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

[DXB]

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

 

Surface corrosion noted on a cam lobe after pulling a jug isn’t great, but in isolation it is a terrible reason to split a Lycoming case – it may need overhaul soon, or maybe not...  My illustrative anecdote: >10 years ago, a newb pilot/owner buys an M20C with a 600hr SMOH O-360 that had spent some time sitting and needs a little cylinder work at the outset.  After pulling a couple jugs, the well-known Mooney A&P/IA who did the prebuy finds surface corrosion on multiple internal components and even some pitting on cam lobes but normal lobe height and lifter faces. The A&P sends pics of the extensive pitting and says newb owner you must overhaul.  The collective “wisdom” of the internet also says that the newb pilot/owner/doctor must overhaul and asserts that he is just another vertical-tailed doctor killer-buying sucka. His demise in a smoking crater seems likely to all because he is arrogant enough not to accept A&P or internet guidance without asking questions.  The sucka pilot then takes deep dive into authoritative literature by Mike Busch and others, learning that (1) there is zero danger in running it until cam/lifter truly spalls and (2) it’s easy to monitor the situation through the filter so that once it happens, further damage to the engine can be prevented. Armed with real knowledge, the arrogant newb pilot/owner pushes back hard against the A&P/IA, getting him to sign it off under some duress.  Though this newb’s demise seems imminent to all, he flies regularly, uses Camguard, and keeps engine dry in winter by heating it uniformly and continuously. He also cuts and scrutinizes his filter nervously at every oil change, finding nothing for many hundreds of hours.  He gets a good engine monitor, learns how to use a borescope, and learns spark plug maintenance, allowing him to monitor the top end as well.  He does oil changes and oil analysis religiously but ultimately quits oil analysis after learning enough about it to conclude that it’s utterly useless for real world maintenance decisions on this engine.  Finally, after flying for another 1200 hours over 10 years and accumulating ample instrument time, he notices a subtle but consistent decrement in TAS on recent flights.  At the next oil change, he finds ample ferrous shavings in the filter (pic).  He had become lazy recently and was only cutting every other filter, and so he pulls the previous filter off the shelf and also finds similar metal, realizing that he flew about 80-90 hours with cam lobe(s) and lifter face(s) coming apart.  He flys it one last time to a major engine shop 3hrs away for overhaul, marveling at the aircraft’s fast and smooth performance during this serene flight and wondering if he is making the right decision. At overhaul, the suspected cam/lifter spalling is confirmed, and metal shavings are flushed from the oil cooler and governor.  The prop is IRAN’d/resealed, and the hub is found to be in fine condition, as is the crankshaft.  He’s finally back in the air and still (mostly) enjoying the continuous learning involved in the Mooney ownership journey….

[EricJ]

Recently on a FB page somebody posted a pic of a Lycoming cam with a little bit of corrosion on one of the flat faces.   Out of well over a hundred responses the majority were of the tone that 'you and everyone you love will die a horrible fiery death if you don't overhaul that engine immediately!'   I exaggerate...slightly.   Kudos to you for handling it sensibly.

Edit:   Lol...I see you posted in the same FB group.   Nicely done!   

[PT20J]

I’d have done what you did. No reason to tear it down if it’s not making metal. I would use x/c victory oil and be especially careful about cold starts if I had a cam that was beginning to show signs of distress, but I’d just watch it until I found metal in the filter. 

[jetdriven]

We had one last year. They had fantastic metal numbers, but we wanted to change a cylinder and my coworker ran the bore scope down the top of the cam and the very last cam lobe was about half the height.  The cratered lifter literally polished the lobe down over the course of probably a couple hundred hours. Interestingly, the previous annual, a MSC put a longer pushrod on the exhaust of #4 because they said the lash was way too great. That was a missed opportunity.  This engine had 1200 hours on it. 

