Engine rebuild question please.

[Mcstealth]

We are about to put on two new cylinders and pistons from Aircraft Specialties, and re-use two original cylinders and pistons. The A/P says that there is no need to hone the existing cylinders, just put them both back on and without new rings either. When I was racing my four cylinder motorcycles, I would never had considered reusing rings and not honing the cylinders. 

Please advise. 

PS.

Yes he has the big Lycoming rebuild book, for my reading please, but it makes my eyes cross with boredom.

[Pinecone]

I am like you, never reused rings, and touched up the honing if the piston was out.

But it seems to be fairly common in aviation to just reuse the rings in the same cylinder.

[jetdriven]

Lightly hone the cylinder and put new rings. Check the end gaps. Do the job right 

[Igor_U]

^^^ THIS ^^^^

Years ago my IA insisted on honing and and new rings for R&R cylinder on my IO360.

[kortopates]

I agree its best to hone and install rings but although it may seem trivial to, its actually too much of a challenge for many A&P's. Plus the OP didn't mention what kind of cylinders. Chrome barrels may not take a hone. But even with Steel, many would have to rely on taking it to the cylinder shop to get that work done. Besides properly cleaning the piston ring groves the rings will often need trimming to fit properly plus having a hone tool for the barrel. What the OP can do is offer to take it to nearby aircraft engine shop to get new rings installed and honed, or ship to someone that can turn it around quickly. Personally it’s speciality job that you don’t want an A&P doing anyway. Even if it takes a couple days it will be better and safer in the long run. Your breaking in to two cylinders anyway.
 

[jetdriven]

we have a medium grit flex hone that's perfect for these barrels. And often a new piston is just about as easy as cleaning up the old one if it's heavily carboned up. . You can do the job right or you can pass on it.

[N201MKTurbo]

If you just pull the cylinder and put it back on, it should work just like it did before you pulled it. The rings and cylinders don't know they have been apart.

That being said, whenever I put new rings in I just took the cylinders to the local engine shop and have them hone them. Half the time it is free, the other times it is like $50.

When I pulled the cylinders on my Cessna's 0-300, I cleaned the pistons according to the Continental service manual using bristle brushes, string and comet cleanser. It is kind of tedious, but it eventually gets them squeaky clean without damaging the metal. It specifically says in bold print to not abrasive blast them. Not even with walnut shells. I just rented a hone from the Auto Zone. It worked fine and they broke in just fine. And yes I did put new rings on.

[M20F]

13 hours ago, N201MKTurbo said:

If you just pull the cylinder and put it back on, it should work just like it did before you pulled it. The rings and cylinders don't know they have been apart.

 

Breaking in new cylinders involves seating the rings.  Once those rings are set and you pull the piston apart you are never going to get the rings set back in exactly the same space.  They are either going to need to wear back into their new position or if they and the barrel are worn too much in that alignment spot they will leak a bit (or a lot).  They also need to seat into the landing on the piston which is why most will say change the piston and rings together. 
 
Depending on things the old ring/pistons may work again, they may not, or they may work for awhile.  They definitely will know they were apart though.  
 
My advice to all is if you pull a cylinder and decompress the rings, new rings at minimum and preferably new rings/piston/hone. 

[N201MKTurbo]

2 hours ago, M20F said:

Breaking in new cylinders involves seating the rings.  Once those rings are set and you pull the piston apart you are never going to get the rings set back in exactly the same space.  They are either going to need to wear back into their new position or if they and the barrel are worn too much in that alignment spot they will leak a bit (or a lot).  They also need to seat into the landing on the piston which is why most will say change the piston and rings together. 
 
Depending on things the old ring/pistons may work again, they may not, or they may work for awhile.  They definitely will know they were apart though.  
 
My advice to all is if you pull a cylinder and decompress the rings, new rings at minimum and preferably new rings/piston/hone. 

That makes no sense at all and is not my experience. 
 

