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Oil leak from vacuum pump drive adapter

Lkitson

Well Known Member
When doing the oil change on my 8 last week as part of the condition inspection I noticed oil on the accessory housing near the right hand mag. It appears to be leaking from the lower right hand stud on the vacuum pump drive adapter . My engine has a cover plate installed there so I ordered a replacement gasket to install before I finish up the condition inspection.

Looking at the parts list below, in addition to the gasket (item 15) I see that there's an oil seal (item 55) for the gear shaft (item 1). Should I replace that seal in addition to the gasket? It seems to me that there shouldn't be much oil to leak past the gasket if the oil seal is doing it's job.
 

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remove cover you will see if seal is leaking
I had to repair this leak on 2 engines and it was not a seal
1) it was wrong suds used on one and oil was leaking through the treads in accessory case ( it must be interference fit studs)
2) gasket was bad
in both cases
I removed adapter because it is not used cut new gasket
and installed cover on accessory case
 
Should be no need for a good seal IF the gasket is holding. It is very possible that the gasket is good and oil is weeping past the threads on the stud. The hole for the studs are through drilled and exposed to splash oil. There is a slinger for the tach drive in that area, so A LOT of oil near those studs. Lycoming uses oversized threads on the stud to prevent this, however they can become loose. If it is the stud, remove it, clean the stud and hole and reinstall with red loctite.

Larry
 
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Should be no need for a good seal IF the gasket is holding. It is very possible that the gasket is good and oil is weeping past the threads on the stud. The hole for the studs are through drilled and exposed to splash oil. There is a slinger for the tach drive in that area, so A LOT of oil near those studs. Lycoming uses oversized threads on the stud to prevent this, however they can become loose. If it is the stud, remove it, clean the stud and hole and reinstall with red loctite.

Larry

Thanks Larry. How do I know if the stud is what's leaking? Is it just a matter of, if the gasket looks good the leak must be coming from the stud?

Lee
 
Should be no need for a good seal IF the gasket is holding. It is very possible that the gasket is good and oil is weeping past the threads on the stud. The hole for the studs are through drilled and exposed to splash oil. There is a slinger for the tach drive in that area, so A LOT of oil near those studs. Lycoming uses oversized threads on the stud to prevent this, however they can become loose. If it is the stud, remove it, clean the stud and hole and reinstall with red loctite.

Larry

Larry is exactly right - and I’d openly add that (especially in the experimental world), you can;t assume that the studs on your engine came from Lycoming. I have seen folks run to the hardware or auto parts store for these studs when they suddenly need one because of a conversion (or whatever), and those studs aren’t made the way the Lycoming studs are…buttons they are a LOT cheaper….
 
Just to add to what has already been said, which is all good info, from my experience the primary cause of oil leaks around the vacuum pump drive is a leak at the gasket. It seems to not be commonly known that there is actually a port at the vacuum pump drive that has full engine oil pressure. This is from the ancient times of using vacuum pumps that were actually lubricated by engine oil. Those went away decades ago but all engines are still capable of supporting them. That is the reason the gasket has so many small holes in it. It is to ensure that regardless of how the gasket would be installed it wouldn’t block the oil passage. My recommendation for anyone not using a vacuum pump or an alternator at this location is that they just remove the entire drive unit, and install a block off plate mounted against the face of the accessory case. If you wish to do this and don’t want to exchange the long studs for shorter ones you can simply make standoff spacer bushings that are the same thickness as The vacuum pump drive.
Most of the demo aircraft at vans have been flying for many years this way with no problem.
 
Larry is exactly right - and I’d openly add that (especially in the experimental world), you can;t assume that the studs on your engine came from Lycoming. I have seen folks run to the hardware or auto parts store for these studs when they suddenly need one because of a conversion (or whatever), and those studs aren’t made the way the Lycoming studs are…buttons they are a LOT cheaper….

My engine was built from a complete Superior Kit so the studs should be the right ones (I hope). I'll pull the cover plate and see if I can tell if the leak is coming from the stud.
 