[DXB]

On 1/19/2025 at 6:42 PM, jetdriven said:

We had one last year. They had fantastic metal numbers, but we wanted to change a cylinder and my coworker ran the bore scope down the top of the cam and the very last cam lobe was about half the height.  The cratered lifter literally polished the lobe down over the course of probably a couple hundred hours. Interestingly, the previous annual, a MSC put a longer pushrod on the exhaust of #4 because they said the lash was way too great. That was a missed opportunity.  This engine had 1200 hours on it. 

This type of borescope inspection of any cam lobes/lifters that aren't directly visible should be standard whenever a single jug needs to be pulled for any reason.  I suspect it rarely gets done though the necessary tools are widely available and relatively inexpensive.

[GeeBee]

If there is a crash from a corroded cam, I do not know of it. Generally the engine slowly stops producing rated power. 

 

 

[M20F]

I would negotiate it as a totally run out engine to buy and then IRAN.  There are a lot of great buys out there, why would I pay a premium for a corroding cam?   Did I miss the point here?

[DXB]

7 minutes ago, M20F said:

I would negotiate it as a totally run out engine to buy and then IRAN.  There are a lot of great buys out there, why would I pay a premium for a corroding cam?   Did I miss the point here?

I wouldn't split the case to IRAN.  Negotiating it as a high time or run out engine is reasonable, but the jug got pulled after the plane was bought and the discrepancies were being worked through. The filter/screen were clean at prebuy, Most sellers won't let you pull a jug to look at the bottom end on a prebuy (and neither would I).  

[M20F]

31 minutes ago, DXB said:

 Negotiating it as a high time or run out engine is reasonable, 

I don’t know the relevance of anything else you wrote. 

[DXB]

6 minutes ago, M20F said:

I don’t know the relevance of anything else you wrote. 

Can't negotiate it as such very easily if you don't have evidence of corrosion on the bottom end.  You can walk away if there's metal in the filter/screen, but you can't collect evidence of an issue unless you can pull a jug as part of prebuy.  Most sellers won't let you pull a jug.  These are relevant considerations during a prebuy.

[wombat]

Some engines you can look at the cams by pulling the valves.  Or draining the oil and sticking a borescope up into the pan from the bottom maybe?

 

While I am normally of the attitude of "If you are selling your plane it's almost not even yours already so as long as the potential buyer hires a licensed A&P to do the work, let them do whatever they want"... But pulling a cylinder....  Maybe have a $5k non-refundable portion of the deposit for that.  It's a little risky, even for a good A&P.  Not very, but a little....

 

[Fly Boomer]

13 hours ago, wombat said:

It's a little risky

2014-06-01 Cylinder Work - Be Afraid.pdf

 

[AJ88V]

There's no way I'd allow a cylinder to be pulled for a pre-buy.  Just move on.

Think I know the answer to this, but asking this because I read the Mike Busch columns in AOPA where shops do unscrupulous things....

QUESTION:  Let's say you're selling your plane and you've just completed annual with a shop you've used for years.  The potential buyer has a pre-buy done by a mechanic of their choice.  Can the A&P performing the pre-buy inspection ground the airplane if they find (claim) a discrepancy?

I imagine this could turn very ugly.

 

[wombat]

3 minutes ago, AJ88V said:

Can the A&P performing the pre-buy inspection ground the airplane if they find (claim) a discrepancy?

 

No they cannot.      Any more than some A&P wandering around the airport can just randomly 'ground' planes they see as they are walking down the line.

[DXB]

24 minutes ago, AJ88V said:

There's no way I'd allow a cylinder to be pulled for a pre-buy.  Just move on.

Think I know the answer to this, but asking this because I read the Mike Busch columns in AOPA where shops do unscrupulous things....

QUESTION:  Let's say you're selling your plane and you've just completed annual with a shop you've used for years.  The potential buyer has a pre-buy done by a mechanic of their choice.  Can the A&P performing the pre-buy inspection ground the airplane if they find (claim) a discrepancy?

I imagine this could turn very ugly.

