The marriage between piston rings and cylinder wall

Q: I am an A&P, and have been in aviation since 1990. I once worked with another mechanic who told me that if you pull a piston out of a cylinder, you must replace the rings and hone the cylinder. He is the only tech I have heard say this — until I read your article What’s best? A flush or overhaul?

You stated: “If oil starvation is suspected, you may want to remove the #1 cylinder (leave the piston in the cylinder so you don’t have to hone the cylinder and install new rings) and remove the connecting rod from the crankshaft.” Can you explain this? Is there a Service Bulletin or other directive that explains this practice? Thanks for sharing your knowledge with us.

TOM THROSSEL, via email

A: Tom, let me start off by thanking you for your question. For the life of me, I can’t recall any Lycoming publication that specifically tells you to do what I stated about leaving the piston in the cylinder while doing an internal inspection in this particular situation. In this case it was for the suspicion of foreign material in the engine. However, as I think back in my career, I honestly believe this is one of those maintenance procedures that one learns from experience.

Let’s take a look at the logical side of the equation. I hope you noticed that the person making the inquiry did not mention how many hours were on the engine in which he found bits and pieces of paper towel. Therefore, I assumed the engine was beyond the normal break-in period of, say, 25 to 50 hours. Given that, I also assumed that if this was the case, then the piston rings would have been seated or bedded in and the oil consumption stabilized. If the piston were removed from the cylinder at this time, the result would be that we disturbed the marriage between the piston rings and the cylinder wall, which is a result of proper engine break-in and which we know has occurred when the engine oil consumption stabilizes. Once the piston is removed from the cylinder barrel, you can never reassemble it and have the same ring to barrel marriage.

The bad part about this is you could end up with that cylinder having high oil consumption following return to service because the rings are not seated and the chances of having them seat on an engine that has, say, 100 hours on it or more is unlikely.

The result, then, to fix the high oil consumption and possibly oil-fouled spark plugs on that cylinder requires the cylinder to be removed and honed and the installation of new piston rings. Then once again, it’s like starting out with a fresh engine for break-in and you go back to mineral base oil until the oil consumption stabilizes. As a matter of fact, Lycoming Service Instruction 1014M at the bottom of page 2 states “Mineral oil must be used following the replacement of one or more cylinders or until the oil consumption has stabilized.”

Now, I know someone will jump on this and say “but we didn’t replace a cylinder, we just honed it and installed new rings.” True statement, but the end result is the same as replacing the cylinder when it comes to seating the rings.

I guess what I’m asking here is for you to just take this old man’s word for it that it’s a heck of a lot less expensive — both in time and money — to do it right the first time.

Getting back to the main point, if you don’t remove the piston from the cylinder, then you don’t have to worry about disturbing the marriage I spoke of, and honing and installing new rings is not required. This has got to be a money-saving deal in both time and materials, regardless of how you look at it. However, this does not apply if you have other concerns regarding the overall condition of the cylinder, piston, or rings.

When confronted with a situation like this one Tom, you’ve got to use a little logic and common sense. This is the approach I took in this case and I’m confident the end results will be fine. I hope I’ve clarified the situation for you and you now understand where I was coming from.

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

 

 

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Comments

  1. James Finlayson says

    When and engine with chromed cylinders comes into a shop with oil leaks and the source has been identified as through bolts and cylinder base o-rings how should the cylinder removal and reinstallat6ion be handled? Also is there any wisdom in turning the oil scrapper ring upside down in chromed cylinder to help control oil consumption?

  2. Bob says

    When running plain old cast iron rings re-honing is not necessary when the rings are removed from the cylinder. They can be safely and assuredly put right back into service without any trouble assuming that they were properly seated to begin with. I’ve done this many times without any problems. However, when running moly or chrome rings, they usually can’t be re-used after they have been removed from the cylinder because the moly/chrome coating fractures when the ring expands as they are removed from the bore requiring replacement of the rings and thus honing of the bore to break the new rings in.

  3. Dennis Reiley says

    That was scary. Replacing rings and honing the cylinder is basic – and required – in every engine – aircraft, car, stationary – where the piston has been removed from the cylinder. I would even be concerned where the cylinder has been disconnected and not removed, as there will likely be a slight shift in position when the cylinder is reconnected. How many aircraft are flying where this procedure was not followed?

    • Pankaj says

      Hello all,
      I am working on Honing process for my dissertation work. I will be doing experimentation on Honing machine of Gehring make. But, I got struck when came across Honing pressure parameter. I have read the research paper named “The influence of of honing process parameters on surface quality, productivity, cutting angle and coefficients of friction”. I am little bit confused about the parameters that the author has taken in his study i.e. specific cutting pressure (Phd) in one paper and Ph and Pd in the other one. He stated that Phd is specific cutting pressure and Ph and Pd are specific cutting pressures during pre-honing and finish honing respectively. My confusion is Phd means which specific pressure i.e. pre-honing or finish honing that he has taken in his study.
      And one more doubt about pre-honing and rough honing, I think that pre-honing are rough honing are the same terms.
      Whether I am right or wrong?
      I am considering three factors for experimentation i.e. Rough honing pressure, Finish honing pressure, cutting speed but got struck at the honing pressure parameters. The machine on which I will be performing the experimentation is having pressure 1 (PR1) and pressure 2 (PR2) settings as a input parameter at each stage of honing i.e. rough honing as well as finish honing have PR1 and PR2 values, hence having total 4 pressures. The confusion is that what is the function of PR2 values.
      Whether Pressure 1 both the stages (i.e. PR1 of rough and finish stage) are doing cutting and Pressure 2 in both the stages are improving the surface finish by removing peaks like plateau honing??
      I will be using 2 Factorial design for DOE and Central Composite Design which is a Response Surface Methodology (RSM) for prediction of optimal process parameters condition.
      Anybody please help me out…!!!!

      • Ace says

        not sure about Phd\Pd\Ph or if pre-honing and rough-honing are interchangeable terms… I can tell you though, rough-honing is used purposely to remove material and get your bore close to the finished size… you see (this is practical use, not theory), after you bore a cylinder there are deep grooves, or machine marks, left in the metal… the cutting tool actually chips the material away and in the process leaves little gouges and areas of compressed metal, not to mention as the tool works it’s way down the bore it spins around leaving a “screw” effect… these valleys can be .005″ deep and will need to be cleaned before a finish can be cut into the wall… typically a rough hone will be used to remove .003″ to .004″ and a finish hone will be used for the last .001″… the finish hone is used to clean up the bore and smooth out the peaks and valleys… as far as pressure used, there is no specific number… it depends on the sample you’re working on (cast iron, carbon steel, nikasil, etc.) amongst other factors (heat cycles during honing process, cooling media, abrasive, speed, etc.)… best way to dial that in is to test surface roughness (Ra, typically)… there are several methods for this… do the research, it’s pretty easy to find… then dial in what gives the best finish for your application… you have to keep in mind, also, the purpose of honing a cylinder is more about allowing the engine’s oil something to cling or “bite” to in order to reduce the instances of metal-to-metal contact between the piston skirt and the cylinder wall than it is about seating rings… as an earlier poster said, you can get a set of cast-iron rings to seat in a work-polished bore… hope this helps

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