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To Hone or Not To Hone?

DanH

DanH

Legacy Member
Mentor
In another thread, a respected individual commented "Normal practice, anytime the piston comes out if the cylinder, and especially when new rings are used, is to hone the cylinder as well."

Honing when installing new rings is obvious enough.

However, consider pulling a jug for any sort of inspection, finding nothing out of the ordinary, and re-installing it over the same rings and piston.

Why would it be necessary to hone? Please be specific.
 
Recently I had a conversation with Continental Motors Inc. specifically on this.

They had a discussion about it with the cyl shop specialists and said that if the rings are already well seated and there is no problems with the rings or cyl, there is a better chance of it maintain a good ring seal by re-using the existing rings than to re-hone and start again with new rings.

I would be very interested in hearing a full explanation of how they arrived at this conclusion and any data they have to support it. It is sort of a surprise to me, but I am sure they know a lot more than me about the results of each scenario.
 
DanH said:
In another thread, a respected individual commented "Normal practice, anytime the piston comes out if the cylinder, and especially when new rings are used, is to hone the cylinder as well."

Honing when installing new rings is obvious enough.

However, consider pulling a jug for any sort of inspection, finding nothing out of the ordinary, and re-installing it over the same rings and piston.

Why would it be necessary to hone? Please be specific.
Click to expand...

I can not think of a single reason.
 
Honing

I've always wondered why the 'accepted' practice was that if the rings came out of the cylinder that they must be replaced and a hone done.I know its not a Lycosaurus,but in Rotax engine school taught by Eric Tucker we were taught to disassemble the rings from the piston,clean out carbon,measure ring side clearance and end gap,and if within the much tighter specs than Lycosaurus,reinstall in an unhoned cylinder.I've done it many times in various engines and it has worked well.Why not in a Lycoming?
Randy Tubbs A+P
RV8 180hp
 
Hold over

Thats a holdover from automotive days of yore. Cost effective preventative mx. Why? 'Cause pistons come out the top and any ridge broke a ring usually, iron rings were (and still are for those motors) cheap, honing was done with a drill in the garage, along with a said ridge ream.

In general tho, I also dont see how it applies for a Lyco or any jug style motor (ie HD, VW, Corvair etc) without due cause.

Dont fix what aint broke.

That said, ever seen a cylinder bore without thrust line scratches?
 
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I see it as an inspect and repair as needed (IRAN). It will depend on time, wear and type of use. Each case is unique.

If one or more ring has broken, I think it makes sense to hone prior to installing new. Having read the other thread, measurements should be made to ensure the cylinders are within limits, along with other wear parts.
 
Honing

I have removed and reinstalled cylinders many times without replacing rings or honing if all other parameters were good. In the case of a cylinder with 1000 or more hrs it always got honing, new rings, a valve & guide inspection and a new exhaust valve if the customer was willing.

Don Broussard A&P, IA

RV-9 Rebuild in Progress

57 Pacer
 
What a terrific discussion topic!

My experience has always been to follow that "if the rings leave the cylinder bore, replace them and hone the cylinder" rule. This has been done, not because of any good science, but more because of folklore.

What I wonder is, if the piston has been removed from the cylinder bore, how does one ensure that each little groove that has been wear-machined between the rings and cylinder walls is re-aligned when the piston is re-inserted into the cylinder? I know that when I've removed pistons the rings have always had a chance to shift around a bit, so even if the conn rod / wrist pin holds the piston in the same rotational plane, the same can't necessarily be said for the rings. Is this at all an important consideration (beyond the obvious concern about ring gaps not aligning)?
 
Just wondering outloud.
Just because a ring set is well-seated, does that mean that if it is turned some on the piston before reassembly, it will still be well-seated?

Is it important, if you are going to keep the old rings and not hone, to be sure to maintain the clock-position of the rings on the piston, so they go back in close to the same orientation as where they came out?

Or, are the wear surfaces of the rings happy enough to mate to new areas of the cylinder?

Ha! Canadian_JOY beat me to it with the same question.
 
Rings rotate in service. There is no set clock position.

The exception is many two-stroke rings, which are pinned to prevent rotation so the ends won't hang up in the port openings.
 
Ummm... why didn't I think of that?!?!?! Yes, of course rings rotate in service. Sometimes they rotate to the point where the ring gaps align, and compression takes a dive. Geez, I should have thought that question through a little better...
 
Canadian_JOY said:
What a terrific discussion topic!

My experience has always been to follow that "if the rings leave the cylinder bore, replace them and hone the cylinder" rule. This has been done, not because of any good science, but more because of folklore.