 

Interesting question - I'm not sure.  In my case, I chose to turn the prebuy inspection into an annual (which is when the cylinder was pulled and the cam corrosion was discovered).  In that context he certainly did have ability to ground the plane.  As an aside, he mentioned several vintage Mooneys that had come to his shop for prebuy over the years and never left due to discovery of serious spar cap or steel cage corrosion.  He showed me an old C with corrosion of the cage that was languishing in the back of his hangar after the prospective buyer getting the prebuy walked and the owner was unwilling to pay the substantial repair cost. I wonder what  the dynamic there was. Also BTW despite my one substantial disagreement with him, my impression was that he and his crew run an excellent operation overall, and I would go back to him in a second if I were in that part of the country.  

 

[Fly Boomer]

1 hour ago, AJ88V said:

Can the A&P performing the pre-buy inspection ground the airplane if they find (claim) a discrepancy?

Instead of getting into a fight, one way to avoid the question is to not use the word "inspection" either spoken or written.  Call it a pre-purchase "examination" or some other word you are comfortable with.  And since the pre-purchase process is undefined, the buyer must specify everything they want examined, and what kind of report they want following the process, and who will have access to that report.  As much as we talk about this, you would think "pre-buy" would be well-defined -- but it isn't.

It's tempting to ask for an "inspection" because that has more meaning.  For example the process of an "annual inspection" is defined by the FAA (or at least the minimum).  But I wouldn't use the word "inspection" if I wanted an expert opinion regarding the condition of an airplane I was thinking of buying.  And the seller probably should be clear that no "inspection" is being done.

And of course, many buyers will arrange with the mechanic to convert the "pre-buy" into an annual inspection if the purchase is completed.

[EricJ]

1 hour ago, AJ88V said:

Can the A&P performing the pre-buy inspection ground the airplane if they find (claim) a discrepancy?

Just to add a little bit, a mechanic does not have the authority to ground an airplane.   A mechanic can't even determine whether an aircraft is airworthy or not, although a pilot can.   

It is, however, not unusual for a mechanic to point out something and state that it is unairworthy and shouldn't be flown, but it is still up to the pilot to ultimately determine airworthiness.    If that weren't true any mechanic about to miss a boat payment could just arbitrarily ground the next airplane that comes along and hold it hostage.  Obviously it's prudent for a pilot to take advice about an "unairworthy" item under serious consideration, but a mechanic's opinion is not authoritative regarding airworthiness.     There are some scenarios that are exceptions, like an IA doing an annual inspection, or if a mechanic finds a serious issue in the course of a repair and is unable (or unwilling) to complete the repair and return the aircraft to service.   

[AJ88V]

Thanks, guys.  Interesting article from Mike Busch:

https://www.aopa.org/news-and-media/all-news/2016/may/pilot/pe_savvy

It's still not entirely clear how one would handle a "pre-buy" on an airplane.  I imagine that if the A&P takes apart anything and puts it back together (e.g., removes the spinner to inspect the hub), then that needs to be recorded in the logbook and signed. (that way, if the spinner comes loose and flies off, there's a record in the logbook to show who worked on it.)  But then the article writes:

• But what if the inspection uncovers one or more discrepancies that make the aircraft unairworthy? And what if the owner is unwilling to have those discrepancies repaired? Can the inspecting mechanic simply refuse to sign the maintenance record entry and hold the aircraft hostage until the owner cries “uncle”?

  No, he can’t. FAR 43.11 provides guidance to the mechanic who finds himself in this situation. It requires the mechanic to “sign off the inspection with discrepancies” as follows: “I certify that this aircraft has been inspected in accordance with an annual inspection and a list of discrepancies and unairworthy items dated [date] has been provided for the aircraft owner or operator.”

  The mechanic then hands the owner a signed-and-dated list of airworthiness discrepancies found during the inspection. At that point, the annual inspection is complete and the inspecting mechanic’s job is done. His signature disapproving the aircraft for return to service attests (at least in theory) that every atom and molecule of the aircraft is airworthy except for the items on the discrepancy list he handed the owner.