I have probably pulled 10 cylinders for different reasons, mostly oil leaks or to inspect inside the engine. I have put them back on and they had the same compression and the same oil consumption as before. Why wouldn’t they? Nothing has changed. The same ring metal is touching the same cylinder metal at the same place as before. The same ring metal is touching the same piston metal in the same place as before. Our rings don’t have anti rotation features and so they naturally rotate some, so rotation doesn’t  matter and besides they really don’t rotate when you remove a cylinder.

 

So, how many airplane cylinders have you personally put back on with the same rings that failed? And what exactly happened?

[M20F]

29 minutes ago, N201MKTurbo said:

So, how many airplane cylinders have you personally put back on with the same rings that failed? And what exactly happened?

None because I never done it, but I have certainly done it on a lot of cars. The cylinder doesn’t stay round when it gets hot.  The rings when new wear down and seat into the pistons and to the nature of the barrel.  You will never get them back in the same place. 
 
Folks are always free to do what they want but for me I wouldn’t reuse a set of rings.  Depending on the piston I might reuse it but for the most part if the cylinder is coming apart it is because the rings or piston have issues, if one ring is bad why reuse 2 others that are probably going to be equally bad or getting ready to be bad. 
 
My 2 cents.  

[N201MKTurbo]

One thing to keep in mind is the rings seat as much or more onto the piston as the cylinder walls. I don’t like putting new rings on a used piston. With the cylinder wall seal, the rings are expected to wear more than cylinder wall. With the piston, the piston will wear more than the ring. So the used piston lands fit the old rings. Granted, if you run it long enough, the ring land seal is mostly on deposits, not metal, but it takes hundreds of hours for that seal to form.

So if you are going to replace the rings, you should replace the piston too.

So let’s look a a fairly simple job like replacing a base o ring to cure an oil leak. 
 

.5 hour to remove the intake and exhaust risers.

.25 hour to remove the baffling.

.25 hours to remove the rockers and pushrod tubes.

.25 hours to loosen the nuts and pull the cylinder.

.25 hours to clean the cylinder base and crankcase and replace the o ring.

.5 hours to replace the cylinder and torque the nuts. The through bolts must be torqued in sequence, this requires a helper.

.5 hours to replace the pushrod tubes and rockers.

.5 hours to replace the risers and baffles.

 

3 hours and $20 worth of gaskets.

To replace the rings too:

.25 hours to remove the piston.

.5 hours to hone and clean the cylinder.

1 hour to check and adjust the ring gap (they are all correct these days), install rings on piston.

.25 hours to clean piston pin and install piston on rod.

So an additional 2 hours of labor and $500 in parts for little or no benefit.

 

Of course this is assuming the cylinder was healthy to start with. If it had low compression or was using oil, that is a different matter. And you might as well do a valve job too. 

 

[N201MKTurbo]

26 minutes ago, M20F said:

None because I never done it, but I have certainly done it on a lot of cars. The cylinder doesn’t stay round when it gets hot.  The rings when new wear down and seat into the pistons and to the nature of the barrel.  You will never get them back in the same place. 
 
Folks are always free to do what they want but for me I wouldn’t reuse a set of rings.  Depending on the piston I might reuse it but for the most part if the cylinder is coming apart it is because the rings or piston have issues, if one ring is bad why reuse 2 others that are probably going to be equally bad or getting ready to be bad. 
 
My 2 cents.  

I’ve never put a used spark plug back into an auto engine either, but I do all the time on airplanes, mostly because they cost 10 times as much and I can’t get new ones from the auto parts store down the block.

[EricJ]

22 minutes ago, M20F said:

None because I never done it, but I have certainly done it on a lot of cars. The cylinder doesn’t stay round when it gets hot.  The rings when new wear down and seat into the pistons and to the nature of the barrel.  You will never get them back in the same place. 

I think this is one of the many differences between a liquid-cooled block of cylinders and individual air-cooled finned cylinders.   They behave differently in a lot of ways.

