CAM LOBE CORROSION AT PREBUY?! A DULL TALE OF 1200 HRS SAFE LYCOMING OPERATION

[wombat]

I had a shop find several airworthiness issues with repair estimates that I disagreed with.   In some cases it was their assessment that the condition was an issue at all (1) and in others it was their estimated cost to fix it. (2)   I paid them for their time so far including the inspection and took my plane back.   I then got a ferry permit and had a mobile A&P certify that it was safe for the flight to my preferred A&P.  I then flew it there and he completed work as necessary to resolve each discrepancy. 

 

1:  An example is that the plane had been flipped in a windstorm in the early 60's and the left wing skin had been damaged.   The wing was repaired and the logbook entry and 337 said something along the lines of "Left wing repaired in accordance with Cessna maintenance manual and acceptable practices according to part 43".      The annual inspection note said something like "Left wing repair documentation is insufficient." and they recommended pulling the skin off the wing so they could see exactly what was done and document that.  They estimated $10k for this.   My A&P's note in the logbook was "Found logbook entry documenting left wing repair in logbook and 337 dated (whatever the date was)" 

#2:  They found that the jackscrews for the elevator trim were bent and offered to buy me a new pair from Cessna at a total cost of $47k.  The plane was only worth about $35k to begin with.     I found new replacements from McFarlane for $1,500 for the pair.

[Pinecone]

22 hours ago, A64Pilot said:

I’m sure that would render the case unairworthy, but I know you were kidding.

If you we’re really serious an STC would be how it’s done, involving at least a powerplant DER

 

Yes, I was joking.

And yes, I understand it could be done with an STC.  

Hmm, I wonder what the market would be for a 1AMU STC to put in a port to inspect cam and lifters???

[A64Pilot]

I don’t think corrosion is the primary driver in Lycoming cam disease.

I base that on two unsubstantiated beliefs,

1. it hasn’t always been that way, by that I mean for decades cam failures were almost unheard of. But for the last twenty or so years it’s become a pretty common failure with Lycoming fielding two fixes, so it is real, it’s worse than back in the day because Lycoming fielded these fixes relatively recently, not 50 or more years ago.

2. Anyone will tell you that corrosion is a variable, some of the variables are location, Fl is much worse than Arizona, frequency of use, aircraft that fly several times a week are almost immune to internal engine corrosion. Aircraft stored in doors, especially in humidity controlled hangars are also well protected from corrosion compared to that Fl airplane tied down on the coast.

But cams fail just about as frequently regardless of location or storage environment, even flight school cams fail, and those I doubt have any corrosion at all.

One thing I have found out is that Lycoming used to source their cams from Crane Cam, but they went out of business 

about 20 years ago? I don’t know where they get them now, nor do I know when they switched

Note: Crane being their Cam supplier is from memory, I don’t have a Lycoming document stating that, so that’s not substantiated either.

I’m not saying that corrosion won’t make a cam fail, if it’s bad enough and on the higher loaded lobe then it certainly can, just that I don’t think corrosion is the primary driver is all.

[jetdriven]

It’s not the cam.  It’s the lifters. 

[MikeOH]

55 minutes ago, jetdriven said:

It’s not the cam.  It’s the lifters. 

Curious why you say that vs. the cam.

[jetdriven]

Because the lifters get little spots of rust on them and they form little pits and then pits turn into spalling and then spalling turns into cratering and then cratering  wears down the camshaft. The cam is a symptom. It’s not the cause. Continental engines we frequently pull the lifters out of them whenever we change the cylinder and sometimes just preemptively look for a pre-buy. They always have little micro pits and some are pretty torn up actually, and the cams are always good, and then you swap out new lifters and you pretty much save the engine. Where lycoming is more of a hopes and prayers kind of Strategy. 

[DCarlton]

When were DLC tappets first made available?  

[A64Pilot]

Lifters and cam wear into each other if for example you move the lifters around it’s likely the wear will get excessive and you lose the cam and lifters

It’s normally called cam wear eventhough it may well have been precipitated first by the lifter because coming from the automotive world on old school engines lifters are easy and cheap to replace, a cam is harder. Newer design engines usually run overhead cams and they usually have cam followers and not lifters, but there too people often refer to lifter tick, even when there are no lifters, so common usage is often incorrect 

For some reason you can’t run DLC on DLC, so either the cam or lifters can be coated but not both.

The cam / lifter interface is by far the most heavily loaded part of an engine, nothing else even comes close to those pressures, for that reason it has to be hardened steel on hardened steel, no kind of bearing material could survive.

Higher RPM, more aggressive cam profiles or bigger and heavier valves require stronger springs to prevent valve float, stronger springs increase the pressure on the cam and lifter often beyond what they can tolerate, that’s why decades ago the automotive world came out with roller lifters, a rolling surface can take more pressure.

But aircraft are low RPM motors, without radical cam profiles and therefore shouldn’t be too hard on cams and lifters, yet they do wear excessively, but didn’t used to. Something changed, but I can’t figure out what.

It could be fuel, 100LL is different than the old 100 but LL came out in the 70’s and the cam problem came later?

Crane went under in the 2000’s but the failures started long before then?

So what is it? I’m thinking maybe LL changed?

I think DLC lifters are relatively recent and even just a couple of years ago were tough to get?