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.