[M20F]

9 minutes ago, EricJ said:

I think this is one of the many differences between a liquid-cooled block of cylinders and individual air-cooled finned cylinders.   They behave differently in a lot of ways.

Certainly the barrels do a lot more interesting things on an individual air cooled cylinder. 

[M20F]

9 minutes ago, N201MKTurbo said:

I’ve never put a used spark plug back into an auto engine either, but I do all the time on airplanes, mostly because they cost 10 times as much and I can’t get new ones from the auto parts store down the block.

What does that remotely have to do with replacing or reusing rings.

[N201MKTurbo]

1 hour ago, M20F said:

What does that remotely have to do with replacing or reusing rings.

My point was there are a lot of things you just do working on a car engine, because they are cheap and easy. Not quite so with airplanes.

[Pinecone]

On 9/23/2023 at 9:40 AM, M20F said:

None because I never done it, but I have certainly done it on a lot of cars. The cylinder doesn’t stay round when it gets hot.  The rings when new wear down and seat into the pistons and to the nature of the barrel.  You will never get them back in the same place. 
 
Folks are always free to do what they want but for me I wouldn’t reuse a set of rings.  Depending on the piston I might reuse it but for the most part if the cylinder is coming apart it is because the rings or piston have issues, if one ring is bad why reuse 2 others that are probably going to be equally bad or getting ready to be bad. 
 
My 2 cents.  

The rings rotate on the piston.  

[M20F]

13 minutes ago, Pinecone said:

The rings rotate on the piston.  

They rotate in a path based on wear patterns.  It is not a smooth surface after break in.  When you reinstall used rings both the rings, cylinder, and the pistons will make metal again to find their place.  They will continue to do some of that over the initial break in which is why at some point they all fail.  

[N201MKTurbo]

19 minutes ago, M20F said:

They rotate in a path based on wear patterns.  It is not a smooth surface after break in.  When you reinstall used rings both the rings, cylinder, and the pistons will make metal again to find their place.  They will continue to do some of that over the initial break in which is why at some point they all fail.  

I think some of what you are seeing on car engines has to do with the way pistons are typically removed. On a car, you remove the rod cap and drive the piston out the top of the cylinder. This pushes the rings past the wear and deposit ridge at the top of the cylinder. It seems like this would damage the rings, also, when you put them back in they go past it again, unless the cylinder has been bored and honed, in which case the rings would be the wrong size anyway.

The piston is pulled out the bottom of the aircraft cylinder, there is no ridge at the bottom of the cylinder, just smooth barrel.

[M20F]

5 hours ago, N201MKTurbo said:

The piston is pulled out the bottom of the aircraft cylinder, there is no ridge at the bottom of the cylinder, just smooth barrel.

I think we will just agree to disagree.  I am not a fan of reusing rings in any circumstances, I am 98% not a fan of reusing a piston, and I am 90% not a fan of doing a rehone.  
 
Your opinion clearly differs, you do you and I will do me. Fly safe. 

[MikeOH]

9 hours ago, N201MKTurbo said:

I think some of what you are seeing on car engines has to do with the way pistons are typically removed. On a car, you remove the rod cap and drive the piston out the top of the cylinder. This pushes the rings past the wear and deposit ridge at the top of the cylinder. It seems like this would damage the rings, also, when you put them back in they go past it again, unless the cylinder has been bored and honed, in which case the rings would be the wrong size anyway.

The piston is pulled out the bottom of the aircraft cylinder, there is no ridge at the bottom of the cylinder, just smooth barrel.

Agree; you can break the rings, or worse, the piston lands removing a piston in a car engine.  That's why you need one of these: https://www.summitracing.com/parts/lil-36500?seid=srese1&ppckw=pmax-tools&gclid=Cj0KCQjwvL-oBhCxARIsAHkOiu0K4un8fSv2kC8IY-1Ds4IcIFMTIlJYD1SXUTJv1OECAcuu3wFvEDQaAsm4EALw_wcB

No such issue that I can see with aircraft piston removal.

