IO-520 cylinder issues

[Cubswede]

Hello, I am helping a friend that just bought a well used 185 over here in Sweden.
He said that two of the cylinders had bad compression. I had a look with the borescope, and found that the exhaust valves on these two cylinders has heat damage. Parts of the valvefaces is green, indicating exhaust gas leak and subsequent overheating. The engine has around 1100 hrs since factory reman done in 1995. New cylinders was installed. I wonder why the valves of these two cylinders has started to leak, and not all of them. I found something that could be a leading clue. The rocker arm thrust washers has eaten into the metal of the rocker bosses on the thrust side. But the wear step is not the same depth around the circumference. It is very deep on one side, and almost no wear on the other side. Could the hole for the rocker shaft be misaligned from the factory? If so, this could be the cause for the valve to have been slightly “sideloaded” and now after 1100 hours the valve guide is enough worn so that the valve does not seat properly? What do you think about my theory?
I did try to find any wear limits for the rocker shaft bosses but have not found anything yet. We are planning on honing and replace/reface the valves on all cylinders, but I need to find out if they are serviceable before starting any work on them. Any inputs is appreciated.
(I am a A&P, but with limited experience on the IO-520)

[Formandfunction]

Nobody resurfaces the rockers when they do a top and it wears the guide egg shaped. Then the valve gets hot from not seating well. It's all in the conti assembly notes.

[Formandfunction]

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

[Cubswede]

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

Ok, probably so but this engine has not been touched since factory reman. We removed the cylinders just now due to the findings. I had a quick look at the rocker faces, and they all seem ok with normal wear pattern. But can you see the difference in wear from the thrust washer? R/H side it has eaten in to the boss almost the entire thickness of the washer. On the L/H side it barely touches the boss material. For me, it seem something is not square from manufacture? I could be wrong though.

[Formandfunction]

Looks bad but if it has 1100 hours count yourself lucky and do a top overhaul. Definitely resurface or replace the rockers while you're at it.

[bubb2]

Change of subject but just wondering. What is the cost of aviation gasoline in Sweden? Do you have 100LL?

[Cubswede]

Change of subject but just wondering. What is the cost of aviation gasoline in Sweden? Do you have 100LL?

We have 100LL and two brands of 91/96 unleaded Avgas. 100LL is around $ 11,50 per gallon and 91/96 around 10,30. But the prices vary a lot depending on location. A couple of airports charged over 14 per gallon this summer!

[Cubswede]

Well, I did some additional measuring on the cylinders. To say the least, I am not impressed with the longevity of the material in this engine. At 1100 hours all cylinders is out of limit in diameter and/or out of round. The exhaust rocker shaft/rocker bushing play is out of limit. A deep wear step in the shafts. All valves are sloppy in the guides.
and look at the wear on this intake valve! It is hard to believe it was rebuilt only 1100 hours ago. I am quite new on big continentals, but is this kind of wear normal?


sorry for the tilted picture.. ��

[Grant]

I'm not certain on the larger Continentals but on the smaller ones the rockers themselves are physically the same but the exhaust has an oil hole designed to shoot oil onto the exhaust valve stem. If you reverse them during assembly the engine will run fine but the exhaust valves will not get the proper cooling. Since they are removed during installation it is possible they were installed incorrectly during the cylinder replacement.

[Steve Pierce]

I was going to say your exhaust guides are worn. Probably some corrosion in the bore of the guide. Been some material changes since 1995 if memory serves me correctly. 1100 hours in 27 years can lead to a little corrosion. They are known to wear the guides as well.

[stewartb]

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.

[OLDCROWE]

My experience is limited to a couple thousand hours on 520's as an Owner but I plan/budget/schedule for cylinders at 900-1000 hours. I was surprised at first but after to talking with a good friend who runs a fleet of them it's the norm.

[Charlie Longley]

When I did the Continental engine school a few years ago I met another attendee from South America. He ran a fleet of 206’s on missionary work. He told me they did a complete top end at mid-time and that got them to recommended TBO.

[bubb2]

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?

[frequent_flyer]

I plan/budget/schedule for cylinders at 900-1000 hours. I was surprised at first but after to talking with a good friend who runs a fleet of them it's the norm.

That's horrible! Three of the cylinders have been on my Lycoming O-360-A4A for over 3,000 hours. One came off at 2,800 hours and was replaced by a good used one.

[Steve Pierce]

That's horrible! Three of the cylinders have been on my Lycoming O-360-A4A for over 3,000 hours. One came off at 2,800 hours and was replaced by a good used one.

Lycoming O-320/360 vs Big Bore Continentals.

