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Old engine bearings

UrbanM

Active Member
Here is some quick background; Purchased flying RV with documented engine rebuild with performance mods. After two intake valve failures (One with the previous owner and one with me) I decided to have an IRAN done by the local shop. I have posted before about some of the questionable stuff like the regrind of the factory camshaft etc.
Something that I found interesting is what appears to be the use of some old questionable parts during the overhaul process. I've attached some photos of the main and connecting rod bearings. These are on a 400HR 0-320. I am somewhat new to owning this model of aircraft engine and wondering if this shows excessive wear for a low time engine.
Thanks!
Kirk
 

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.... I have posted before about some of the questionable stuff like the regrind of the factory camshaft etc.
....

Why is this part questionable? This will open debate about quality of recent factory new versus older versions, etc. That said, nothing wrong with a proper camshaft (or crank) regrind. Now, a bad job that doesn't leave proper surface hardness is a quality issue. I assume most (the vast majority?) will get DLC application these days. Just asking. Maybe I'm misunderstanding this part of your statement.
 
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WHile a couple shell have spots that have the tin layer gone, and that points to wear. Whether or not they are problems cannot be done visually. You need a bore gauge to determine the clearance and it either falls into spec or it doesn't. You also don't know if that wear was there at the overhaul or occurred post overhaul. You cannot assume that the old bearings were re-used just because you see wear at this tear down. That much bearing wear (referring only to those that have copper showing) on a low time engine points to too tight of a clearance between the bearing and the crank or an oil flow problem in that journal. Shinny tin is not indicative of meaningful wear. It just looks that way compared to the dull tin that got no contact.

I dont think anyone in aviation is actually re grinding camshafts (involves welding on new metal and regrinding to spec). They are simply taking off a couple thou as they surface finish them back to flat and the proper angle at the lobe tip. Usually the cams are case hardened to a thickness of 10 thou and would hope that a competent shop would be checking the surface hardness post grind to ensure thet didn't grind through the hard layer. This produces a cam that is pretty much good as new, as they have limits on lobe height post finishing. You would never notice a performance hit on these engines from a lobe that was 3 thou less than spec in height. It is also likely that the overhaul shops re-harden them, as they all have the ovens as all cranks must be hardened post grinding when undersizing.

Larry
 
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Why is this part questionable? This will open debate about quality of recent factory new versus older versions, etc. That said, nothing a proper camshaft (or crank) regrind. Now, a bad job that doesn't leave proper surface hardness is a quality issue. I assume most (the vast majority?) will get DLC application these days. Just asking. Maybe I'm misunderstanding this part of your statement.

DLC coating is just on the lifters, not the cam lobes, and I don't think lyc is putting these lifters on new engines, though I don't know. Given how hard they are to find, I doubt it. I put them on my 540 when I overhauled it.
 
DLC coating is just on the lifters, not the cam lobes, and I don't think lyc is putting these lifters on new engines, though I don't know. Given how hard they are to find, I doubt it. I put them on my 540 when I overhauled it.

Yep. Brain-farted that one.

He stated rebuild but I assumed 3rd party OH based on the "performance mods" comment; something I associate with 3rd parties. Thank God it's Friday.
 
Thanks for the responses. What I meant in part about "Questionable Parts" is twofold. The cam is listed in the log as a "Phase 3" a thorough internet search did not produce any insight. the shop doing the IRAN (40 years experience) could not come up with any answers either. Upon close comparison with a stock cam it looks like the P3 was reground to shorten the duration. As I understand cams this will improve low range torque. How does this help an aircraft application?
The other aspect of my comment was how many of the parts were close to or outside of Lycoming specs at 400 hrs. Many of the exhaust valves for example are at or past limits. Same with the cylinders. The engine was built as an experimental
with some new and some yellow tag parts. Here again, as my understanding goes a yellow is for a part that is within limits. The problem is that if the part is right at limits at the time of installation it could be outside or the limits in a few hundred hours. So the question and sort of comment was about the use of old parts. Up until the valve issue the engine didn't leak or burn oil and ran great.
I'm planning on a new top end, Cam, Cam followers, and bearings. This is not just because I don't trust some of the old stuff but because I want a full TBO out of the engine and not to have to worry out parts being out of spec a few hundred hours from now.
Thanks,
Kirk
 
Thanks for the responses. What I meant in part about "Questionable Parts" is twofold. The cam is listed in the log as a "Phase 3" a thorough internet search did not produce any insight. the shop doing the IRAN (40 years experience) could not come up with any answers either. Upon close comparison with a stock cam it looks like the P3 was reground to shorten the duration. As I understand cams this will improve low range torque. How does this help an aircraft application?
The other aspect of my comment was how many of the parts were close to or outside of Lycoming specs at 400 hrs. Many of the exhaust valves for example are at or past limits. Same with the cylinders. The engine was built as an experimental
with some new and some yellow tag parts. Here again, as my understanding goes a yellow is for a part that is within limits. The problem is that if the part is right at limits at the time of installation it could be outside or the limits in a few hundred hours. So the question and sort of comment was about the use of old parts. Up until the valve issue the engine didn't leak or burn oil and ran great.
I'm planning on a new top end, Cam, Cam followers, and bearings. This is not just because I don't trust some of the old stuff but because I want a full TBO out of the engine and not to have to worry out parts being out of spec a few hundred hours from now.
Thanks,
Kirk

Performance profile on a cam? Can help in a limited RPM range from what I recall. Torque usually required a bore or crank throw increase. Changing the profile would most certainly require re-nitriding. Probably not what happened here. If I had to guess, that cam had two re-grinds @ ~ 3 thou each. That said, it’s just another unknown.

Have it (the whole power plant) OH’d to new limits. You want to be able to trust this machine. My guess with your current approach -> you’ll be throwing good money after bad. Be safe, Sir.
 
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Old parts

Overhaul covers a wide range from at service limits to all new. A field overhaul to minimum limits with old worn parts may be within spec when done but out of spec 200 hours later. Documentation, or lack thereof, and inspection by a qualified mechanic will tell the story.

In my case we Sent the case to Divco and were able to use the crank and camshaft after inspection and regrind. Put in lots of new parts in to replace the worn items.
 
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The other aspect of my comment was how many of the parts were close to or outside of Lycoming specs at 400 hrs. Many of the exhaust valves for example are at or past limits. Same with the cylinders.

When I tore mine down and we were looking at crankcase fretting, scoring on the crank journals, lifter wear that looked very close to spalling, and polishing on the outer surface of some of the bearing shells that suggested they’d been moving, my engine builder said every time you dismantle them you can find a good reason to do it, even if they’re factory-new. But if they’re assembled properly they’re virtually bulletproof.

You can’t measure the internals once the engine is built, so the factory spec is only for pre-assembly, and it’s not outrageous for components in running engines to be outside what’d go into a new engine. That’s why we do compression checks, boroscope inspections, max static RPM checks, wobble tests, etc: When an engine is in-service, its measurable ability to function is more important than whether a part is a few ten thousandths of an inch out of true.

- mark
 
Returning to the photos...torque the case halves together and carefully measure the main bearing bores for roundness.
 
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