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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