[OLDCROWE]

That's horrible! Three of the cylinders have been on my Lycoming O-360-A4A for over 3,000 hours. One came off at 2,800 hours and was replaced by a good used one.

totally different animal

[Formandfunction]

The 520's I've come across seem to average about 800 hours before a top overhaul and almost always make tbo with one top. Most of the time it's the valve guides becoming egg shaped and burning the valves. I have a theory that if a person replaced the rockers and shims every 500 hours the cylinders would last a lot longer. I've never came across a conti top overhaul that didn't have flat spots on the rockers but it's possible rocker geometry might just be the limiting factor also.

[bubb2]

Any Millennium cylinder experience out there for comparison to OEM? Sounds like they couldn't be worse.

[DENNY]

Name most any aircraft major part Crank, cam, Cylinders, and they all have issues over the years. Check the AD lists for the engine you are buying them for the one that has fixed a recent problem may be better than the one that has not found the problem yet. It usually takes years for a problem to work itself up to the top so everything is a crapshoot. DENNY

[stewartb]

My Millenniums are on the one run and done AD, but they work great and compression history is better than any Continental cylinders I’ve owned. I don’t know how Continentals are doing now that they’re essentially ECIs. That was a good acquisition for Continental. They weren’t very good at cylinders before.

[BC12D-4-85]

Hmmm..I ran three IO-520's in a C-185 to TBO last Century. No cylinder change outs. EGT single probe no less than -100F lean of peak at 23 squared. Averaged 14 GPH. Maybe current metal has gotten less durable? Single weight Shell oil the whole trip.

Gary

[OLDCROWE]

My Millenniums are on the one run and done AD, but they work great and compression history is better than any Continental cylinders I’ve owned. I don’t know how Continentals are doing now that they’re essentially ECIs. That was a good acquisition for Continental. They weren’t very good at cylinders before.

I'm hopeful, my factory re-man is still fresh.

[wireweinie]

Hmmm..I ran three IO-520's in a C-185 to TBO last Century. No cylinder change outs. EGT single probe no less than -100F lean of peak at 23 squared. Averaged 14 GPH. Maybe current metal has gotten less durable? Single weight Shell oil the whole trip.

Gary

But how much calendar time in each TBO?

Web

[BC12D-4-85]

But how much calendar time in each TBO?

Web

About 6 years each 1979-98. Never parked indoors. Always preheated in winter. Probably 3/4 on floats so got warmer than conventional gear. Lots of 55-65% operation if cylinder temps allowed, not sure what %. They ate alternators, vacuum pumps, and exhaust mufflers but no cylinders. Compression never approached lower limits. Just lucky.

Gary

[wireweinie]

My opinion here, but I think the harder an engine works the better they wear. Short calendar time between TBO seems to equal less repairs.

Web

[TcraftF21]

I had an IO-520 on my last Bonanza. Overhauled my me with Millenium cylinders, match flowed and ported and roller bearing rocker shafts. No problems and ware after 800 hours andI don’t think the current owner has had to touch a thing in the last ten years he has owned it.

[BC12D-4-85]

Good baffling and use of cowl flaps helps. Kept the cylinders warm and didn't over lean.

Gary

[Cubswede]

I was going to say your exhaust guides are worn. Probably some corrosion in the bore of the guide. Been some material changes since 1995 if memory serves me correctly. 1100 hours in 27 years can lead to a little corrosion. They are known to wear the guides as well.

Hello Steve. Thanks for the input. I removed all exhaust valves, and all of them had quite big play in the guides. Even the ones that show normal heat signature. From talking to older technicians in sweden with 520 experience tey say: that´s how they become after a while..
I am more familiar with Lycoming, and where this engine show cylinder wear (about everywhere) Lycomings does not even come close to wear this quick. Strange. I mean, they are built quite the same technically speaking.

[Cubswede]

Nobody resurfaces the rockers when they do a top and it wears the guide egg shaped. Then the valve gets hot from not seating well. It's all in the conti assembly notes.

Hello, Thanks for the input. I have looked through the overhaul manual and searched through SB/SI info on the subject and have onlu found SI 86-1-1 from ECI that adress the topic of valve rocker-to-valve tip alignment. The limit is 0.005"
That seems a LOT to me. Im not sure you can see the picture, it turned out pretty small from what I can see.

[Cubswede]

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

[Cubswede]

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?

[BC12D-4-85]

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.

Gary

[Cubswede]

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?

[Steve Pierce]

Looks even to me. The uneven ones I had chipped the face away.

[Formandfunction]

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.

[JohnnyR]

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.

[DENNY]

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.
DENNY

[BC12D-4-85]

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?

Gary

[JohnnyR]

What did the builder say about this?

Gary

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.