The International Cessna^® 170 Association

Hi Ho again from New Orleans,

The cylinder I removed two ago for low compression came back cracked/red tag from J+J Airparts (nice folks!). It was cracked along the fins at the intake port side and the exhaust valve guide was worn as well. I'm not sure of how much time the cylider had on it, but couldn't have been or 500hrs since it was off in 1998 by the previous owner. It's the Middle cylinder, left side (yes, I know we numbered them on the intake gasket post but I've slept since then).

So my questions are these;
1) What are some of the causes of cylinder head cracking
2) How do I prevent this in the future
3) Can I detrmine if the others are cracked without having to pull each one
4) Any other tidbits for ensuring you C-145 makes it to TBO

This is my third, and so far longest annual (6 weeks and counting). And to answer Blue Leaders statement in the intake gasket thread, yes it's getting to be an expensive annual.

Thanks in advance,
Lee

Hi Lee,

My experience is from a Franklin engine but they are both old engines with old parts.

I lost a cylinder between Edmonton and Whitecourt AB that way. From when the vibration started until the engine died completely was only a few minutes, not sure exactly how long but suffice it to say it was about 20 seconds shorter than I needed to make the nearest runway After I was towed out of the hayfield and I and the friendly neighborhood Canadian mechanic (eh?) removed the cylinder it came apart in three pieces and the intake manifold was cracked (ripped, really) to boot.

Who knows what causes these things? Metallurgical flaw when manufactured? Best guess is bolts loosening themselves (or incorrect torquing), backing out and vibration from the piston stressing the loosened cylinder case.

Finding new Franklin cylinders is analogous to searching for two dollar bills in general circulation. We just keep overhauling them and putting them back on year after year. Does that increase the risk of a cracked cylinder? Donno. My completely unexpert opinion.

Richard,
Thanks for sharing your expericance, too bad about the 20 seconds, but glad it turned out ok in the end. As a short time aircraft owner when there's a problem like this my reaction is "did I do something?". I have a egt/cht gage for the aft most cylinder and use it to monitor temps and I keep the oil temp in the green arc, so I wasn't sure if there is something I was missing.
Again, thanks for the input and safe landings.

Cheers,
Lee

Cylinder cracks can be caused by a multitude of things. The most common is probably age and re-use of cylinders. Things can only be heated up and cooled down so-many times before the elasticity of the materials give up the ghost.
The most common areas of cracks are in the exhaust -port areas, due to the temperature extremes and resultant erosion that take place there. Many times a cylinder shop may offer to weld-up these cracks in an effort to return them to service, and it works for awhile, but it's only a matter of time before the failure occurs again. Whenever you can afford it, I believe it's best to use new cylinder assy's for replacements.
Another consideration is the dissimilar materials at the conjunction of the steel barrel and aluminum head of these cylinders. The steel barrel has male threads upon which the female-threaded head is screwed onto. Aluminum and steel not only disagree with each other from an electrolytic-corrosion standpoint but also as a feature of dissimilar expansion/contraction rates. It's only a matter of time before a leak or a crack occurs at that joint. (I had a head completely blow off the cylinder during take-off/initial-climb in my C-206. On another airplane (Baron) during an inspection, we noted a cylinder which held 70/80 compression, but heard a slight hissing. When we painted the cylinder with soapy water, the cylinder head-to-barrel joint foamed up quite nicely! It was a good catch! I initially thought the hissing noise was simply the compressed air being used to feed the compression-test.... and so now you have a good method to inspect cylinders, don't you... Paint them with soapy water during compression tests.)
Ordinarily, the only real method to check them is removal and dye-penetrant checks. Sometimes those cracks are very difficult to detect except by that method.

It's widely-believed that cracking is encouraged by large power changes made of short periods of time, and by "super cooling" caused by high-speed, low-power descents. I'm not personally convinced of it being any more so than by the application of takeoff power at initial takeoff, however. But it never hurts to be gentle with your engine's power changes and attempt to keep operating temperature/pressure changes from making wild excursions unnecessarily. (And it's good technique at any rate.)

I agree with most of what GAHORN has to say with one additional item.
I have always been told / taught that the single biggest, PILOT induced cause of blowing a cylinder is detonation caused running TOO LEAN, and being unable to hear the "pinging" as you might in an older car. Newer cars have a knock sensor that controls timing. There is an extreme difference between fuel burning at a rate which the moving, (decending), cylinder and head are designed to cope with, and the EXPLOSIVE force caused by detonation.
Detonation keeps "flexing" the cylinder, specially at the head line. Flexing the metal accelerates the temperature rise and metal fatigue.

I installed an E.I. ub16 engine monitor which gave me precise cylinder temperature information. I've since adjusted and added baffles (Lycoming) to even out the cylinder temps. The only problem with this monitor is now you know too much information.