What I wonder is, if the piston has been removed from the cylinder bore, how does one ensure that each little groove that has been wear-machined between the rings and cylinder walls is re-aligned when the piston is re-inserted into the cylinder? I know that when I've removed pistons the rings have always had a chance to shift around a bit, so even if the conn rod / wrist pin holds the piston in the same rotational plane, the same can't necessarily be said for the rings. Is this at all an important consideration (beyond the obvious concern about ring gaps not aligning)?
Click to expand...

Rings/walls don't mate that way; they don't develop a complex mating arrangement. Final honing leaves thousands of ridges/valleys. The scraping of the rings against the walls during the initial seating wears down the sharp peaks to flat plateaus on top of the peaks. This leaves a smooth surface for the ring to smoothly slide up and down and a portion of valleys remain to hold oil. Without these valleys, you burn oil. Chrome rings don't wear, though they will polish off the scratches that come from the initial break in. Also, the rings often rotate around the piston over time. The ring/wall interface is set by the boring/honing process and the break in is wearing away WAY less then .001"

Larry
 
Dan,

I too couldn't understand why so many in aviation recommend a re-ring any time a jug came off. I thought an argument could be made that the rings would get scratches when dragged over the lower part of the cylinder during removal, but that wouldn't cause oil burning or other problems.

I asked Mahlon for his opinion and he told me there was no reason to re-ring due to pulling a jug and he had never experienced a problem re-installing without a re-ring/hone.

Larry
 
Canadian_JOY said:
Sometimes they rotate to the point where the ring gaps align, and compression takes a dive.
Click to expand...

This is also an old wives tale. The air under pressure has absolutely no problem taking a turn in order to find the next ring gap. There is plenty of space between the two rings. If this were not true, an air compressor with 90* fittings wouldn't work. Further, you lower ring gap sets your leak down/compression not the top, assuming a good ring seal and not counting the leakage past the ring/land interface. It doesn't matter what gets past the top ring. The only air that gets through as leakage is through the second ring, ignoring poor ring seating and ring/land leakage. I say this in general terms. There is, in fact, a slow down when the air has to traverse a longer path to the second ring gap and at high RPMs will make a difference in helping to hold the initial compression and therefore worth a couple ponies at high RPM. However, for a leakdown test it makes no difference.

Larry
 
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In my previous employment I was the manufacturing engineer for piston ring manufacturing for turbine jets. (Rolls Royce, Pratt, GE, etc.) Not a piston per se, but the theory is the same.

Rings are actually made out of round in the free state. This allows for tension in the ring while in the mating part. This demands a lot of solutions on how to machine/form the ring round, yet have tension. In our case we machined most of our rings from exotic material oversize via a lathe. We then cut the gap. From there the rings were put into a master cylinder and inspected for roundness via light, looking for light between the wall of the master and ring. It would then be adjusted via manual method to achieve a % of light passage. (20% light acceptable for example)

As the ring is used the "high" spots will wear down or the break-in period. The teeth from the hone help this process.

Our aviation rings are mostly likely rolled from strip or cast and then follow on machining process.


Hopefully this gives some insight in how/why rings work/made.
 
ty1295 said:
In my previous employment I was the manufacturing engineer for piston ring manufacturing for turbine jets. (Rolls Royce, Pratt, GE, etc.) Not a piston per se, but the theory is the same.

Rings are actually made out of round in the free state. This allows for tension in the ring while in the mating part. This demands a lot of solutions on how to machine/form the ring round, yet have tension. In our case we machined most of our rings from exotic material oversize via a lathe. We then cut the gap. From there the rings were put into a master cylinder and inspected for roundness via light, looking for light between the wall of the master and ring. It would then be adjusted via manual method to achieve a % of light passage. (20% light acceptable for example)

As the ring is used the "high" spots will wear down or the break-in period. The teeth from the hone help this process.

Our aviation rings are mostly likely rolled from strip or cast and then follow on machining process.


Hopefully this gives some insight in how/why rings work/made.
Click to expand...

What you describe was my understanding from the auto world, using cast and other steel rings. Does this apply to the lycomings that use Chrome plated rings? It was my understanding that the chrome is much harder and won't wear to fit near as much as cast rings.

Larry
 
lr172 said:
What you describe was my understanding from the auto world, using cast and other steel rings. Does this apply to the lycomings that use Chrome plated rings? It was my understanding that the chrome is much harder and won't wear to fit near as much as cast rings.

Larry
Click to expand...

We didn't do really any chrome plated rings. Ours was high temp alloys (Inconel, hanes, etc.) From my personal extensive automotive experience I agree the chrome is much harder, and typically much more difficult to perform the proper break-in. I never used them on any thing I built, including a top fuel bike I was involved in.

One possible way would be to chrome plate and then do a grind operation after to get roundness, or to send the ring out round for plating and hope it comes back round.
 
lr172 said:
What you describe was my understanding from the auto world, using cast and other steel rings. Does this apply to the lycomings that use Chrome plated rings? It was my understanding that the chrome is much harder and won't wear to fit near as much as cast rings.

