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IO-520 cylinder issues

Early-mid ‘90s Continental cylinders were well known to have problems, but most of that was the honing and early loss of compression. All Continental big bores have a reputation for poor valve guide life. Some of that from poor rocker geometry. The odd wear at the rocker support is a new one for me. That warrants inspection by a good cylinder shop. I’d appreciate hearing what they say.

Thanks for the input Stewart. As you mentioned with the honing, in theese cylinders you can easily see the wear with the bare eye. The only honing pattern left is at the very bottom where no rings go.. The rest is all mirror finish with clearly visible wear steps at ring reverse areas.
We will install new Cylinders, so maybe these will be machined and used as crankcase hold down plates for maintenance or something :)
Was this engine run with a digital six-cylinder EGT/CHT monitor system of some kind or the typical single probe EGT/CHT with an almost impossible to read analog gauge? You think at $11.50 a gallon someone might get a little aggressive at leaning, especially with poor engine data?

It was nun with normal Cessna gauges. I believe it has been operating as per the POH, where proper fuel flow is shown in relation to RPM/Manifold pressure to get wanted percent power. I dont think it has been "over leaned", and if operated below 65% I believe you can lean until it stops without harming the engine. But maybe continentals are more sensitive to leaning than Lycomings? At least for the barrel wear I guess that leaning would be beneficial for longevity due to less pressure from the rings against the barrel? Just my thought.. maybe not correct?
This is where an oil analysis program, if established early and routinely maintained, might have detected abnormal levels of wear metals and silicon (ingested dirt). As far as leaning...a quick look at the exhaust outlet for color of debris can promote further exam for each cylinder if concerned.

Sorry,resurface the rockers where they make contact with the valve. That's the sideload.

From the look of the pictures, is this wear considered to be severe enough to have an effect on the geometry? Enough to have the rockers resurfaced?




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They also have a tendency to top load when the valve opens. Install them and with the valve open/closed verify they're contacting the center of the stem.
We just rebored and sleeved all valve guides for all six cylinders in a purportedly “perfect” engine from a very well known engine builder after compressions began to plummet at 250 hrs.
All were off in both X and Y axis, causing accelerated wear and rapidly worsening compressions. The guides were likely shipped from manufacturer out of spec and it’s probable that the engine shop never checked before they built with the second-run cylinders. A $50K engine all of a sudden became a $70K engine with 3 months of commercial downtime because folks got complacent/sloppy.
I had my cylinders for the new cub build sent to PAP Performance Aircraft for inspection and porting. Seth said it was not uncommon to find misaligned valve guides/seats in factory new cylinders. I replaced all the Exhaust guides on my factory Lycoming engine at 500 hours I think the issue was high CHT'S fixed with proper baffling.
We just rebored and sleeved all valve guides for all six cylinders in a purportedly “perfect” engine from a very well known engine builder after compressions began to plummet at 250 hrs....

What did the builder say about this?

What did the builder say about this?


Blame game until faced with valve guide measurements done by another highly reputed builder, plus all engine operation data (there’s a vote for having an engine monitor with logging!). Some $$ returned, but still a major loss on our end since the plane was down for essentially the whole Alaska summer season.

The takeaway is that an engine builder should accurately assess whether the valve guides are straight prior to assembly. Don’t assume or bypass this step. I’m wondering how many Continentals have had premature top work because of this issue that leads to rapidly accelerating wear.