I have also added an engine ahalyzer, specifically to track temps.
My experience is that the "lean find" mode does not balance the temps as well as I can do by the old process of lean till it coughs, enrichen a bit, and then I can balance 1&2 to mostly match as well as 3&4 to mostly match, but at a higher temp. Oh, yeah, I have the 4 cyl.engine,

I found that if I let the analyzer do the lean find, and use that, then all 4 cylinders have a different temp. I FEEL better, when I can balance the front two cylinders together and simultaneously balance ther rear two cylinders. Even if a wee bit towards the rich side.
Fuel is still cheaper than a new engine.

Chris Christensen wrote:I agree with most of what GAHORN has to say with one additional item.
I have always been told / taught that the single biggest, PILOT induced cause of blowing a cylinder is detonation caused running TOO LEAN, and being unable to hear the "pinging" as you might in an older car.

I agree that detonation is a real problem, however I have also read that it is now almost unheard of in our O-300s running 100LL with its higher octane unless you really abuse the mixture control.

Is this really the case ot is it another aviation fairy tale?

Thanks

All,

Thanks for inputs and observations, just what a new aircraft owner is looking for. I did replace the cylinder with a new one, it went on yesterday and I'm hoping to break it in this weekend. I'm finding aircraft and college are the same in a few areas....all learning is good, but can be expensive.

Cheers,
Lee

There are usually two ways to damage engines by improper use of mixture: Valve erosion/burning .... and detonation (which more likely damages pistons and cylinders.)

It's extremely difficult to get our C-145/O-300 engines to damage valves with temperature due to the common fixed pitch props we ordinarily use and with the 100 octane avgas we usually find at airports. In order to seriously damage these engines in that fashion (valves, due to extreme leaning) it's necessary to have very high power settings, usually more than 75%, and that can only be accomplished with these props at/near sea level where high manifold pressures and high rpms can be achieved, (possibly in cruise or descent but less likely in climb unless the mixture enrichment function of the carb is overridden.) Once above 5,000 msl it's almost impossible to achieve such high power settings.

Detonation is another matter. Detonation can be caused by improper ignition timing and/or poor quality fuel and/or a combination of both and.....(Sorry guys, but it's true)..... it is much easier to accomplish conditions conducive to detonation with mogas than with avgas. (That's not a kneejerk slap at mogas users. It's intended to be a helpful suggestion that if you use mogas to be aware of it, so you can avoid the problem.)

In a properly set-up engine, running on Avgas, and normally aspirated, you will not have to contend with detonation--ever. Preignition is a wholly different matter. That can trash your engine in minutes if not seconds. Preignition is commonly caused by a cracked insulator on a spark plug or a helicoil tang sticking in the cylinder. Both then function as a glow plug.

You cannot run too lean, so long as it's not rough. (Well, you can run too lean. It's called idle cut-off.) The leaner you get, the cooler the cylinders become. Cooler is a good thing.

Run you cylinders below 380 degrees and they'll last a long time.

Shock cooling is a myth. If it's not, why isn't there such a thing as shock heating? Cylinders change temperature a lot faster on take-off than after a sudden power reduction. But George has it right--no sudden power changes is good practice because there are lots of moving parts that have to slow down or speed up.

Some believe running lean of peak is bad for valves or seats. Not true. The hottest is peak. Valves lose most of their heat while closed (about 75 % of the time they're closed), thru the valve seat. Running LOP does two good things--reduces cylinder temperature and lowers internal cylinder pressure. Both help extend cylinder life. But if you compare 50dF rich of peak with 20dF lean of peak, it's true that EGT's are hotter, but ICP's are lower, and the cylinder (typically) runs cooler. Again, cooler is a good thing.

It is hard to run LOP on carbureted engines because fuel flows to the cylinders are poorly balanced. Thus the poster is right--sometimes it's better to just do it the old fashioned way: lean until it shakes; then richen up a bit. Another way is lean until it shakes, then add a touch of carburetor heat. That often improves fuel distribution and smoothes it out.

Okay flamers, let 'er rip!

robert.p.bowen wrote: You cannot run too lean, so long as it's not rough. (Well, you can run too lean. It's called idle cut-off.) The leaner you get, the cooler the cylinders become. Cooler is a good thing.

Okay flamers, let 'er rip!

One question from me then, how was it that on a particular day I was slowly descending while coming out of a mountain range and did not think to richen my mixture. When I glanced over at my EGT it was pegged to the right, engine running smooth. I immediately richened the mixture and things began to cool back down. I do run MOGAS by the way, and a friend of mine with a PA-12 / Lycoming says he can not get high EGT's with 100 LL.

Corey,
That happened because you were rich of peak in cruise. As you let down, the mixture got leaner and the percent of power increased--the result was you were likely running your engine at the worst possible place: not lean enough to have cool cylinder temperatures and not rich enough to do the same thing.

The folks at GAMI and those who teach at the Advanced Pilot Seminars at GAMI/Ada, Ok say lean of peak is fine and so is rich of peak. But if ROP, it needs to be REALLY rich, not just 50dF or so rich.