Re-installing Lycoming cylinder

For those gurus who have done this before, how common is to see some oil leak at the base of the cylinder after reinstalling it.

I had to pull a cylinder for some work and installed it back with all new o-ring and other gaskets. Torqued the cylinder based on SI 1029D and did a two hour flight to break/set the rings. After the two hour flight, I see a bit of oil (maybe three drops worth) at the based on the cylinder.
I couldn't determine if the oil is coming from the cylinder, oil return lines or oil tube (this was #3 cylinder) and it is all possible that is some leftover for lubricating the bolts, etc.

It's flying... Wipe and watch. Base "O" rings are fairly reliable.

Check and re torque ASREQ

Howdy everyone. My guess is it is residual nut/bolt lube used during assembly.

I have seen one of the through bolt/studs leak from the center of the case. Depending on which cylinder it is, either the front two or rear 2 studs go all the way across to the opposit cylinder on the other side of the engine. Oil from one of the main bearing journals can follow the "bolt" to the cylinder base causing a leak.

It’s most likely coming from the through bolt when the torque was relaxed from it with the removal of the cylinder and or the re-torquing (stretching) of the through bolt breaks the sealing of the case half “O” rings and a little bit of oil will seep through the tolerance of the threads.
You can remove the offending nut spray it out as clean as possible with break cleaner with the little straw that comes with the can. Poke the straw in the bit of gap between the cylinder base and the leaking through bolt and let it “rip” and flush out the oil. Blowing out the residual break cleaner out with compressed air. Let it dry over night. Squirt a generous gob of high temperature RTV silicone around the now clean and dry through bolt. Place a way too large “O”ring over the through bolt and another one that will just fit around the inner one as a reinforcement. Place a finder washer on the through bolt ( you might have to make a custom one or stack several. Install the nut and tighten it enough to force the silicone into the void between the through bolt and the case, don’t tighten so tight that you blow out the “O” ring. Let the silicone set up overnight, remove the finder washers & “O”rings trim away all cured silicone off the cylinder base. Reinstall the proper washer and torque the base nut to it’s appropriate value.
I’ve used this process on many engines after a cylinder replacement with great success .

The most important thing is to make sure there is no silicone betwee the nut and the flange,or between the mating face of cylinder base and the case. Many cylinders have been blown off the engine because silicone was under the nut or the mating face, because the silicone lets the bolt cycle in it's loaded stretch, making it sooner or later break. This is a subject gone over at almost every IA renewal seminar I've been to.

Assuming you put on a new o ring and also oilled that o ring. If you put a generous amount of oil on the o ring at installation, it is not surprising that as it got hot / thinned it created a couple of drops.

If it continues to leak, I would assume that you either cut or nicked the oring during installation or nicked the chamfered edge of the case hole, which is what the o-ring is pressing against. I have done a few with no leaks in that area. Replacing a cylinder is the same as installing a cyl at build time and this is not a common leak area. Quite unlikely that retorquing will resolve it, as it is an oring sealed area and they don't require a lot of torque to work. I would hope you used a torque wrench on the hold downs, so quite unlikely they are under torqued.

Leaking from the through bolts only happens if there are no case orings retrofitted AND there is some wear in the bolt / case interface (should be very light press fit near the bearing web). However, I don't see how removing and re-torquing the nut on the end can cause it to START leaking. It is a LONG bolt and it is only stretching 3-4 thou over 8". The case may relax after nut removal and cause a few more thou of movement against the bolt. However, just don't see how that kind of movement can take you from leak free to 3 drips per hour.

Larry

BoydBirchler said:

The most important thing is to make sure there is no silicone betwee the nut and the flange,or between the mating face of cylinder base and the case. Many cylinders have been blown off the engine because silicone was under the nut or the mating face, because the silicone lets the bolt cycle in it's loaded stretch, making it sooner or later break. This is a subject gone over at almost every IA renewal seminar I've been to.

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+1

That is not a fix I would undertake. The is the MOST stressed area in the engine and I would want a perfectly flat and tight interface between the case and cyl flange. Any protrusion will prevent that solid interface and over time it will start rocking and loosening things. If the through bolt leaks, fix it the right way with a reamer and oversize through bolt.

Cylinder base leak

BoydBirchler said:

I have seen one of the through bolt/studs leak from the center of the case. Depending on which cylinder it is, either the front two or rear 2 studs go all the way across to the opposit cylinder on the other side of the engine. Oil from one of the main bearing journals can follow the "bolt" to the cylinder base causing a leak.

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Likely not the cause of the leak but make sure the torque on the through bolt (opposite cylinder) maintains its proper torque; ie torque the bolt on the opposite cylinder through bolt nut.

BoydBirchler said:

The most important thing is to make sure there is no silicone betwee the nut and the flange,or between the mating face of cylinder base and the case. Many cylinders have been blown off the engine because silicone was under the nut or the mating face, because the silicone lets the bolt cycle in it's loaded stretch, making it sooner or later break. This is a subject gone over at almost every IA renewal seminar I've been to.

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Since I am a beginner when it comes to this type of work, I would not even know how one would get silicone between the nut and flange but I am pretty sure I have not. We just lubricated the bolt head generously with oil and put things back together..

lr172 said:

Assuming you put on a new o ring and also oilled that o ring. If you put a generous amount of oil on the o ring at installation, it is not surprising that as it got hot / thinned it created a couple of drops.

If it continues to leak, I would assume that you either cut or nicked the oring during installation or nicked the chamfered edge of the case hole, which is what the o-ring is pressing against. I have done a few with no leaks in that area. Replacing a cylinder is the same as installing a cyl at build time and this is not a common leak area. Quite unlikely that retorquing will resolve it, as it is an oring sealed area and they don't require a lot of torque to work. I would hope you used a torque wrench on the hold downs, so quite unlikely they are under torqued.
Larry

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Larry, I agree with your thoughts here and your assumption about oiling the oring and the base is correct. If the oring has got nicked, I should know soon but one would think more oil would come out. I sure hope not.

Bavafa said:

We just lubricated the bolt head generously with oil and put things back together..

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Well, there you go. That oil will pool in the cavity between the bolt and flange hole ID. It will slowly leach out and show as a few drips. I would keep cleaning and observing before worrying much about it.

Larry

The studs that hold the cylinder on are not supposed to be wet torqued to my knowledge. You can use a wax, but not oil. I had a leak that I had a hard time finding and it turned out to be one of the small base studs that are screwed into the case. george

georgedouglas said:

The studs that hold the cylinder on are not supposed to be wet torqued to my knowledge. You can use a wax, but not oil. I had a leak that I had a hard time finding and it turned out to be one of the small base studs that are screwed into the case. george

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The service instruction that I used from Lycoming specifically recommends that it needs to be wet and it list the type of lubrication it can be used.

Bavafa said:

The service instruction that I used from Lycoming specifically recommends that it needs to be wet and it list the type of lubrication it can be used.

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I must have missed that on all the cylinders and field rebuilds I have done over the years. The wax I referred to was for the Catto prop bolts, not engine cylinder bolts. I just inspected and reinstalled my prop for my condition inspection and that accidently stuck in my mind. george