Interesting to not is that the cylinders all had around 78 compression except for the one with two broken compression rings of course which had 0. The one cylinder that had one broken ring showed a compression check reading of around 73 before removal.
 
wear spots

Its not uncommon to see broken piston rings with worn-out cylinders. As the cylinders wear, the rings form wear steps at the top and bottom of the cylinders, and over time as the rings pass over these they become fatigued and break. You see it more often on chrome cyllinders since the rings are more brittle due to them being cast iron.

How many hours would it take to create ring wear groves deep enought to break the rings? One would hope that it would be more than the suggested overhaul time! Would greatly depend on the type of cylinder/barrrel surface too I would surmise.
 
dunno, couple hundred hours depending on the wall material. I have seen broken rings on an engine that had an induction leak which ran for quite sometime without being fixed (rental airplane.) In that condition the cylinder got really hot but how hot nobody knows since there were no CHT probes and engine instrumentation. When I pulled the jug the rings fell on the floor in about 5 pieces.
 
Don't the mag and engine shops have some responsibility since they assembled the mag and engine wrong???
 
I wish...

Don't the mag and engine shops have some responsibility since they assembled the mag and engine wrong???

I had a different engine shop do the repairs. I am seeking some restitution from the other engine shop but there is question as to wether it's still in business or not from what I hear. Original shop was recommended to me and has/had been in business a long time.
 
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