Just to add to what has already been said, which is all good info, from my experience the primary cause of oil leaks around the vacuum pump drive is a leak at the gasket. It seems to not be commonly known that there is actually a port at the vacuum pump drive that has full engine oil pressure. This is from the ancient times of using vacuum pumps that were actually lubricated by engine oil. Those went away decades ago but all engines are still capable of supporting them. That is the reason the gasket has so many small holes in it. It is to ensure that regardless of how the gasket would be installed it wouldn’t block the oil passage. My recommendation for anyone not using a vacuum pump or an alternator at this location is that they just remove the entire drive unit, and install a block off plate mounted against the face of the accessory case. If you wish to do this and don’t want to exchange the long studs for shorter ones you can simply make standoff spacer bushings that are the same thickness as The vacuum pump drive.
Most of the demo aircraft at vans have been flying for many years this way with no problem.

Thanks. I didn't know about the pressure port but it makes sense. With the adapter removed won't there still be full oil pressure behind the cover plate coming from the accessory drive? Also when you say that the primary cause of leaks in that area are from the gasket are you referring to the one between the adapter and cover plate or the adapter and the accessory drive?
 
Thanks. I didn't know about the pressure port but it makes sense. With the adapter removed won't there still be full oil pressure behind the cover plate coming from the accessory drive? Also when you say that the primary cause of leaks in that area are from the gasket are you referring to the one between the adapter and cover plate or the adapter and the accessory drive?

Yes, there is still potential for leak because you are still sealing the pressure port with the gasket and the cover plate, but it is one potential leak source instead of three. By getting rid of the Vacuum Drive you no longer have a shaft seal that could leak And you only have one split point that is sealed by a gasket instead of two. As for a potential leak point, if it is the gasket leaking it could leak either at the accessory case or at the cover plate. I have seen it in both places. I think the benefit of getting rid of the governor drive other than not having to worry about the shaft seal is that you then have only one gasket under compression. It seems as though overtime the combination of the two gaskets and whatever amount of shrinkage they might have overtime seems to reduce the preload on the studs and I think that is what allows it to start to leak. If you have only one gasket then that is that much less likely to happen.
 
Follow up - I ended up replacing the cover plate gasket and after two ground runs and a short test flight I don't see any signs of leakage but I'll keep an eye on it.
 
Just to add to what has already been said, which is all good info, from my experience the primary cause of oil leaks around the vacuum pump drive is a leak at the gasket. It seems to not be commonly known that there is actually a port at the vacuum pump drive that has full engine oil pressure. This is from the ancient times of using vacuum pumps that were actually lubricated by engine oil. Those went away decades ago but all engines are still capable of supporting them. That is the reason the gasket has so many small holes in it. It is to ensure that regardless of how the gasket would be installed it wouldn’t block the oil passage. My recommendation for anyone not using a vacuum pump or an alternator at this location is that they just remove the entire drive unit, and install a block off plate mounted against the face of the accessory case. If you wish to do this and don’t want to exchange the long studs for shorter ones you can simply make standoff spacer bushings that are the same thickness as The vacuum pump drive.
Most of the demo aircraft at vans have been flying for many years this way with no problem.

Have you ever herd that removing the vacuum gear drive may cause oil to be slung to the breather tube causing
excessive oil loss? I herd this happened to someone operating a smaller lycoming engine.
 
Have you ever herd that removing the vacuum gear drive may cause oil to be slung to the breather tube causing
excessive oil loss? I herd this happened to someone operating a smaller lycoming engine.

I have never heard of that nor have I seen any indication of that on engines I have removed the drive from.
 
Have you ever herd that removing the vacuum gear drive may cause oil to be slung to the breather tube causing
excessive oil loss? I herd this happened to someone operating a smaller lycoming engine.

If you study the pics and diagrams, you will see that this is NOT the case. One could make the argument that fewer spinning parts creates less slinging of oil and therefore would possibly reduce it. Fact is there is a shroud around the breather exit to prevent slung oil from directly contacting the exit
 
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