Larry
Click to expand...

High volume makes machining processes not so cost effective. They are investment castings then plated with chrome or plasma faced. They do not "break in" well as they are hard and not for wear in. High volume, precision rings like heavy duty, are potted/lapped at the end of processing. I don't know the exact process, but maybe maybe something like a precision CBN cylinder. Just to get the plasma or chrome even. Then they are packaged in "cylinders" that slide over a tapered mandrel and pushed off the end onto the precise position for the piston being assembled. You will have to use your geometric imagination to visualize.

There is some wear in, but if they are not already round, and the bore is round, there will be some additional blow by for uppers, and oil control issues for lower.

I just happened to learn this in solving a weird production oil control problem for a medium truck engine. In looking at the light reflection on the face of the oil rings, the rails showed gaps in concert with there the slots were cut. The gaps were maybe in the 10 micron level from touching the walls, but definite increase in oil use even in a 50 hour production test. No pressure to (like top and scraper rings) to encourage conforming to the bore.

There is some break in between the ring and wall. My sense of this is a fine polishing of asperities and conformance, not any real wear. i.e. Down to the plateau of the hone. Probably the most critical issue might be slightly higher friction and heat generation at the ring, but little actual dimension changes. A reason for full rich to keep piston cool too.

Processes change, but this might give some insight to processing of production rings. At least at a point in time.
 
DanH said:
Rings rotate in service. There is no set clock position.

The exception is many two-stroke rings, which are pinned to prevent rotation so the ends won't hang up in the port openings.
Click to expand...

I've often heard that rings are supposed to rotate, but do they really? What is the source of torque that makes them rotate? I always figured as soon as the slightest bit of carbon build-up occurs in the ring groove, it probably pins the ring in one spot.
 
My engine builder told me that rings can rotate, but also cylinders and rings get out of round with wear. So rings will wear out of round with the cylinder and then more or less stay in position. So removing them disrupts this alignment, which affects the fit of the ring and blow by, oil consumptions etc

You hear all this stuff from people who work on engines all the time, but how do they really know? Perhaps the certified shops have contact with tech reps from the mfg and this information gets passed on informally? I don't know. It is really hard to sort out what is real data and what is musings from the coffee shop gurus.

Having said that, honing, if you have the tool, takes about 30 seconds, so why wouldn't you do it?
 
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scsmith said:
I've often heard that rings are supposed to rotate, but do they really? What is the source of torque that makes them rotate? I always figured as soon as the slightest bit of carbon build-up occurs in the ring groove, it probably pins the ring in one spot.
Click to expand...

I have no idea why they rotate, but there is probably an answer somewhere if we dug enough.

Here's nice little paper that turned up in quick spin on Google. Rotation rate and direction varied with speed and load.

https://www.kns.org/jknsfile/v31/A04803285773.pdf
 
If rings didn't rotate then upon disassembly they should not be found in the random order of end gaps that are always seen.Rings that don't rotate, or move in the grooves are a problem as they don't maintain a good seal with the cylinder wall.Line the end gaps up where you want;in a properly operating engine they will always be moving and be somewhere else.I've always found this in a disassembled engine.The gaps lined up and compression is low? Don't worry, they will be not lined up shortly after startup.Zero gap [Total Seal] rings only provide marginal reduction in leakdown.The end gaps of maybe .030"in a 5.125" cylinder only amount to .003 percent of the 16" circumference of the ring. doesn't allow much leakage.And some engines [not aircraft]only use one compression ring anyway and don't seem to suffer for it.
Randy Tubbs A+P
Engine machinist since early 1980s
 
scsmith said:
I've often heard that rings are supposed to rotate, but do they really? What is the source of torque that makes them rotate? I always figured as soon as the slightest bit of carbon build-up occurs in the ring groove, it probably pins the ring in one spot.
Click to expand...

I am not a physicist or Engineer, so can't tell you why, I just know they do. Each engine manufacturer gives a recommended ring orientation and they rarely come out at disassembly in that orientation. Free movement of the ring is key to good compression. If your rings stick, you'll see it in poor compression readings. The pressurized air from above the piston works into the ring/land interface and pushes against the back of ring, increasing the force of the ring against the wall. Sometimes this effect is enhanced by using a tapered ring, as some lyco's use (the tapered ring can harness some of the more prevalent downward force) Without this, dynamic compression and power will be down. A stuck ring will eventually cause serious compression issues, as the ring will be inhibited from forming a seal against the wall.

Larry
 
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Wow, this subject is a real rabbit hole and just used up a couple of hours of my time. :)Try Googling some of the references used in the Korean paper Dan listed.
 
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