My next question is, could you request that the pre-buy only provide a list of discrepancies separate from the logbook?  But then wouldn't the pre-buy A&P be writing things like "removed and installed spinner" in the logs?

[EricJ]

3 minutes ago, AJ88V said:

Thanks, guys.  Interesting article from Mike Busch:

https://www.aopa.org/news-and-media/all-news/2016/may/pilot/pe_savvy

It's still not entirely clear how one would handle a "pre-buy" on an airplane.  I imagine that if the A&P takes apart anything and puts it back together (e.g., removes the spinner to inspect the hub), then that needs to be recorded in the logbook and signed. (that way, if the spinner comes loose and flies off, there's a record in the logbook to show who worked on it.)  But then the article writes:

• But what if the inspection uncovers one or more discrepancies that make the aircraft unairworthy? And what if the owner is unwilling to have those discrepancies repaired? Can the inspecting mechanic simply refuse to sign the maintenance record entry and hold the aircraft hostage until the owner cries “uncle”?

  No, he can’t. FAR 43.11 provides guidance to the mechanic who finds himself in this situation. It requires the mechanic to “sign off the inspection with discrepancies” as follows: “I certify that this aircraft has been inspected in accordance with an annual inspection and a list of discrepancies and unairworthy items dated [date] has been provided for the aircraft owner or operator.”

  The mechanic then hands the owner a signed-and-dated list of airworthiness discrepancies found during the inspection. At that point, the annual inspection is complete and the inspecting mechanic’s job is done. His signature disapproving the aircraft for return to service attests (at least in theory) that every atom and molecule of the aircraft is airworthy except for the items on the discrepancy list he handed the owner.

My next question is, could you request that the pre-buy only provide a list of discrepancies separate from the logbook?  But then wouldn't the pre-buy A&P be writing things like "removed and installed spinner" in the logs?

As with most things like this there are a variety of opinions, but I can give my take on it.   FAR 43.9 requires "maintenance" to be recorded in the aircraft records, and the definition of "maintenance" in FAR 1 includes inspections.   So a 43.9 maintenance record entry for an "inspection" may just include a statement of items that were inspected on that date, and nothing more than that.   How much disassembly was performed is not really a required detail, and many people lean toward making maintenance entries as short and terse as possible.   If the "inspection" isn't a 100-hour or annual inspection done by an IA, or some other inspection required under Part 91, then there's no requirement for a discrepancy list or statement of airworthiness.  

Generally, 43.11 is only taken to apply records of 100-hour or annual inspections or other required inspections, although the wording says "any inspection performed in accordance with Part 91".   Pedants may take that to mean more than it is generally taken to mean, which is why some people advise that a pre-buy not be described as an inspection.   Maybe it's an "investigation" or a "look" or something, but "inspection" is fairly broadly defined elsewhere, so this is one of those areas that can become a conversational minefield.   In 43.11(a)4 where it says "insert type", the options for "type" are generally "100-hour" or "annual".   For the most part many pre-buy inspections are done without considering them maintenance or inspections and the only record is whatever report is given to whoever is paying for it.   YMMV, consult carefully with whoever is doing the work as well as whoever is managing the records for the aircraft.    The owner of the aircraft is responsible for maintaining the records, so their input in this area is important as well.

Even when an inspection is an official 100-hour or annual inspection with a list of discrepancies, there is no requirement that that list is or becomes part of the aircraft records.  The only requirement, per 43.11, is that the inspector "must give the owner or lessee a signed and dated list of those discrepancies."   The list can be, and often is, completely separate from the usual aircraft records.   It is then the owner's responsibility to manage compliance, per the discrepancy list, to have the aircraft returned to service.   Many IAs cover their own liability interests by keeping a copy of the list, and some insist on entering the list in the aircraft record with the 43.11 inspection statement.  If an owner has a preference on how the list is handled, that should be sorted out with the IA ahead of time.

Individual mechanics and IAs will have their own opinions and practices in these areas, so it's always good to ask questions first to manage expectations.