I still can't follow how, if rings rotate (which I am convinced they do), that they can take any kind of 'set' that would be lost if the old rings are put back?

[N201MKTurbo]

5 minutes ago, MikeOH said:

Agree; you can break the rings, or worse, the piston lands removing a piston in a car engine.  That's why you need one of these: https://www.summitracing.com/parts/lil-36500?seid=srese1&ppckw=pmax-tools&gclid=Cj0KCQjwvL-oBhCxARIsAHkOiu0K4un8fSv2kC8IY-1Ds4IcIFMTIlJYD1SXUTJv1OECAcuu3wFvEDQaAsm4EALw_wcB

No such issue that I can see with aircraft piston removal.

I still can't follow how, if rings rotate (which I am convinced they do), that they can take any kind of 'set' that would be lost if the old rings are put back?

Yes, a ridge reamer is a last chance thing to do to a tired engine you want to get a few more miles from. Any proper repair will bore the cylinder to a new size which requires new pistons and rings, of course. 
 

This has nothing to do with pulling a perfectly good aircraft cylinder for some non cylinder related problem and just reassembling it again.

[MikeOH]

12 minutes ago, N201MKTurbo said:

Yes, a ridge reamer is a last chance thing to do to a tired engine you want to get a few more miles from. Any proper repair will bore the cylinder to a new size which requires new pistons and rings, of course. 

It's not just a 'last chance' for CBs, my experience is that you can't remove the piston if there is a decent ridge!  You need the reamer to disassemble the engine!  I guess if you hit the rod hard enough you could probably break the rings/lands and eventually get the piston out...

[dzeleski]

10 hours ago, M20F said:

They rotate in a path based on wear patterns.  It is not a smooth surface after break in.  When you reinstall used rings both the rings, cylinder, and the pistons will make metal again to find their place.  They will continue to do some of that over the initial break in which is why at some point they all fail.  

Piston rings generally rotate unless we are talking about 2 smokes or the odd ball 4 stoke engine that uses piston ring pins. If the rings did not rotate you would get adverse wear patterns on the cylinder walls, and the ring lands would have a harder time staying clean. Remember that cross hatching on the cylinder is passing those rings at several thousand feet per second, on top of the fact that the piston is actually quite side loaded due to the off center nature of a crank shaft at the half way point. That is what causes the rotation. It’s generally agreed that rings rotate somewhere around a few single digit RPM. But it’s not an exact science, sometimes they spin a lot, sometime less, but they generally always spin at some amount.

Plenty of racers are bending rods and replacing the rod while using the same exact piston and rings. If the ring gap is still correct and the piston is healthy it’s completely fine.

There was a study done where a sensor was placed inline with the rings and two radioactive isotopes were placed in different spots on the rings. They found that generally rings do rotate around 2-8rpm. But sometimes they don’t spin as much or at all. Installation tolerances and honing matters, as well as operating loads.

Ive never put rings on a piston to find them in the same spot again after disassembly. Nor any engine builder I know.

Edit: https://www.jstor.org/stable/44548521?typeAccessWorkflow=login

[EricJ]

When I bought my airplane it only had about 40 hours since a complete IRAN at a local engine shop.   The oil consumption was inconsistent and could go way up at times and then settle for a while, go up again, etc.   It eventually settled down to consistent normal consumption.  I had an opportunity to ask a rep from the engine shop about the inconsistent oil consumption and he said it wasn't unusual during break-in when the rings are rotating the gaps can occassionally line up and cause increased consumption.

I just looked at the IRAN record and it indicates that they re-honed the cylinders and put new rings in.   One cyl got a new valve guide and lapped.   Other than that the same cylinders and pistons were reinstalled.   At the time they had about 1300 hours on them.   They have about 2100 now and they're still all operating fine.