[PT20J]

I believe that there are FAA-required inspections and there are informal inspections. Inspections required by regulation (for part 91, annual and AD mandated inspections) are maintenance activities and require appropriate logbook entries. An informal inspection such as a pre-purchase inspection is merely a professional opinion of the condition of the aircraft. It is not maintenance and there is thus no requirement for a logbook entry. 

[DXB]

2 hours ago, EricJ said:

As with most things like this there are a variety of opinions, but I can give my take on it.   FAR 43.9 requires "maintenance" to be recorded in the aircraft records, and the definition of "maintenance" in FAR 1 includes inspections.   So a 43.9 maintenance record entry for an "inspection" may just include a statement of items that were inspected on that date, and nothing more than that.   How much disassembly was performed is not really a required detail, and many people lean toward making maintenance entries as short and terse as possible.   If the "inspection" isn't a 100-hour or annual inspection done by an IA, or some other inspection required under Part 91, then there's no requirement for a discrepancy list or statement of airworthiness.  

Generally, 43.11 is only taken to apply records of 100-hour or annual inspections or other required inspections, although the wording says "any inspection performed in accordance with Part 91".   Pedants may take that to mean more than it is generally taken to mean, which is why some people advise that a pre-buy not be described as an inspection.   Maybe it's an "investigation" or a "look" or something, but "inspection" is fairly broadly defined elsewhere, so this is one of those areas that can become a conversational minefield.   In 43.11(a)4 where it says "insert type", the options for "type" are generally "100-hour" or "annual".   For the most part many pre-buy inspections are done without considering them maintenance or inspections and the only record is whatever report is given to whoever is paying for it.   YMMV, consult carefully with whoever is doing the work as well as whoever is managing the records for the aircraft.    The owner of the aircraft is responsible for maintaining the records, so their input in this area is important as well.

Even when an inspection is an official 100-hour or annual inspection with a list of discrepancies, there is no requirement that that list is or becomes part of the aircraft records.  The only requirement, per 43.11, is that the inspector "must give the owner or lessee a signed and dated list of those discrepancies."   The list can be, and often is, completely separate from the usual aircraft records.   It is then the owner's responsibility to manage compliance, per the discrepancy list, to have the aircraft returned to service.   Many IAs cover their own liability interests by keeping a copy of the list, and some insist on entering the list in the aircraft record with the 43.11 inspection statement.  If an owner has a preference on how the list is handled, that should be sorted out with the IA ahead of time.

Individual mechanics and IAs will have their own opinions and practices in these areas, so it's always good to ask questions first to manage expectations.

Outstanding, detailed guidance, particularly for any owner taking a plane to a prebuy.  Speaking about this stuff in advance to the A&P involved seems critical to prevent a lot of potential pain.    

[201Steve]

Sounds to me like someone needs to build a custom boroscope and write the instructions on how to inspect a Lycoming cam from accessing thru an oil port, dip stick hole, oil drain whatever. It can’t be that hard. Would answer a lot of questions for a lot of people. 
 

has anyone gone done this road before? Seems like it would be easiest to map out with engine on the bench about to be disassembled to view and think about the angles and routing. 

[jetdriven]

Well, you can’t get to it from the dipstick hole in the case because you have to make 160° turn, go thru the slots where the case halves come together and get past the crankshaft and then you have eight lifters and six cam lobes to view. You can’t really go up through the drain port where the oil comes out because of the same issue, you have these quarter inch wide slots that are on the center line of the case halves that you have to go through and then go around the crank and then bend back towards the middle of the case to see the cam lobes. You can however, pull a cylinder off and you can see all of them pretty easily, but that’s the only way I know of. 

[PT20J]

Here’s a picture of what Byron’s talking about (view from the bottom with the oil pan removed. Dipstick hole is in the upper right corner).

[201Steve]

Gotta go dipstick tube.
Put a group of engineering students together, with all of their materials and tools, offer a grand prize of a couple thousand bucks, making a tiny camera do a 180 and telescope a little further up wouldn’t be asking the impossible. I’ll throw down some prize cash.